FORMATION AND MAINTENANCE OF CORPORA …...FORMATION AND MAINTENANCE OF CORPORA LUTEA IN LABORATORY...

FORMATION AND MAINTENANCE OF CORPORA LUTEA IN LABORATORY ANIMALS

G I L B E R T S. G R E E N W A L D 1, 3

University oJ Kansas Medical Center, Kansas City

AND

I R V I N G R O T t t C H I L D 2, 3

Western Reserve University, Cleveland, Ohio

T HE past decade has witnessed a revival of interest in the mammalian corpus

luteum. The availability of new techniques is in part responsible but a more important factor has been the increasing use of cows, sheep, pigs and even women as experimental species. The use of these mammals has uncovered, to a much greater degree than was expected, a diversity in the physiology of the corpus luteum, as well as in mechanisms regulating other reproductive phenomena. This has been the main impetus for the renewed attack on problems of luteal physiology.

For the common laboratory species--guinea pig, hamster, mouse, rat and rabbit--a long history of histophysiologic studies is available, dating back to the pioneering investiga- tions of the first 25 yr. of this century. In fact, we are deluged with an embarrassing wealth of information, which is gradually and unfortunately being given less attention than it deserves in the training of younger in- vestigators.

The purpose of this paper is to summarize, on a comparative basis, knowledge of the corpus luteum and the factors involved in its growth and regression in the common laboratory species. Topics to be considered by other speakers at this symposium have been deliberately excluded from this review.

The five species under discussion differ from one another in the characteristics of their estrous cycle and in the conditions under which the corpora lutea are formed and function (for details and references see Eckstein and Zuckerman, 1956). The mouse, hamster and rat have cycles of 4- to 6-days' duration,

1 Department of Obstetrics and Gynecology. 2 Department of Obstetrics and Gynecology. aThis is a contribution from the Research Professorship in

Human Reproduction (G.S.G.) The authors' research incor- porated in this review has been supported by grants from the U.S.P.H.S. (HD 005 96) and the Ford Foundation (G.S.G.) and the U.S.P.H.S. (HD 000 28) and the institutional grant to Western Reserve University for the study of aging (I .R.) .

during which corpora lutea are formed spontaneously at each ovulation period. These corpora, however, either do not function at all as significant sources of progesterone secretion, or, more likely, secrete progesterone at a very low rate and for less than the full duration of the estrous cycle. The guinea pig has a cycle of about 16 days, ovulation and corpus luteum formation are spontaneous, and the corpora lutea show histologic and physiologic signs of activity for a period of about 12 to 13 days. The rabbit ovulates only in response to coitus (or an equivalent stimulus) and the corpora lutea formed at ovulation also function for a period of about 16 to 18 days.

In the mouse, hamster and rat, periods of luteal activity equivalent to those seen in the guinea pig and rabbit occur after sterile coitus, cervical stimulation or similar stimuli. This period of prolonged and heightened activity of the corpora is called pseudopregnancy but it is physiologically homologous with the luteal phase of the guinea pig and similar animals. The duration of pseudopregnancy in the hamster is about 9 days and in the mouse and rat about 13 days.

Pregnancy lasts 16 days in the hamster, 22 days in the rat, 19 to 20 days in the mouse, 32 days in the rabbit, and 64 days in the guinea pig. Lactation lasts for approximately 3 to 4 wk. in the rat, mouse and hamster, 50 to 60 days in the rabbit, and for variable intervals in the guinea pig. Postpartum ovulation occurs spontaneously in the rat, mouse and guinea pig, and after coitus in the rabbit. I t does not occur at all in the hamster. The lactation period is accompanied in each species by characteristic changes in ovarian activity which differ enough from one another so that their description will be deferred untiI the appropriate section.

Definitions of a few important terms are also in order here. Luteotrophin (or its adjectival form) is used only in a generic sense

139

140 GREENWALD AND ROTHCHILD

and not as a synonym for any particular pituitary or placental hormone. I t signifies a substance which by whatever means stimulates a corpus luteum to grow and to secrete progesterone. (Whether the definition of luteotrophin should apply to stimulation of secretion of related steroids (e.g.: 20a or 20fl- hydroxy-pregn-4-ene-3-one) is an open question).

Luteolysin (or its adjectival form), similarly, signifies a substance which causes a corpus luteum to regress in size or to stop secreting progesterone or related steroids, or both.

There is no convenient term (or one gener- ally agreed on) for a substance which stimulates a corpus luteum to secrete estrogen. A luteotrophin may, under certain conditions, also stimulate estrogen secretion, but this is not why it is a luteotrophin. In other words, the term luteotrophin implies the ability to stimulate progesterone secretion, regardless of whatever else may also be stimulated.

The term gonadotrophin is a generic one for substances which act in a stimulatory way on a gonad or on its parts. I t therefore includes follicle stimulating hormone (FSH), luteinizing hormone (LH) or interstitial cell stimulating hormone (ICSH), and prolactin, among the pituitary hormones, and human chorionic gonadotrophin (HCG), human placental lactogen (HPL), and pregnant mare's serum gonadotrophin (PMSG). When com- monly used for the pituitary hormones the term implies only FSH and LH, but does not explicitly exclude prolactin. When used here, however, it will have the generic connotation only.

Folliculotrophin is a term proposed to serve as a specific generic term for either FSH or LH, or both (Rothchild, 1960a, b). In other words, this term specifically excludes prolactin. In the text, the latter is referred to as such, or as a luteotrophin, when that is the context in which it is used.

Cellular Origin of the Corpus L u t e u m

Whether the corpus luteum is derived solely from granulosa or theca cells or from both types of cells has been a recurring question in reproductive biology (for early references see Corner, 1919). The possible functional significance of a dual derivation of luteal cells was apparent to Leo Loeb (1906), who wrote: "Although these two varieties of cells (granulosa and theca) cannot be distinguished morphologically after the r luteum has

been formed, we cannot say with certainty that there might not exist some chemical or physiochemical differences." The studies of Falck (1959) and of Short (1962) have produced evidence suggesting that the theca cells (in conjunction with granulosa cells) may be the more important source of estrogen secretion, while luteal cells derived from granulosa cells may be the major source of progesterone secretion.

In some mammals, e.g., the human female (Nelson and Greene, 1953) and cattle (Donaldson and Hansel, 1965), a histologic distinction between luteal cells of thecal or granulosal origin can be made even late in the life span of a corpus luteum. The problem is complicated in the laboratory species by the fact that even in the best of material, morphologic and histochemical distinctions between the two cell types are soon lost.

The corpus luteum of the rat is the best documented example among the laboratory species of a dual cellular origin. This has been demonstrated with Azan staining, although by 36 hr. after ovulation it is difficult to distinguish between the theca and granulosa derivatives (Pederson, 1951). Histochemical localization of alkaline phosphatase (Malone, 1960) and dehydrogenase activity (Malone, 1957) also indicate that theca cells form an integral part of the corpus luteum of the rat. The invasion of the granulosa layer by the theca blood supply with accompanying theca cells (Bassett, 1943) is additional evidence for a thecal contribution to the rat corpus luteum. The pattern of luteinization of cystic follicles (Engle and Smith, 1929) also involves primarily the theca cells. Since the granulosa cells tend to degenerate in such follicles, however, such evidence for a thecal contribution to the normal rat corpus luteum must be interpreted with caution, although the fact that preovulatory thecal luteinization is a wide- spread phenomenon (see Rothchild, 1965a) makes it a likely possibility.

The corpus luteum of the guinea pig is also derived from theca and granulosa cells (Loeb, 1906; Rowlands, 1956) but by 32 hr. after ovulation, luteinization has progressed to the point where the precursor cells can no longer be distinguished from one another. A much more cautious view on the origin of the guinea pig corpus luteum minimizes the contribution of theca cells to the definitive corpus luteum (Stafford et al., 1942).

In the other laboratory species [hamster (Nakano, 1960); mouse (Deanesly, 1930a) and rabbit-(Deanesly, 1930b)] the genera~

CL IN LABORATORY ANIMALS 141

consensus is that the corpus luteum is derived solely from granulosa cells. In the mouse and rabbit the theca interna becomes reduced and disappears during vascularization of the corpus luteum. If ovulation is induced in the rabbit by large doses of exogenous gonadotrophins the granulosa cells degenerate and the theca interna proliferates towards the central cavity (Yasuda and Doi, 1953). Similarly, in the immature rat given excessive amounts of gonadotrophins, thecal luteinization and the ultimate degeneration of the granulosa layer is observed (Corner, 1938).

Are the differences in cellular origin of the corpus luteum reflected in the ability of luteal tissue to secrete estrogen as well as progesterone? There is some evidence that the corpora lutea of the rat can secrete estrogen. Greep et al. (1942) showed that the ovaries of hypophysectomized rats with luteal tissue induced by exogenous gonadotrophins secreted estrogen in response to purified LH as evidenced by vaginal cornification. This did not occur if the ovaries lacked corpora lutea. It has recently been shown that LH stimulates estrogen secretion from corpora lutea of the rat regardless of whether such corpora are also secreting progesterone (Mac- Donald et al., 1966). Biochemical and biological evidence indicates that heavily luteinized ovaries of PMS-HCG treated rats secrete appreciable quantities of estrogen (Richardson et al., 1962; Armstrong and Greep, 1965).

The only evidence for estrogen secretion by the corpus luteum of the guinea pig is provided by histochemical localization of 17fl- ol dehydrogenase in the corpora lutea of pregnant animals (Davies et al., 1966).

Incubation of rabbit corpora lutea with suitable precursors results in the production of androgens (Gospodarowicz, 1965a) and progesterone (Gospodarowicz, 1965b) but not estrogen. Whether the corpora lutea of the mouse and hamster are also capable of estrogen synthesis has not been established.

Corpus L u t e u m of the Es t rous Cycle

We will describe here the characteristics of the corpora lutea of the estrous cycle of only the mouse, hamster and rat. In the next section the corpora lutea of pseudopregnancy of these three species will be compared with those of the rabbit and guinea pig.

In the case of the rat (Long and Evans, 1922) and mouse (Allen, 1922) three or more generations of corpora lutea can be present in

the ovaries during the estrous cycle. These represent the corpora of successive ovulation cycles; hence, each set of corpora lutea per- sists for approximately 12 to 14 days. In the rat, corpora lutea of different ages are distinguishable by their size (Deane, 1952; Boling, 1942; Long and Evans, 1922) vascularity (Bassett, 1943) and histochemical characteristics (Everett, 1945; Deane, 1952; Pupkin et al., 1966).

The newly formed corpora lutea reach their maximal size by early diestrus (Boling, 1942) when they have an approximate volume of 790 x 106 ~3 (Deane, 1952); they are maintained at this size through metestrus of the next cycle (Deane, 1952). Between metestrus and diestrus day 1 (i.e., during the second cycle of their existence) the corpora lutea abruptly regress. This regression (in the second cycle) coincides with several other events, namely: closure of blood vessels and development of areas of degeneration and leucocyfic infiltration (Bassett, 1943); eleva- tion of the content of hydrolytic enzymes (Banon et al., 1964; Lobel et al., 1961); increased content of 20 a-hydroxysteroid dehydrogenase (Pupkin et al., 1966); and increase in cholesterol concentration (reflected in development of a moderately intense Schultz reaction) (Everett, 1944, 1945; Deane, 1952; Dawson and Velardo, 1955).

Freshly formed corpora lutea show differences in cholesterol deposition which are most likely relate8 to strain differences. Maximum cholesterol deposition, as evidenced by histochemical techniques, is usually correlated with minimal secretory activity of the corpus luteum; for example, cholesterol is frequently absent during pseudopregnancy (Dawson and Velardo, 1955) and pregnancy (Everett, 1947) when progesterone is being secreted in appreciable quantities.

Deane (1952) reported the absence of Schultz-positive material at any stage of development of first generation corpora lutea in the Sprague-Dawley rat. On the other hand, Everett (1945) working with the Vanderbilt strain, observed a positive Schultz reaction which began during diestrus; this was followed by a transient depletion dnring the subsequent proestrus, and in turn by a renewed intracellular deposition of cholesterol on the following day (full vaginal cornification). Everett (1944) therefore suggested that the corpora lutea of the preceding ovulation might be the source of the progesterone secreted during proestrus. Further work

142 GREENWALD AND R O T H C H I L D

(Everett, 1947) suggested that LH causes cholesterol deposition in the presence of prolactin, but that an excess of prolactin prevents this action of LH on the corpus luteum. Thus, the LH/prolactin ratio may determine whether cholesterol is stored or depleted. In this connection, it is now known that the ovulatory surge of LH secretion occurs at proestrus (Everett, 1961, 1964; Schwartz, 1966); whether prolactin is also discharged at this time has not yet been determined.

In a particularly astute leap into the unknown, Everett (1947) postulated: " I f local cholesterol synthesis (or its accumulation from an extra-luteal source) is an essential initial step in progesterone formation, it follows that any agent such as lactogen, which by itself is capable of maintaining corpus luteum secretion, must have a certain minimal capacity to produce cholesterol. I t also follows that under ordinary conditions of full corpus luteum activity, as in pregnancy or natural pseudopregnancy, cholesterinization is nicely bal- anced with the later steps of progesterone synthesis. No storage of cholesterol then occurs. I f minimal cholesterol production is simply one of several attributes of a single entity, lactogen, then it would appear that LH is capable of substituting quite exclu- sively for this one attribute. Granting this, one might logically expect that other agents may be found which may substitute exclu- sively with respect to later steps in progesterone synthesis. Combined with LH such a substance might produce the full effect of maintaining corpus luteum secretion, yet by itself be ineffective." The possible significance of this statement will be considered in a later section of this paper.

The estrous cycle of the rat usually varies in length from 4 to 6 days. However, females injected with i0 ~g. of estradiol on diestrus I and caged with males the next day will ovulate prematurely if mating takes place (Aron et al., 1963). The duration of the cycle in the rat can be shortened or prolonged by single injections of estrogen or progesterone depending on the stage of the cycle at which the steroids are administered (Everett, 1948).

The corpora lutea of the estrous cycle of the mouse have been studied much less ex- tensively than those of the rat. In the mouse, histologic accumulation of fat in lutein cells is presumptive evidence for a slowing down of cellular metabolism but it is not necessarily synonymous with degeneration. The corpus luteum of the mouse has a diameter of 560

at metestrus; it reaches its maximum diameter (about 600 ~) by the next estrus. The corpora lutea of the e~trous cycle begin to accumulate fat when 2- to 3-days old. However, by two days after metestrus, the corpora lutea of the previous cycle contain hardly any fat or lipid (Deanesly, 1930a). This is in marked contrast to the aging corpora lutea of the cyclic rat.

The corpora lutea of the cyclic hamster show several interesting differences from those of the mouse and rat. The estrous cycle of the hamster has a remarkably constant duration of 4 days (Deanesly, 1938). In contrast to the mouse and rat, there is never more than one set of corpora lutea in the hamster's ovaries. If metestrus is designated as day 1 of the cycle, the first signs of luteolysis are apparent by day 3 (diestrus); these consist of a characteristic leucocytic infiltration, the appearance of free and esterified cholesterol and the accumulation of triglycerides (Guraya and Greenwald, 1965). By day 4 the corpora lutea are avascular and their cells contain pycnotic nuclei and vacuolated cytoplasm. I t is interesting that the enzyme 3fl-ol dehydrogenase can be demonstrated in cyclic corpora lutea on days 1 to 3 but not on day 4. By day 4, 3fl-ol dehydrogenase activity is limited to the theca interna, especially of large preovulatory follicles (Wingate and Greenwald, unpublished data). This suggests that the preovulatory source of progesterone, which is essential for behavioral estrus in the hamster, comes from extra-luteal tissue.

Unpublished experiments on hypophysectomized cyclic hamsters shed some light on the factors responsible for the rapid degeneration of the corpora lutea (Greenwald). When hamsters are hypophysectomized on day 1 of the estrous cycle, the histology and growth of the corpora lutea on subsequent days is the same as in intact animals. Thus, the development and regression of the corpus luteum in cyclic hamsters seems to be independent of the pituitary. Luteolysis during the estrous cycle can thus be attributed to the lack of pituitary luteotrophic factors rather than to the effect of a specific pituitary luteolytic hormone or hormones.

The corpora lutea of the cyclic hamster can be brought to full secretory activity as late as 54 hr. after ovulation (Greenwald, 1963). The corpora lutea of the cyclic rat also retain the capacity to be activated for a similarly prolonged period (Nikitovitch-Winer and Everett, 1958).


The cyclic corpora lutea of the rat and mouse secrete variable amounts of progestins. The content of progestins in ovarian vein blood during the estrous cycle has been measured chemically for the rat (Eto et al., 1962; Telegdy and Endrd~czi, 1963). Both studies indicate that during the estrous cycle, more . 20a-hydroxy-pregn-4-en-3-one (20-OH- P) is secreted than is progesterone. The peak values for progesterone content in the blood occur during proestrus while 20-OH-P content is highest at estrus. The interconversion of 20-OH-P and progesterone is mediated by a TPN ~linked enzyme, 20a-hydroxysteroid dehydrogenase. The enzyme varies in content during the estrous cycle (Wiest et al., 1963; Balo~h arL8 Wiest, 1966). The highest values are found during proestrus and diestrus; by histochemical endpoints, the enzyme is con- centrated in the corpora lutea (Pupkin et al., 1966). A dramatic increase in lutein 20a-hydroxysteroid dehydrogenase occurs after administration of LH or HCG to immature, superovulated rats (Kidwell et al., 1966).

Plasma levels of progestins have been measured in the cyclic mouse by the Hooker- Forbes assay (Guttenberg, 1961), which does not distinguish between progesterone and the 20-OH-P. The values reported, therefore, are considerably higher than for progesterone determined chemically. In the cyclic mouse, the minimum plasma level of progestins occurs during diestrus; maximum activity is observed during proestrus and estrus (four progesterone equivalents/ml.), and is followed by a sharp drop. The preovulatory levels reported for the mouse are higher than for monkeys, women or rabbits (Guttenberg, 1961): Whether this is a true species difference or one due to technical problems has not been established.

There is no evidence for progesterone secretion by the cyclic corpus luteum of the hamster. The extreme regularity of the cycle suggests that if progesterone is secreted at all, it is produced in very small quantities, and only during a sharply circumscribed time of the cycle.

Corpus L u t e u m of Pseudopregnancy and P regnancy

The corpora lutea of the estrous cycle of the mouse, hamster and rat, following sterile coitus, cervical stimulation, or similar stimuli, go through a period of growth and secretory activity which is physiologically homologous

to that which takes place in many other mammals (e.g., guinea pig, sheep, cow, horse, monkey, women etc.) spontaneously. In the mouse, rat and hamster, this phase of luteal activity has been called "pseudopregnancy", while in the other mammals it is simply known as the luteal phase of the cycle. The rabbit shows a similar period of luteal phase activity following ovulation, but in this species, as already noted, ovulation itself is dependent on coitus. The corpora lutea of the ovulation cycle of the guinea pig and rabbit, therefore, are more properly to be compared with the corpora of pseudopregnancy of the mouse, hamster and rat.

Dubreuil (1962) has classified functional corpora lutea into Corps progestati/s (progestational corpora lutea) and: Corps gestatiJs (gestational corpora lutea). The former type includes corpora lutea that secrete progesterone from the time of mating (sterile or fertile) (i.e., from ovulation) until luteolysis or ova-implantation. Examples of progestational corpora lutea in the rat would be the corpora of pseudopregnancy, of delayed implantation, the corpora lutea maintained by an ectopic pituitary transplant (see below), and the corpora lutea of the first third of normal pregnancy. Gestational corpora lutea describe the type of corpora lutea which characterizes the period from implantation to parturition.

This distinction is a useful one, at least for the species under discussion, since, as will be seen below, progestational corpora lutea are usually not as large nor do they secrete as much progesterone as do those of pregnancy. The gestational corpus luteum thus represents a condition of optimal luteal activity.

The distinction, however, is not universal. For example, the corpora lutea of hysterectomized guinea pigs resemble more the corpora lutea of pregnancy than of the luteal phase of the cycle (Rowlands and Short, 1959); in several monestrous mammals (e.g., dog, cat, ferret) in which a 4 to 8 wk. period of pseudopregnancy is characteristic, the corpora of pregnancy do not differ appreciably from those of pseudopregnancy (Eckstein and Zuckerman, 1956; Amoroso and Finn, 1962). The major value of D~breuil's classification is that it draws attention to the role of the pregnant uterus (regardless of whether the fetus, the placenta, or the uterus itself is the important factor) in the maintenance of the corpora lutea.


In the rat, the corpora lutea of pseudopregnancy do not store cholesterol (Dawson and Velardo, 1955) and do not apparently contain 20a-hydroxysteroid dehydrogenase (Wiest et al., 1963). The corpora lutea of pseudopregnancy are about 1.5 ram. in diameter (Weichert and Schurgast, 1942) and reach a maximum weight of about 1.6 mgm. (Roth- child and Schwartz, 1965). The injection of estrone or of other estrogens increases their diameter to the size of gestational corpora lutea (Weichert and Schurgast, 1942; Des- clin, 1949b; Maekawa, 1956; Alloiteau, 1965). On day 4 of pseudopregnancy, the progesterone/20-OH-P ratio shifts sharply in favor of progesterone (Fajer and Barraclough, 1967). This coincides with a maximum reaction of the corpus luteum for various lipid stains (Dawson and Velardo, 1955) and is also the time when the size of the corpus luteum first exceeds that of the corpus luteum of the estrous cycle (Toyoda, 1961).

In the pregnant rat, cholesterol was not detected histochemically through day 16 of gestation (Everett, 1947). Claesson and Hil- larp (!948) reported that Schultz-positive sterols could be demonstrated in only small amounts on the 19th and 20th day of pregnancy; 20a-hydroxysteroid dehydrogenase was also absent in the corpora lutea during the last week of gestation (Balogh, 1964); presumably this enzyme was also absent during the earlier part of pregnancy. However, a recent abstract indicated that the enzyme appears in the corpora lutea of pregnancy near term (Kidwell and Wiest, 1967). This observation is consistent with biochemical data (see below).

There is a considerable amount of mitotic activity in the corpora lutea of rats during the first 5 days of pregnancy; a second peak occurs between days 12 and 15 (Bassett, 1949). The size of the corpora lutea begins to increase rapidly on the 10th to l l th dav (Weichert and Schurgast, 1942; Bassett, 1949) and reaches its peak on day 15; a steady decline occurs thereafter (Bassett, 1949).

The gestational corpora lutea of the rat, commencing at days 10 to 11, have increased amounts of succinic dehydrogenase (Meyer et al., 1947) and alkaline phosphatase (Staf- ford et al., 1947) ; these findings suggest that a period of increased metabolic activity begins at this time.

The concentration of progestins in ovarian venous blood during pregnancy in the rat agrees with other parameters of luteal activity.

T h e content of progesterone in the blood reaches a maximum on day 15 and then drops sharply during the rest of pregnancy (Eto et al., 1962); in contrast, the content of 20- OH-P is markedly low during the first half of pregnancy, but increases abruptly during the last week.

There have been relatively few studies of functional corpora lutea in the mouse. The corpora lutea of pseudopregnancy in the mouse are only slightly larger (720 ~ diameter) than the corpora lutea of the estrous cycle (Deanesly, 1930a). By 6 to 7 days after their formation, the corpora lutea of pseudopregnancy are laden with fat and it is possible that their functional life is nearing its end. The change in the nuclei of en- dometrial stromal ceils indicate that progestin secretion is relatively high between days 3 and 7 of pseudopregnancy and begins to decline by day 8 (Atkinson and Hooker, 1945).

During pregnancy the corpus luteum of the mouse begins to increase in size on day 8 and reaches its maximal diameter on day 13. I t is interesting that between the 10th and 12th days of pregnancy all the old corpora lutea, regardless of their age, degenerate rapidly and simultaneonsly. The corpora lutea of pregnancy begin to accumulate fat at day 18 and by parturition their mean diameter has decreased to 864 t~ (Deanesly, 1930a). The plasma level of progestins (determined by the Hooker-Forbes assay method) is elevated between days 9 and 12 of pregnancy in the mouse. A second peak occurs between days 13 and 15. The plasma level of progestins drops very sharply at day 16, i.e., almost 4 days before parturition (Forbes and Hooker, 1957). This evidence for a peak of progesterone secretion at day 13 presumably reflects the luteotrophic effect of the placenta, since hypophysectomy at this stage, but not earlier, is compatible with the maintenance of pregnancy (Gardner and Allen, 1942).

In the hamster pseudopregnancy lasts 8 to 10 days, with a 9 day interval predominat- ing (Kent and Atkins, 1959). A decidual response is elicited two full days after sterile mating and the response is maximal during the seventh day, with necrosis commencing near the end of the 8th day (Turnbull and Kent, 1963). The corpora lutea are still vascular and histologically normal 7 days after sterile mating, but 2 days later regressive changes are apparent and the corpora lutea are grossly avascular (Greenwald, unpublished data).

In the hamster the corpora lutea of preg-


nancy enter a second growth phase on day 12, reaching their maximal size (1044 to 1061 /~ diameter) by days 14 to 16 (Green- wald et al., 1967). Also beginning on day 12, there is a remarkable increase in the number of large antral follicles (Greenwald, 1964). Vaginal mucification--an indication of the interaction of estrogen and progesterone--is first evident on day 3 of pregnancy and is maintained through day 12; however, on day 15 the pattern of mucification is altered, pos- sibly because of a drop in progesterone secretion (Weitlauf and Greenwald, unpublished data). There are, unfortunately, no biochemical determinations of progesterone available for either the cyclic, pseudopregnant or pregnant hamster.

In the guinea pig, the corpora lutea attain their maximum size on about the 10th to 12th day after ovulation (Rowlands, 1956, 1961) ; they begin to degenerate histologically between days 10 and 12 (Loeb, 1911). This time also coincides with a drop in progesterone concentration in the corpora, i.e., from 16.2 ~g./gm. on day 6 to 7.7 i~. /gm, between days 11 and 13 of the cycle (Rowlands and Short, 1959). The changes in histolo~v and progesterone concentration are probably associated with the early stages of the preovlflatory phase of the next cycle, since at this time also, unilateral ovariectomv does not lead to any change in the number of eggs ovulated at the next ovulation (Hermreck and Greenwald, 1964).

In contrast to the corpora lutea of the estrous cycle of the guinea pig, those of pregnancy continue to grow until the 18th or 20th day to a diameter of 6 to 7 mm. They remain at this size for the rest of pregnancy (Row- lands, 1956). The maximum weight (4.9 m~.) is also reached at about this time and is also maintained to term (Rowlands and Short, 1959). From day 35 on the corpora lutea of pregnancy undergo a gradual aging, charac- terized bv a blurring of the outlines of the luteal cells, and~ a progressive infiltration of fibrous connective tissue cells.

On day 11 to 13 of pregnancy in the guinea pig, the mean progesterone concentration of the corpus luteum (14.9 ~g./gm.) is not signi- ficantly different from its concentration on day 6 of the ovulation cycle (16.2 /~g./gm.), and increases further to 25.0 ~g./gm. on days 21 to 23. The latter concentration is maintained until term (Rowlands and Short, 1959). The differences in size and progesterone content are undoubtedly related to the activity of the

placenta. Implantation occurs on the 6th to 7th day post coitum (p.c.) (Deanesly, 1960b) and the chorio-allantoic placenta is completely established by day 21 to 23 of pregnancy.

Systemic levels of progesterone have recently been determined in pregnant guinea pigs (Heap and Deanesly, 1966). The concentration of progesterone in intact animals reached a peak at 30 to 45 days p.c. (265 ng./ml.) and declined to 140 ng./ml, prior to delivery. In pregnant guinea pigs ovariectomized 28 days p.c. or later, progesterone levels were lower except at the end of pregnancy. This indicates that the ovarian contribution of progesterone is initially appreciable but the placenta becomes increasingly important as a source of progesterone as pregnancy progresses.

In the rabbit ovulation occurs 10 to 12 hr. after coitus or the intravenous injection of HCG or LH (Harper, 1963). The changes in progestin secretion associated with ovulation are reflected in a massive mobilization of free and esterified cholesterol from the interstitium, which can be demonstrated biochemically (Claesson and Hillarp, 1946) as well as histochemically (Guraya and Greenwald, 1964). Within 30 rain. after coitus there is an abrupt increase in the progestin concentration of the peripheral blood (Forbes, 1953). The principal component is 20-OH-P which is present at a concentration approximately 10 times greater than that of progesterone (HiUiard et al., 1963). The same changes in progestin secretion occur even if the ovaries are de- prived of corpora lutea and large follicles, a finding which implicates the interstitium as the source of the steroid secretion. The interstitium of the rabbit ovary is an extremely versatile tissue which has hydroxysteroid dehydrogenase activity towards steroids with OH groups in the 17fl, 20a and 20fl positions (Davenport and Mallette, 1966). The blood levels of 20-OH-P are elevated for several hours after coitus but are nndetectable during and immediately following ovulation itself (Hilliard et. al., 1964).

By the 6th day of pseudopregnancy, the corpus luteum of the rabbit is fully formed and remains well vascularized until day 15 (Hughes and Myers, 1966). The first signs of luteal regression are evident on day 12 and consist of irregularities in the outline of the nuclei, an abundant influx of leucocytes and incipient fatty degeneration of some luteal cells. These features are accentuated by day 15.

i46 GREENWALD AND ROTHCHILD

In the pregnant rabbit, the weight of the corpora lutea increases until day 28 (Pincus and Berkman, 1937). If one uterine horn is sterilized by ligating the utero-tubal junction before mating, progestational proliferation (an effect of progesterone) is present in the sterile horn only until day 18. This corres- ponds to the duration of pseudopregnancy. The disappearance of the reaction at this time may be due not so much to a drop in cir- culating progesterone but to a marked increase in estrogen secretion (Beerstecher, 1942). A sharp rise in serum progestins, determined by the Hooker-Forbes assay method, occurs between days 4 and 12 of pregnancy. Thereafter, a plateau (at 6 to 8 ~g./ml.) is observed until day 24 and there is subsequently a slight increase until parturition (Zarrow and Neher, 1955). Actual measurements of progesterone by chemical methods in ovarian venous blood indicate that a peak value of 2.36 ~g./ml. is attained by day 15 of pregnancy. This is followed by a steady decline; a consistent drop--to 0.44 t~g./ml.-- has been observed 2 to 3 days before delivery (Mikhail et. al., 1961).

Role of the Placenta in Lutea l Func t ion

The five species under discussion vary widely in their dependence on the pituitary for the maintenance of both functional corpora lutea and gestation itself. Pregnancy in the rat (Pencharz and Long, 1933) and the mouse (Gardner and Allen, 1942; Selye et. al., 1933) is maintained after hypophysectomy on the l l t h to 12th day or later. Hypophy- sectomy of the guinea pig at 34 to 36 days of pregnancy or earlier results in fetal resorption within 2 days, but at 40 to 41 days or later, results in viable young at term (Pencharz and Lyons, 1934). On the other hand, pituitary extirpation at any time during gestation in- terrupts pregnancy in the hamster (Green- wald, 1967a) and the rabbit (Smith and White, 1931; Firor, 1933; Robson, 1937b).

In all five species, the placenta has some luteotrophic activity since the retention of the placentae after fetectomy maintains functional corpora lutea (guinea pig: Klein, 1939; hamster: Klein, 1938; rabbit: Klein, 1933; mouse: Deanesly and Newton, 1941; rat: Klein, 1935).

I t has been amply demonstrated that, with the exception of the rabbit, the placenta in the four other laboratory species produces a lactogenic substance. Thus, after hypophy-

sectomy there is a slight postpartum production of milk in the pregnant guinea pig (Pen- charz and Lyons, 1934), mouse (Newton and Lits, 1938; Newton and Richardson, 1940), and hamster (Greenwald, 1967a). Rat placentas have been demonstrated also to have mammotrophic and lactogenic effects as well as crop-gland (pigeon) stimulating effects (Averill et al., 1950; Ray et al., 1955; Cani- venc, 1956).

The luteotrophic effect of the placenta has been shown in several ways. Thus, cyclic mice injected daily with homogenates of two to three placentas (day 12) show a persistent vaginal mucification (Cerrufi and Lyons, 1960). Saline extracts of rat placentas injected into pseudopregnant rats after hypophysectomy on the 4th day sustained enough luteal activity to elicit deciduomata, although fresh placental extracts injected from the 19th day of pregnancy onward did not delay parturition (Astwood and Greep, 1938). How- ever, 0.5 mg. or more of prolactin, injected from the 16th to 20th day of pregnancy did delay parturition (Meites and Shelesnyak, 1957). Twelve-day rat placentas injected into cyclic rats for 10 days produced vaginal mucification and excellent lobulo-alveolar development of the mammary glands (Ray et al., 1955). An interesting experiment involved the maintenance of pregnancy, in rats hypophysectomized on day 6, with daily implantation of rat placentas (Averill et al., 1950). Neither day 10 nor day 15 placentas were effective, but day 12 placentas (5 to 8 /d / ra t ) maintained pregnancy in eight of 10 recip- ients. However, one day-12 placenta per day plus 1 ~g. of estrone every 48 hr. was an equally effective treatment.

Estrogens could not be demonstrated in the rat placenta (Canivenc, 1956) as evidenced by its failure to produce vaginal cornification when transplanted to the vagina. Both rabbit and guinea pig placenta preparations failed to produce estrogens on incubation with C19 sub- strates; thus, the aromatizing and 17fl-ol- dehydrogenase systems appear to be lacking (Ainsworth and Ryan, 1966). However, ovariectomized pregnant guinea pigs maintained with progesterone have mucified vaginas at day 20 comparable to control females killed at the same stage of pregnancy (Deanesly, 1960a). In the latter instance, the possibility can not be excluded that the adrenal serves as a source of estrogen.

With the exception of the guinea pig (Heap and Deanesly, 1966), there is little direct evi-


dence for progesterone secretion by the placentas of the laboratory species. In ovariectomized pregnant guinea pigs, progesterone is high in the uterine vein but low in placental tissue. No progestafional activity can be detected biochemically in the rat placenta (Wiest, 1959), although pregnancy can be maintained to term, even after castration if all the fetuses but one are removed, and all the placentas are left in situ (Haterius, 1935, 1936). Following ovariectomy of the pregnant rabbit, there is a prompt return of the plasma progestin content to pre-gestational levels (Zarrow and Neher, 1955). Bilateral ovariectomy leads to the interruption of pregnancy in all of the laboratory species except the guinea pig (for references, see Amoroso and Porter, 1966). This indicates that the amount of progesterone produced by the placenta must be less than enough to maintain pregnancy except under special conditions, such as those of Haterius' experiment.

The last topic to be considered in this section is the effect of production of new corpora lutea on the time of regression of the corpora lutea of pregnancy. In the guinea pig (Row- lands, 1956), hamster (Greenwald, 1967c) and mouse (Burdick and Crump, 1951) both sets of corpora lutea regress simultaneously and the time of delivery is unaffected. Con- versely, when ovulation is induced in the rabbit at day 25, parturition is delayed until day 40 and the induced corpora lutea have a normal life span and regress asynchronously (Snyder, 1934). Ovulation could not be induced in pregnant rats injected with HCG (Greenwald, 1966).

The Corpus L u t e u m of Lac ta t ion

The corpora lutea of lactation of the rat are approximately the same size as the corpora lutea of pseudopregnancy (Weichert and Schurgast, 1942). Injection of prolactin fails to increase them to the dimensions of gestational corpora lutea; it would be interesting to test the effects of estrogen. The corpora lutea of pregnancy are readily distinguishable from the corpora lutea of lactation on the basis of their intense birefringence which is minimal or nonexistent in the latter set (Buno and Hekimian, 1955). On the other hand, both sets of corpora lutea have been reported to be Schultz-negative up to the 30th day (Claesson and Hillarp, 1948).

Several lines of evidence indicate that sub- stantial amounts of progesterone are secreted

during lactation in the rat. This has been shown directly by the chemical determination of progesterone in ovarian venous blood. The amount of progesterone in blood is influenced by the size of the litter; more progesterone is released by mothers nursing six pups than by mothers nursing two (Eto et al., 1962). I t is of interest that the mother must eat the placentas after delivery; otherwise progesterone levels in plasma are reduced (Grota and Eik-Nes, 1967). Deciduomata can be produced from day 4 to 17 in the lactating rat (Lyon and Allen, 1938). This contrasts with pregnancy and pseudopregnancy in which a decidual reaction can be elicited for only a brief time (Yochim and DeFeo, 1963; Selye and McKeown, 1935). Since estrogens can prevent the development of deciduomata (Rothchild and Meyer, 1942), it is reason- able to assume that estrogen levels are very low during the first two weeks of lactation (Long and Evans, 1922).

The factor ultimately responsible for the steroids secreted by the ovary during lactation is undoubtedly the suckling stimulus. Suckling causes prolactin secretion and sup- presses the secretion of the folliculotrophins. The conditions of lactation in the rat are thus conducive to the secretion of progesterone by the corpora lutea and the suppression of estrogen secretion by the remainder of the ovary (Rothchild, 1960a; Rothchild and Dickey, 1960).

In the mouse, the corpora lutea of lactation can be readily distinguished from those of pregnancy in Mallory-Azan stained material. The cells of the former group have definite cell outlines and their cytoplasm is finely granular and less basophilic than are the corpora lutea of pregnancy. The corpora lutea of lactation at their maximum size (660 ~) are smaller than the corpora lutea of pregnancy at the time of parturition (730 /~) (Greenwald, 1958a). The corpora lutea of lactation increase in size at day 11; one day later the vagina becomes fully mucified. Dur- ing the first half of lactation (I0 days) progesterone and negligible amounts of estrogen are present, as evidenced by vaginal histology. From the l l th day onward increasing levels of estrogen are secreted, in association with a significant increase in the growth of antral follicles (Greenwald, 1958a).

The histology of the nuclei of the endo- metrial stromal cells indicates that progesterone is secreted until about day 16 post-partum in all lactating mice and in some animals


until day 21. Despite this evidence for progesterone secretion, deciduomata cannot be produced in the lactating mouse (Greenwald, 1958b) except after treatment with an amount of progesterone (0.25 mg./day) smaller than that required (0.35 mg./day) to induce deciduoma formation in ovariectomized animals (Greenwald, 1958b).

Post-partum ovulation in the guinea pig occurs within 15 to 18 hr. after delivery (Row- lands, 1956). The corpora lutea of pregnancy show marked signs of histologic degeneration within 6 days after parturition (Loeb, 1911). However, they persist as large structures for at least 15 to 19 days post-partum.

In the rabbit post-partum ovulation does not normally occur, but can be induced by mating. The corpora lutea, in contrast to the condition in the rat or mouse, show a survival time inversely related to the size of the litter (Hammond and Marshall, 1925).

Following delivery, the corpora lutea of pregnancy in the rabbit gradually diminish in size and the presence of stored "lipoid" suggests that they are no longer functional (Hammond and Marshall, 1925). Suckling does not prevent ovulation after coitus from days 1 to 4 of lactation, but does so from about the 8th to 12th days in does nursing large litters. In the first instance, the blastocysts begin to develop normally but soon undergo resorption. This is associated with atrophy of the CL of lactation. On the other hand, if only a small litter of one or two is suckled, pregnancy is only occasionally interrupted (Hammond and Marshall, 1925). Thus, the degree of suckling stimulus determines whether functional CL are maintained.

Unlike the previous species, post partum ovulation does not occur in the hamster (Greenwald, 1965b). The corpora lutea of pregnancy of the rat, mouse and guinea pig can be recognized as such during the first 2 wk. post partum, even though their size de- creases gradually. In the hamster, however, parturition is accompanied by a rapid luteolysis, so that the corpora are soon reduced to scattered remnants of connective tissue. De- generation of the corpora lutea is paralleled by a massive development of the interstitium and the disappearance of all antral follicles. Ovulation can be induced in the lactating hamster by a single injection of PMSG; however, the induced corpora lutea show signs of histologic regression within 4 to 7 days after their formation (Greenwald, 1965b)

The vaginal epithelium of the lactating

hamster resembles that of ovariectomized animals (Deanesly, 1938). A decidual response cannot be elicited following trauma at day 6 post partum (Choudary and Greenwald, unpublished data). There is thus no evidence for either estrogen or progesterone secretion during lactation in the hamster.

Effects of H y p o p h y s e c t o m y on the Corpora Lu tea

The histologic fate of the corpora lutea after hypophysectomy differs considerably among the five species. The corpora lutea of the cyclic rat regress so slowly that they are easily recognizable as late as 9 mo. after hypophysectomy (Smith, 1930), although their progesterone-secreting ability is lost within 24 hr. after hypophysectomy (Evans e,t al., 1941; Astwood, 1941; Desclin, 1949a). However, removal of the pituitary of the pseudopregnant rat (Rothchild, unpublished observations), or of the pituitary transplant of the hypophysectomized rat bearing its pituitary as a transplant beneath the kidney capsule (Everett, 1956) (see further below) results in the rapid decrease in size of the corpora. The latter corpora lutea are, of course, much larger than those of the cyclic rat. The luteal cells of hypophysectomized rats have a less highly developed agranular endoplasmic reticulum and contain more lipid droplets than the cells of active corpora lutea (Enders and Lyons, 1964). I t is interesting that 3fl-ol-dehydrogenase can be demonstrated histochemically in large amounts in the corpora lutea of untreated rats for as long as 11 wk. after hypophysectomy (Levy et al., 1959).

Treatment with prolactin several weeks after hypophysectomy markedly hastens the morpholgic regression of rat corpora lutea; FSH or LH treatment for the same period of time (13 days), however, does not cause luteolysis (Malven and Sawyer, 1966) (but see below).

In the mouse, corpora lutea begin to show a progressive invasion by connective tissue 12 days after hypophysectomy; no corpora lutea were observed 97 days after hypophysectomy (Leblond and Nelson, 1937).

Following hypophysectomy of the non- pregnant guinea pig, the luteal cells are histologically indistinguishable from those of intact animals; 30 days after hypophysectomy, the corpora lutea are 1.7 mm. z in volume (Rowlands, 1956). I t is interesting that 5 to 19 days after hypophysectomy, the progester-


one concentration in the corpora lutea is apparently higher than in the normal cycle or in pregnancy, although systemic levels are reduced (Heap et al., 1965). One wonders whether this indicates autonomous function of the guinea pig corpora lutea or of the effects of incomplete hypophysectomy.

Hypophysectomy of the rabbit early in pseudopregnancy or pregnancy results in a rapid atrophy of the corpora lutea (Smith and White, 1931; Westman and Jacobsohn, 1937). Four days after hypophysectomy (on day 1 of pregnancy) the corpora lutea are avascular, small, and are infiltrated with connective tissue. Removal of the pituitary during the second half of pregnancy is followed by abortion within 24 to 48 hr. (Robson, 1936).

The corpora lutea of the cyclic hamster or of the hamster in early pregnancy also in- volute rapidly following hypophysectomy (Greenwald, 1967a). However, when hypophysectomy is clone on day 12 of pregnancy, large corpora lutea persist and are histologically normal as long as the placentas remain in' situ; in spite of this the pregnancy is terminated by abortion or resorption.

The nature of the pituitary hormone or hormones which maintain the growth and secretory activity of the corpora lutea is fairly clear in some instances and is still a matter of debate in others. What is probably true in all cases (not only for the five species under discussion but for other mammals as well) is that several hormones participate in the maintenance of luteal activity.

The term "luteotrophic substance" was first used by Astwood (1941). He hypophysectomized young rats previously treated with chorionic gonadotrophin and then tested various pituitary extracts for their luteotrophic effect, using as an endpoint mucification of the vagina in response to twice weekly injections of 10 t~g. of estradiol dipropionate. The results indicated that prolactin, apparently free of FSH or LH activity, possessed luteotrophic activity, but the hormone failed to maintain vaginal mucification beyond 15 days.

In another study, which appeared in the same year, deciduoma formation was used as an endpoint of a luteotrophic effect (Evans et al., 1941) ; prolactin was the only hormone of several tested (FSH, LH, PMSG, human pregnancy urine extract) that induced deciduoma formation in normal or hypophysectomized rats.

Perhaps the most satisfactory evidence for

optimal luteal activity is the maintenance of pregnancy in hypophysectomized animals. Rats hypophysectomized between days 1 and 9 of pregnancy were injected' with 1 to 3 mg. daily of prolactin (Cutuly, 1941, 1942). The author interpreted the results as leaving "no doubt that lactogenic hormone is capable of stimulating function of the corpora lutea". However, the few animals used and the in- consistent results have cast doubt on Cutuly's conclusions. A subsequent paper of Lyons et al. (1943) claimed that Cutuly used impure extracts of prolactin which were not of the highest potency. Lyons' study utilized rats hypophysectomized 1 day after mating and injected for 13 to 16 days. Although neither crude nor pure prolactin established pregnancy in otherwise untreated hypophysectomized rats, treatment with estrone in addition to prolactin allowed implantation to occur and pregnancy to proceed. I t must be kept in mind that the hypophysectomies were per- formed before implantation, and it is now known that estrogen is required for implantation in the rat (Shelesnyak and Kraicer, 1963).

I t would be very worthwhile to determine what hormones are required alter implantation to maintain pregnancy in the hypophysectomized rat. Preliminary experiments indicate that .1 to 4 nag. prolactin daily for 4 days will not sustain pregnancy in rats hypophysectomized at day 6 (Greenwald and Johnson, unpublished data). If pregnant rats are hypophysectomized on the 9th day, and treated with LH from the 9th to 12th days, they will remain pregnant (Alloiteau and Bouhours, 1965). This finding suggests that pituitary LH may be an important factor in the maintenance of pregnancy up to the stage when the placental luteotrophin becomes most important. The effect may be through the stimulation of estrogen secretion which could synergize with the less than adequate amount of placental luteotrophin secreted before the 12th day of pregnancy in the rat.

The corpora lutea of hypophysectomized rats lose their capacity to secrete progesterone if replacement treatment with prolactin is deferred for one or two days (Astwood, 1941; Evans et al., 1941; Desclin, 1949a; Malven and Sawyer, 1966). On the basis of ultra- structural changes, it appears that prolactin has a general metabolic effect on corpora lutea of the rat rather than one which affects a specific organelle (Enders and Lyons, 1964).

The ultimate size reached by the corpora


lutea of hypophysectomized rats depends not only on prolactin but on estrogen as well (D:esclin, 1949b). Combined treatment with both prolactin and estrogens leads.to corpora lutea of larger dimensions than does treatment with prolactin alone (see also Everett, 1956). These important experiments indicate that estrogen exerts a direct effect on the corpora lutea in the presence of prolactin. In the absence of the pituitary the estrogens have only a limited ability to increase the size of the corpora lutea. Thus, in pseudopregnant rats, treated for 28 days after hypophysectomy with sesame oil alone, the mean weight of the corpora was 0.76 mg., while in those treated daily with 5.0 or 50.0 ~g. of estradiol, the mean weights of the corpora were 0.83 and 0.96 mg., respectively (Rothchild and Schwartz, 1965). The estrogens, however, in the presence of prolactin, can affect the size of even regressing corpora lutea. For example, LH treatment for a period of 10 days causes about a 60% decrease in the size of the corpora lutea of hypophysectomized rats bearing autotransplanted pituitaries; the same per- centage decrease occurs, if at the end of treatment, the rats are given 150 ~g. of estradiol benzoate over a period of 5 days. In the latter case, however, the corpora lutea of the control rats (i.e., those given the estrogen, but no LH) are about twice as large as those of the LH-treated rats (Rothchild, 1965a).

The injection of prolactin into cyclic rats (Lahr and Riddle, 1936) and mice (Dresel, 1935) will interrupt the cycle and induce vaginal mucification. In the mouse, treatment with LH plus small amounts of prolactin produced a more striking suppression of the cycle than did treatment with prolactin alone (Nathanson et al., 1937). Pseudopregnancy can be induced in cyclic rats by the daily injection of 1 mg. prolactin commencing on the day after estrus (von Berswoldt-Wallrabe et al., 1964). Massive deciduomata were induced, comparable in weight to the decidual cell response produced in rats made pseudopregnant by stimulation with a glass rod.

Kovacic (1964) has thoroughly studied the effects of a large number of gonadotrophin and prolactin preparations in hypophysectomized mice. The term "luteotrophin" was re- served for hormones which caused hypertrophy of luteal cells and hyperemia of the corpora lutea. These requirements were fulfilled by prolactin and growth hormone, although treatment with prolactin, human LH and growth hormone of human or bovine origin, or with

PMSG or HCG, all caused deciduoma formation. The decidual response could have been due to progestogens secreted by interstitial tissue, which would account for the activity of some of the listed hormones. A subsequent note (Kovacic, 1965) indicated that in cyclic intact mice, some of the preparations tested (e.g., human LH, HCG, PMS) increased the number of hyperemic corpora lutea. However, when injected into hypophysectomized mice no hyperemic corpora lutea were found. I t can be concluded that neither human LH, HCG, or PMS has a luteotrophic effect by itself but exerts this activity indirectly.

The possible importance of LH as a syn- ergist with prolactin in the mouse has been well demonstrated by the work of Browning and his co-workers. The functional life span of corpora lutea in the mouse--as evidenced by gross hyperemia--is prolonged to 17 days by treatment with prolactin and LH; LH by itself is ineffective, and prolactin by itself maintains the corpora for only 7 days (Brown- ing et al., 1962 ; Browning and White, 1965a; Browning et al., 1965b).

I t is interesting that in mice hypophysectomized on day 7 of pregnancy, the daily administration of 20 I.U. prolactin fails to maintain pregnancy (Jaitly et al., 1966). That prolactin is involved in maintaining pregnancy in the mouse, through its luteotrophic effect, can be inferred from the interruption of pregnancy by the smell of a strange male mouse (i.e., a male other than the one with which the female copulated) (see Rothchild, 1965a for references to work of Bruce). In- jection of prolactin (Bruce and Parkes, 1960) counteracts the effect of the strange male. Reserpine (Dominic, 1966b) or progesterone (Dominic, 1966a) also have this effect and there is evidence that both reserpine and progesterone stimulate prolactin secretion (see Rothchild, 1967 for references).

I t has recently been shown that the maintenance of pregnancy in the hypophysectomized hamster requires concurrent administration of prolactin and FSH (Greenwald, 1967a). Prolactin is needed for the morphologic integrity of the corpora lutea and FSH for the secretion of progesterone. Neither hormone by itself is effective. After the placentas have been established, exogenous FSH alone is essential for the maintenance of pregnancy in the hypophysectomized hamster. Presumably, the placenta is then responsible for the production of a prolactin-like hormone. Thus, prolactin and FSH constitute the mini-


real luteotrophic complex of the hamster. This conclusion is also based on the fact that treatment of cyclic hamsters with prolactin alone does not interrupt the cycle (Greenwald, 1965a). However, exogenous prolactin and FSH induce pseudopregnancy in cyclic hamsters (Grady and Greenwald, unpublished data). The situation is still further complicated by the fact that in the pregnant hamster LH at low doses synergizes with the luteotrophic complex, but at high doses has an antagonistic effect. Injection of estrogen along with the luteotrophic complex, still further increases the size and vascularity of the corpora lutea.

Further studies have shown that prolactin and FSH are required to maintain pseudopregnancy in hypophysectomized hamsters; deciduoma formation and ovarian histology were used as the endpoints of a luteotrophic effect (Choudary and Greenwald, 1967).

The identification of the luteotrophic complex or process in the guinea pig remains unknown, primarily because it is still not clear to what extent the corpora lutea retain an independent capacity for function following hypophysectomy. I t is at least known that the daily injection of 100 ~g. LH after hypophysectomy does not prevent the regression of the corpora (Deanesly, 1966).

Treatment of the intact guinea pig with prolactin does not prolong the function of the corpora lutea (Aldred et al., 1961; Rowlands, 1962); however, see Rothchild (1966) for a criticism of this and related tests of the luteotrophic effectsof prolactin.

The maintenance of luteal function in the hypophysectomized pseudopregnant rabbit has usually been assessed by progestational proliferation of the endometrium. This poses two problems: the reaction requires less progesterone than the amount necessary to insure implantation (Courrier, 1950); with the exception of Robson, other authors have failed to give even a subjective rating to the magnitude of the response.

I f rabbits are hypophysectomized shortly after mating and then injected with progesterone, pregnancy is maintained for 9 to 10 days before fetal death occurs (Robson, 1937a). The addition of small amounts of estrone to the progesterone treatment did not prevent the embryos from being resorbed, but the endometrium showed marked progestational proliferation. Subsequently Robson (1937b, c) showed that a 2-}- to 3-}- progestafional proliferation response could be

maintained for 7 to 13 days after hypophysectomy by the daily injection of 10 /zg. of estrone and that gonadotrophic hormone treatment of hypophysectomized pseudopregnant rabbits maintained luteal function by stimulating the secretion of estrogens and not by a direct action on the corpora lutea (Robson, 1938). I t was also shown that the luteotrophic effect of estrogens in rabbits was directly on the corpora lutea, since local implants of estrogen were as effective as systemic ones (Hammond and Robson, 1951).

Two recent studies of hypophysectomized rabbits have demonstrated that prolactin treatment alone does not prevent atrophy of the corpora lutea (Kilpatrick et al., 1964; Rennie et al., 1964). The former studies also show that relatively large amounts of exogenous LH maintained the histologic integrity of the corpora lutea, although they were not as large as the corpora lutea of the controls. The combined effects of LH and prolactin have not yet been tested or reported. Progesterone was secreted by the maintained corpora lutea as evidenced by progestational proliferation.

The role of prolactin and of other pituitary hormones in the maintenance of function of corpora lutea has also been studied (inten- tionally as well as otherwise) by the transplantation of the pituitary to a site distant from the hypothalamus, or by section of the pituitary stalk, since either of these procedures leads to a continuous secretion of prolactin, and in general, a diminution, if not the virtual absence, of secretion of other gonadotrophins. Because the pituitary also plays a role in the luteolytic process, however, it would be worthwhile, before discussing the effects of pituitary transplantation or stalk section, to consider the problem of luteolysis.

Lu teo ly t i c Act iv i ty of Steroids and Gonadot roph ins

Regression of the corpora lutea or the cessation of progesterone secretion could be due to either a change in the luteotrophic process (diminution or cessation of secretion of one or more elements of the luteotrophic complex), or to the secretion of luteolytic substances, or to both (Rothchild, 1964). One example of the first method is the regression of the corpora lutea of the hamster during the estrous cycle, since this occurs at the same rate in the intact hamster as it does after hypophysectomy (Greenwald, unpublished data). Another example is the loss of


ability of the corpora lutea of the rat, within 1 to 2 days after hypophysectomy, to secrete progesterone in response to prolactin; regression itself seems to be due to the effects of either LH, prolactin or both (see further below).

Still another example of luteal regression of the first type is the luteolytic effect of estrogen treatment in the hamster. Hamsters injected on day 1 of pregnancy with estrogen (but not with progesterone) have regressing avascular corpora lutea by day 5 (Greenwald, 1965a). The luteolytic effect of estrogen can be reversed by concurrent treatment with FSH but not by treatment with prolactin or LH. Since prolactin and FSH seem to be essential for the luteotrophic process in the hamster the luteolytic effect of estrogen under these conditions, seems to be through its inhibition of secretion of FSH. Later in pregnancy estrogens seem to have a luteotrophic effect, probably because the corpora lutea have reached a different stage of development and are now under the influence of the placental luteotrophin.

The corpora lutea of pregnancy of the mouse can also be affected by steroid treatment. When injections are started on the day of mating, either progesterone (Burdick, 1942) testosterone (Burdick et al., 1940), or desoxycorticosterone acetate (Burdick et al., 1954) cause luteal involution, an effect reminiscent of the effect of early progesterone treatment on the corpora lutea of the guinea pig (Aldred et al., 1961; Nalbandov, 1961). The mechanism of this effect is unknown.

FSH treatment, under some conditions, can be associated with luteolysis in the mouse. After hypophysectomy on day 1 of diestrus, large amounts of F S H- -bu t not of LH or prolactin--induce follicular growth and at the same time reduction of the size of the corpora lutea as well as of their number (Kovacic, 1964). This effect resembles that of LH or HCG in the rabbit (see below).

A luteolytic effect of estrogen has not been demonstrated in the rat, in which, in fact, estrogens seems to be luteotrophic (see Roth- child, 1965a). This effect is not present in the absence of the pituitary (Rothchild and Schwartz, 1965). The effect is probably a triple one (Rothchild, 1965a), namely: the stimulation of prolactin secretion, the suppression of LH secretion, which, in the rat, has a luteolytic effect (Rothchild, 1965b), and synergism with prolactin directly on the corpora lutea (see below).

Progesterone injected into pseudopregnant (McKeown and Zuckerman, 1937; Rothchild, 1960b; Sammelwitz et al., 1961; Rothchild and Schwartz, 1965) or pregnant rats (Sammel- witz et al., 1961) has no luteolytic effect on the size of the corpora lutea. On the contrary, in hypophysectomized rats bearing pituitary transplants the size of the corpora lutea may be increased by progesterone treatment and decreased by unilateral ovariectomy (Roth- child, 1960b; and unpublished findings).

As just mentioned, a luteolytic role for LH in the rat has been shown by Rothchild (1965b). Ten- -bu t not 3 or 5--days of LH treatment induces a significant reduction in the size of the corpora lutea and in signs of progesterone secretion in adult hypophysectomized rats bearing autotransplanted pituitaries. Although evidence for signs of diminution of progesterone secretion were present at the end of the treatment, regression of the corpora did not occur until several days later. I t is worth speculating that the effect could have been due at least in part to the prolactin secreted by the transplanted pituitary, in a way similar, perhaps, to the luteolytic effect of prolactin on corpora lutea of hypophysectomized rats that have lost the ability to respond to the luteotrophic effect of prolactin (Malven and Sawyer, 1966). Such an effect of prolactin, in fact, had been postulated previously (Desclin, 1949a; Alloiteau, 1958b) and had already been shown to be true of the placental luteotrophin in the rat (Alloiteau, 1958b). I t may also help to explain the earlier studies of Desclin (1948, 1949a) in which rapid luteolysis followed treatment of hypophysectomized rats with a prolactin preparation, when the treatment was not begun until 24 hr. after hypophysectomy. In an earlier study, the simultaneous administration of FSH and LH induced rapid involution in the persistent corpora lutea of hypophysectomized rats (Greep, 1938) but the possibility of a significant degree of contamination with prolactin can not be discounted.

In the guinea pig, high doses of progesterone very early in pregnancy led to a partial regression of the corpora lutea (Aldred et al., 1961; Nalbandov, 1961). However, a single injection of estradiol cyclopentylpropionate on the morning of ovulation causes a much more profound destruction of the corpora lutea of the cyclic guinea pig (Greenwald, 1967b) than do multiple injections of progesterone.

As already mentioned, treatment with es-


trogen or with LH in relatively large doses maintains luteal function in the hypophysectomized rabbit. However, the intravenous injection of HCG or LH is luteolytic in the intact rabbit after the 5th day of pseudopregnancy (Foster et al., 1937; Spies et al., 1966; Stormshak and Casida, 1965), but this effect in turn can be prevented by estrogen treatment (Stormshak and Casida, 1965; Spies et al., 1966). In the pregnant rabbit the number of conceptuses offers some protection against the luteolytic effect of exogenous LH, since the corpora lutea of rabbits bearing 6 embryos were larger than those of rabbits bearing two embryos (Stormshak and Casida, 1966). The induction of ovulation in pregnant (Mayer and Klein, 1946) or pseudopregnant rabbits (Spies et al., 1966) results in a rapid breakdown of the previous set of corpora lutea. The latter authors have suggested' that LH may induce luteal regression by inter- fering with the production of endogenous estrogen by the interstitial cells.

Effect of an Ectopic P i tu i t a ry on the Corpus L u t e u m

The inhibiting effect of the CNS on the secretion of prolactin by the pituitary (for a discussion of the evidence with references, see Rothchild, 1965a) has led to the use of the ectopic pituitary in many studies of corpus luteum function. Although the first demonstration of the prolactin secreting ability of the transplanted pituitary was made by Des- clin (1950) (who believed that the simultaneous transplantation of a stilbestrol pellet with the pituitary caused it to secrete prolactin) and by Everett (1954) (who showed that estrogen was not necessary) the credit for the original demonstration that denerva- tion of the pituitary causes it to secrete prolactin really belongs to Westman and Jacob- sohn (1938), although they were unaware that their find~ings actually showed this. They found that pituitary stalk section, in the rat, 2 to 5 hr. before cervical stimulation did not prevent pseudopregnancy, and therefore believed that the cervical stimulation did not require an intact pituitary stalk to be effective. If they had done the same experiment without cervical stimulation, the knowledge that the CNS inhibits prolactin secretion might have been reached 12 yr. earlier.

Everett (1956) showed that corpora lutea in the hypophysectomized rat bearing a pituitary transplant remain in a healthy condition

and secrete progesterone for at least 3 to 4 mo. Regression, however, eventually occurs (Everett, 1956; Rothchild, unpublished data). Estrogen treatment of such rats, although it increases the amount of prolactin in the pituitary transplant (Desclin and Koulischer, 1960) increases the size of the corpora lutea to those of pregnant rats (Everett, 1956; Rothchild, 1965b; D~sclin, 1949b; Alloiteau, 1962, t965) by a direct effect on the corpora lutea; i.e., the estrogen synergizes with the prolactin at the cellular level. In the otherwise untreated hypophysectomized rat bearing a pituitary transplant, the corpora lutea measure 1.5 mm. in diameter (Everett, 1956) and weight about 1.8 mg. (Rothchild, 1960b).

The functional state of the corpus luteum can be demonstrated not only by indirect signs of progesterone secretion (vaginal mucification in response to estrogen treatment, or deciduoma formation) but also by elevated levels of progesterone and low levels of 20-OH-P in ovarian venous blood (Eto et al., 1962).

Prolactin has long been assumed to be the only luteotrophic hormone of the rat. How- ever, it has recently been shown on the basis of in vitro biochemical studies, that LH acts as a luteotrophin in the rat in the sense that it stimulates progesterone synthesis (Arm- strong, 1966). This effect, however, has not been shown to occur in corpora lutea that have been tested long enough after hypophysectomy to have eliminated any influence of endogenous prolactin; the luteotrophic effect of LH, therefore, may be present only in corpora that are still or were very recently under the influence of prolactin. Nevertheless, these findings raise the question of what gonadotrophins, other than prolactin, may be produced by the ectopic pituitary.

In hypophysectomized rats bearing a transplanted pituitary, all ovarian components except the corpora lutea show regressive changes, i.e., atrophy of the interstitium and the absence of large antrum-containing follicles (Everett, 1956). The vaginal smear is of the anestrous type and remains so in the absence of estrogen treatment. After 2 to 3 wk., PAS+cells-- the presumptive source of FSH and LH--a re rarely encountered in the pituitary graft (Nikitovitch-Winer and Everett, 1959). Ultrastructure of the pituitary grafts reveals some scattered cells with the morphologic characteristics of basophils but with cytoplasmic features which are interpreted as signs of degeneration (Rennels, 1962). The


assay of the pituitary transplant has also shown that the amount of LH present is extremely small (Parlow and Rothchild, unpublished data).

On the face of it, therefore, there appears to be little evidence for the production of FSH and LH by the ectopic pituitary. How- ever, four pituitaries grafted under the kidney capsule of immature hypophysectomized rats possess considerable FSH activity when tested 3 wk. to 3 mo. later by HCG augmenta- tion (Hertz, 1960). I t would be interesting to determine whether significant FSH secretion could be detected in on'e pituitary by this method. Conversely, would the injection of exogenous FSH into hypophysectomized rats bearing autotransplanted pituitaries lead to development of antral follicles, a step which presumably requires both FSH and LH? It appears premature to rule out the possibility that small but physiologically significant amounts of LH and FSH are produced by the ectopic pituitary. The eventual regression of the corpora lutea, in fact, may be due to the long term luteolytic effect of minute amounts of LH secreted by the pituitary transplant.

In the intact cyclic mouse, subcutaneous implantation of pituitaries produces a succes- sion of true pseudopregnancies as evidenced by vaginal mucification, positive decidual reaction and well developed corpora lutea (Mfihlbock and Boot, 1959; Browning and White, 1965b; Browning et al., 1965a). A similar effect has been seen in rats (Alloiteau, 1958a; Quilligan and Rothchild, 1960). In mice, the duration of the luteotrophic response, as judged by the duration of the diestrus (Mfihlbock and Boot, 1959) as well as by luteal hyperemia (Browning and White, 1963) is determined by the number of pituitaries transplanted. Ten months after the implantation of two pituitary isografts into intact female mice, prolongation of vaginal diestrus to 11 days was still observed (Hagen and Rawlinson, 1963). To our knowledge, the effects of removal of the in' si tu pituitary on luteal function has not been determined in mice bearing ectopic pituitaries.

The hamster shows an interesting difference from the rat in the luteotrophic effect of the ectopic pituitary. The difference is readily understandable in view of the existence of a luteotrophic complex of prolactin and FSH in the hamster (Greenwald, 1967a). Multiple pituitary implants from males (Turnbull and Kent, 1966) or a single female pituitary transplant (Choudary and Greenwald, 1967)

will induce recurring pseudopregnancies in cyclic in.tact hamsters. On removal of the recipient's pituitary, however, the ectopic pituitary is unable to maintain pseudopregnancy (Turnbull and Kent, 1966; Choudary and Greenwald, 1967) or pregnancy (Orsini and Meyer, unpublished data; Choudary and Greenwald, 1967). However, the injection of FSH into such hypophysectomized hamsters bearing pituitary transplants enables functional corpora lutea to persist (Choudary and Greenwald, 1967).

Intraocular autotransplants of the pituitary of the adult female guinea pig maintained follicles of large size but the ovaries all con- tained degenerating luteal cells 21 to 93 days after hypophysectomy (Schweizer et al., 1937). It was concluded that the ectopic grafts were functional, but were capable of releasing only FSH. If these animals were fully hypophysectomized, the guinea pig would indeed be a most unusual species. I t is remarkable that up to the present, this interesting experiment has never been repeated. However, "partial" hypophysectomy of female guinea pigs results in a pituitary island which is isolated from the hypothalamus (Aron and Petrovic, 1960). Under these cir- cumstances, only small antral follicles are maintained and only remnants of involuting luteal tissue are present.

To our knowledge, the effects of an ectopic pituitary on luteal function in the rabbit have not been evaluated; one may assume, however, that they would be similar to those of stalk section, the discussion of which follows.

Effects of P i tu i t a ry Stalk Transec t ion on the Corpus L u t e u m

In a number of species, sectioning of the pituitary stalk prevents the inhibitory influence of the hypothalamus from reaching the pituitary; the latter, consequently, secretes prolactin continuously at an appreciable rate.

I t is evident from the previous sections of this paper, that prolactin is the principal luteotrophic hormone of the rat. This is also substantiated by the results of stalk sectioning (see introduction to previous section). When the pituitary stalk is severed in cyclic rats when functional corpora lUtea are present, persistent pseudopregnancy is produced. This can be demonstrated by ovarian and vaginal histology (Greep and~ Barnett, 1951 ), or by deciduoma formation as long as 74 days


after stalk sectioning (Nikitovitch-Winer, 1965). Lesions in the anterior hypothalamus of the rat also lead to pseudopregnancy (Mc- Cann and Friedman, 1960; Flament-Durand and Desclin, 1964; Flerko and Bardos, 1966).

The effects of pituitary stalk section in the guinea pig were carried out in the era before it was realized that, without a barrier between the stalk and the pituitary, the pituitary could be revascularized by the vessels of the hypo- physeal portal system. Consequently, it is difficult to evaluate the results of Dempsey and Uotilla (1940). Two to 3 mo. after stalk- sectioning, approximately half of the guinea pigs whose stalks had been sectioned showed vaginal diestrus; the remainder had resumed cyclic activity. Lesions between the optic chi- asma and the median eminence resulted in the absence of large follicles and corpora lutea; none of the hypothalamic lesions caused persistence of the corpora lutea or regression of the other ovarian components (Dey, 1941).

Newly formed corpora lutea in the rabbit degenerate within 5 to 6 days after stalk transection, and transection of the stalk during pregnancy causes degeneration of the corpus luteum and abortion (Westmann and Jacobsohn, 1940). The effect strikingly resembles that of suckling on the corpus luteum and may have the same etiologic basis. Al- though it is not known to what extent prolactin plays a role in the luteotrophic process in the rabbit, it is clear from what has already been said that if it plays one at all, it is one that requires the presence of some other pituitary hormone, probably LH. Since both suckling and stalk section would lead to the secretion of prolactin and a sharp diminution in the secretion of LH, as well as of other pituitary hormones, it is not surprising that both procedures cause involution of the corpus luteum.

Discussion The aim of this paper has been to develop

a panoramic view of luteal regulation in laboratory species. The review of the literature has uncovered several gaps in our knowledge of the corpus luteum.

Considerably more attention has been de- voted to the corpus luteum of the rat than to that of other species. Although the study of rat's corpus luteum can form the basis for developing some generalizations about how luteal activity is regulated there is no a priori reason to assume that the rat is any more representative than is any other species. Such

generalizations, therefore, must be considered tentative until not only the other laboratory species, but many other mammals have been studied at least as intensively as the rat has been. Such studies, moreover, should be di- rected towards the analysis of luteal activity throughout all phases of the reproductive cycle because the accentuation of certain ovarian features at different stages of the cycle (e.g., estrous cycle, pseudopregnancy, pregnancy, lactation) may furnish valuable clues about the factors regulating the growth and regression of the corpus luteum.

The use of a variety of techniques may also be more rewarding than any single one for evaluating luteal activity. A good example is the fact that a much more complete picture of the physiology of the corpus luteum of pregnancy of the rat emerges from the com- bination of measurements of its size and weight, histochemical studies of its enzyme and lipid content and chemical studies of the progesterone content in ovarian vein blood, than from any one of these techniques alone.

The corpus luteum should not be dissoci- ated from the other ovarian elements. Numer- ous examples have been cited in this review where estrogen (hence, presumably LH) in- teracts with luteotrophic hormones to produce optimal luteal function. The rat is an especially good example in which prolactin plus estrogen apparently act in synergy. The secretion of estrogen may be inherent in the corpus luteum or produced by the follicular apparatus or interstitium. The interaction between the corpus luteum and extra-luteal tissue deserves much more attention than it has received.

One of the difficulties in evaluating luteal function is the variety of biologic endpoints which have been used. In the rat, deciduoma formation has often been used to assess luteal function without information on the magnitude of the response. In most instances, it is impossible to determine from the literature whether a minimal or maximal decidual cell reaction was obtained. This is of considerable importance in determining whether the corpus luteum is secreting maximal levels of progesterone. Similarly, the use of progestational proliferation of the endometrium of the rabbit as an endpoint would be more mean- ingful if expressed in objective, quantitative terms (e.g., planimetric determinations).

Perhaps one of the best criteria of luteotrophic activity of any hormone(s) is the ability to maintain pregnancy in hypophysectomized animals before a placental source of hormones is established. However, the di-


rect action of hormones (e.g., estrogen) on the uterus must always be kept in mind.

From the standpoint of the reproductive physiologist, luteotrophic activity of any hormone or hormones, implies the ability for sustained maintenance of progesterone secretion. This differs from the biochemist's in vitro approach, in which of necessity, the efficacy of hormones as luteotrophins is determined in a time scale of hours rather than days. This does not deny the value and valid- ity of the in vitro studies but it indicates the very fundamental difference in methodology and resultant philosophy of the two groups.

Although much remains to be done, it does seem possible to make one valid generalization about the corpus luteum at the present time. The activity of the corpus luteum is quite clearly regulated by the interaction of luteotrophic and luteolytic processes. In the most general terms, the luteotrophic process stimulates the growth of the corpus luteum and its secretion of progesterone, and the luteolytic process causes regression of the corpus luteum and a diminution and cessation of the secretion of progesterone. The luteotrophic process does not necessarily consist of the secretion of a single substance. For example, it seems clear that prolactin is the principle (though probably not the only) luteotrophic hormone in the rat, prolactin plus LH seem to be the essential minimal elements of the process in the mouse, and prolactin plus FSH are luteotrophic in the hamster. Hence, the term luteotrophin should be used in a generic sense with the recognition that it may represent a different hormone or complex in different species.

The character of the luteotrophic process may be different in different phases of the reproductive cycle. Thus, the placenta plays a major role in maintaining growth and progesterone secretion by the corpus luteum in the second half of pregnancy in the rat and mouse, and the placenta and pituitary are essential in the hamster. In other mammals other components of the fetal-maternal complex may be equally important (for references see the papers of Du Mesnil du Buisson, Moor and Rowson, and the papers of Dena- tour and of Melampy presented at this symposium).

The fact that a hormone (or group of hormones) is an essential part of the luteotrophic process does not exclude its participa- tion in tuteolysis. The reverse also holds true; that is, a substance which is involved in luteolysis may also possess luteotrophic ac-

tivity. Several examples have been cited in our review which illustrate this point (prolactin can cause luteolysis, and LH can synergise with prolactin in the luteotrophic process, in the rat). This quality is perhaps one of the most important, as well as one of the most overlooked, of the two processes. The fact that it exists should remind us that the ephe- merality itself of the corpus luteum indicates that its responsiveness to the various elements of its environment can change with age, or with changes in the environment itself.

The ability of a hormone or hormones to exert a luteotrophic effect depends upon the state of reactivity of the corpus luteum and therefore can be evaluated only in relation to the state of activity of the luteolytic process. The reverse of this is also true, namely, the luteolytic effect of a substance will vary in relation to the state of activty of the luteotrophic process and the reactivity of the corpus luteum. Numerous examples of this quality have been cited elsewhere (Rothchild, 1965a, 1966).

The luteolytic process, like the luteotrophic process, does not necessarily consist of the secretion of a single substance. For example: both LH and a uterine substance have a luteolytic effect in the rat, the uterus plays a major role in the luteolytic process in several mammals (see paper of Melampy, at this symposium), and prolactin itself may under some conditions by luteolytic.

The mechanism of action of all substances which participate in either process is not necessarily the same. For example: prolactin by itself has a limited luteotrophic effect in the mouse, while LH by itself has none, yet the latter has a strong synergistic effect when injected with prolactin. The placental hormone and prolactin both have luteotrophic properties in the rat, yet the corpora lutea maintained by prolactin have gross and biochemical characteristics that are different from those being maintained by the placental hormone.

The luteotrophic process may be, in general, an unstable one, or one with an intrinsically limited life span, or one regulated by a system which is basically unstable or temporary. The ability of sheep and pig corpora lutea to function independently of the pituitary for a while may exemplify the first situation; the secretion of a luteo~rophic hormone by the placenta exemplifies the second possibility, and the postulated regulation of prolactin secretion by progesterone, in the rat (Rothchild

CL I N L A B O R A T O R Y A N I M A L S 157

a n d Schwar t z , 1965) is a n example of the th i rd .

T h e lu teo ly t i c process , in con t r a s t , m a y be a c o n s t a n t p a r t of the e n v i r o n m e n t of the co rpus l u t eum, or i ts a c t i v i t y m a y be inf lu- enced b y p rogres t e rone . T h e lu teo ly t i c effect of t he u te rus , in fact , cou ld well be the r e su l t of the ac t ion on i t of p roges t e rone .

T h e p r e v i o u s l y q u o t e d pas sage of E v e r e t t in r e l a t i on to these r ecogn izab le qua l i t i es of the l u t e o t r o p h i c a n d lu teo ly t i c processes , should , we hope , o b v i a t e the need to sea rch for the I u t eo t roph i c ho rmone , or the l u t eo ly t i c h o r m o n e in a n y p a r t i c u l a r m a m m a l . T h e more i m p o r t a n t q u e s t i o n s are first , to define w h a t cha r ac t e r i s t i c s of lu t ea l p h y s i o l o g y are com- m o n to the co rpo ra l u t ea of all m a m m a l s , or even of all v e r t eb r a t e s , a n d second, h o w t he d ive r s i t y of the l u t e o t r o p h i c a n d lu t eo ly t i c p rocesses can be co r r e l a t ed w i t h these com- m o n cha rac te r i s t i c s . I n o t h e r words , g iven the s t rong p r o b a b i l i t y t h a t a v a r i e t y of m e t h o d s h a v e evo lved t h r o u g h wh ich a co rpus l u t e u m can be m a i n t a i n e d , or c an be m a d e to regress , c an we exp la in the r e l a t ive u n i f o r m i t y in the d u r a t i o n of a c t i v i t y of m o s t m a m m a l i a n cor- p o r a l u t ea on the bas i s of ce r t a in bas ic com- m o n qua l i t i e s of all co r po r a lu tea , a n d of all I u t eo t roph i c a n d lu teo ly t i c p rocesses?

L i t e r a t u r e C i t e d

Ainsworth, L. and K. J. Ryan. 1966. Steroid hormone transformations by endocrine organs from pregnant mammals. I. Estrogen biosynthesis by mammalian placental preparations in vitro. Endocrinology 79:875.

Aldred, J. P., P. H. Sammelwitz and A. V. Nalbandov. 1961. Mechanism of formation of corpora lutea in guinea-pigs. J. Reprod. Fertil. 2:394.

Allen, E. 1922. The oestrous cycle in the mouse. Am. J. Anat. 30:297.

Alloiteau, J. J. 1958a. Sur la nature du contr61e de la fonction luteotrophique de l'hypophyse anterieure par l 'hypothalamus chez la ratte. C.R. Acad. Sci. 247:1047.

Alloiteau, J. J. 1958b. Destruction active des corps jaunes chez la ratte gestante hypophysectomis6e avant la nidati~n. C.R. Soc. Biol. 152:707.

Alloiteau, J. J. 1962. Le contr61e hypothalamique de l'adenohypophyse III. Regulation de la fonction gonadotrope femelle. Activite LTH. Biologie Medi- cale, Vol. LI.

Alloiteau, J. J. 1965. Action des oestrog6nes et des androgbnes sur le corps jaune de la ratte. Rev. Europ4en Endocr. 2 : 155.

Alloiteau, J. J. and J. Bouhours. 1965. Poursuite de la gestation chez la ratte hypophysectomis6e re- cevant de l'hormone luteinisante. Reflexions sur les mecanismes successifs assurant le maintien fonctionnel du corps jaune gestatif. C.R. Acad. Sci. (Paris) 26I :4230.

Amoroso, E. C. and C. A. Finn. 1962. Ovarian activity during gestation, ovum transport and implana-

tation. In S. Zuckerman [ed.] The Ovary. Aca- demic Press, N.Y. and London. VoL 1.

Amoroso, E. C. and D. G. Porter. 1966. Anterior pituitary function in pregnancy. In G. W. Harris and B. T. Donovan [ed.] The Pituitary Gland. Vol. 2. Univ. Calif. Press.

Armstrong, D. T. 1966. Comparative studies of the action of luteinizing hormone upon ovarian steroi- dogenesis. J. Reprod. Ferti[. Suppl. 1. 101.

Armstrong, D. T. and R. O. Greep. 1965. Failure of deeiduomal response to uterine trauma and effects of LH upon estrogen secretion in rats with ovaries luteinized by exogenous gonadotropins. Endocrinol- ogy 76:246.

Aron, M. and A. Petrovic. 1960. ContrSle hypothalamique des secretions gonadotrope, corticotrope et thyreotrope dans la glande pituitaire du cobaye femelle. Archives d' Anatoraie Microscopique et de Morphologie Eperimentale 49:363.

Aron, C., G. Asch and L. Asch. 1963. Le rapproche- ment sexual est-il susceptible de provoquer la rupture folliculaire chez les mammiferes dits "a ponte spontanee"? Gynecol. Obst. (Paris) 62:53.

Astwood, E. B. 1941. The regulation of corpus luteum function by hypophysial luteotrophin. Endocrinol- ogy 28:309.

Astwood, E. B. and R. O. Greep. 1938. A corpus ]uteum-stimulating substance in the rat placenta. Proc. Soc. Exp. Biol. and Ned. 38:713.

Atkinson, W. B. and C. W. Hooker. 1945. The day to day level of estrogen and progestin throughout pregnancy and pseudopregnancy in the mouse. Anat. Rec. 93 : 75.

Averill, S. C., E. W. Ray and W. R. Lyons. 1950. Maintenance of pregnancy in hypophysectomized rats with placental implants. Froc. Soc. Exp. Biol. and Med. 75:3.

Balogh, K. 1964. A histochemical method for the demonstration of 20a-hydroxysteroid dehydrogenase activity in rat ovaries. J. Histochem. and Cytochem. 12 : 670.

Balogh, K. and W. G. Wiesl. 1966. Histochemical localization of rat ovarian 20a-hydroxysteroid dehydrogenase activity initiated by gonadotrophic hormone administration. Endocrinology 78:75.

Banon, P., D. Brandes and J. K. Frost. 1964. Lyso- spinal enzymes in the rat ovary and endometrium during the estrous cycle. Acta Cytologica 8:416.

Bassett, D. L_ 1943. The changes in the vascular pattern of the ovary of the albino rat during the estrous cycle. Am. J. Anat. 73:251.

Bassett, D. L. 1949. The /utein cell population and mitotic activity in the corpus luteum of pregnancy in the albino rat. Anat. Rec. 103:597.

Beerstecher, E. 1942. Variations in estrogen content of m-ine of female rabbits. Endoerinology 31:479.

Boling, J. L. 1942. Growth and regression of corpora lutea during the normal estrous cycle of the rat. Anat. Rec. 82:131.

Browning, H. C., G. A. Larke and W. D. White. 1962. Action of purified gonadotropins on corpora lutea in the cyclic mouse. Proc. Soc. Exp. Biol. and Med. III:686.

Browning, H. C. and W, D. White. 1963. Luteo- tropic action of fractional and multiple pituitary isografts in the mouse. Tex. Rpt. on Biol. and Med. 21:176,

Browning, H, C, and W. D. White. 1965a. Luteotropic response in pituitary-ovarian autografts in male mice. Proc. Exp. Biol. and Med. 119:1224.

Browning, H. C. and W, D. White. 1965b. Local stimulation of mammary gland by pituitary and


ovarian grafts in the mouse. Tex. Rpt . on Biol. and Med. 23 : 26.

Browning, H. C,, W. D. White and W. E. Gibbs. 1965a. M a m m a r y gland st imulat ion by pi tui tary isografts in mice with normal or ectopic ovarian tissues. Tex. Rpt. on Biol. and Med. 23:16.

Browning, H. C., A. L. Brown, T. E. Crisp and W. E. Gibbs. 1965b. Response of ovarian autografts to purified FSH, L H and L T H in partially and completely hypophysectomized mice. Tex. Rpt . on Biol. and Med. 23:715.

Bruce, H. M. and A. S. Parkes. 1960. Hormona l factors in exteroceptive block to pregnancy in mice. J. Endocr. 20:xxix.

Buno, W. and L. Hekimian. 1955. Histochemieal s tudy of the ovary of the rat in relation to estrous cycle. Anales de la Facul tad de Medicine (Monte- video) 40:42.

Burdick, H. O. 1942. Effect of progesterone on the ovaries and embryos of mice in early pregnancy. Endocrinology 30:619.

Burdick, H . O., B. Emerson and R. Whi tney . 1940. Effects of testosterone propionate on pregnancy and on passage of ova through the oviducts of mice. Endocrinology 26 : 1081.

Burdick, H. O. and M. Crump. 1951. The effect of induced ovulat ion on pregnancy. Endocrinology 48:273.

Burdick, H. O., J. A. Baird and R. T. Rogers. 1954. Fur ther studies on the effect of desoxycorticosterone acetate on early pregnancy. Endocrinology 55:369.

Canivenc, R. 1956. Le placenta de rat et son activit6 endocrine. Archiv. d 'Anat. , d' Histol. et d 'Embryol . 39:1.

Cerruti, R. A. and W. R. Lyons. 1960. Mammogenic activities of the mid-gestat ional mouse placenta. Endocrinology 67 : 884.

Choudary, J. B. R. and G. S. Greenwald. 1967. Ef- fect of an ectopic pi tui tary on luteal maintenance in the hamster . Endocrinology: (In press.).

Claesson, L. and N. A. Hillarp. 1946. Format ion mechanism of 6estrogenic homones. I. The presence of an oestrogen-precursor in the rabbit ovary. Acta Physiol. Scand. 17 : 115.

Claesson, L. and N. A, Hillarp. 1948. Sterol content of the interstitial gland and corpora lutea of the rat, guinea pig and rabbit ovary during pregnancy, parturi t ion and lactation. Acta Anat. 5:301.

Corner, G. W. 1919. On the origin of the corpus lu teum of the sow from both granulosa and theca interna. Am. J. Anat. 26:117.

Corner, G. W. 1938. Sites of formation of estrogenic substances in the animal body. Physiol. Rev. 18:154.

Courrier, R. 1950. Interactions between estrogens and progesterone. Vitamins and Hormones 8:179.

Cutuly, E. 1941. Implanta t ion following mat ing in hypophysectomized rats injected with lactogenic hormone. Prec. Soc. Exp. Biol. and Med. 48:315.

Cutuly, E. 1942. Effects of lactogenic and gonadotropic hormones on hypophysectomized pregnant rats. Endocrinology 31 : 13.

Davenport , G. R. and L. E. Mallette. 1966. Some biochemical properties of rabbit ovarian hydroxysteroid dehydrogenases. Endocrinology 78:672.

Davies, ~[. R., G. R. Davenpor t , J. L. Norris and P. I. C. Rennie. 1966. Histochemical studies of hydroxysteroid dehydrogenase activity in m a m - malian reproductive tissues. Endocrinology 78:667.

Dawson, A. B. and J. F. Velardo. 1955. A histochemical s tudy of lipids of the corpora lutea of the rat during pseudopregnancy. Am. J. Anat. 97:303.

Deane, H. W. 1952. Histochemical observations on the ovary and oviduct of the albino rat during the estrous cycle. Am. J. Anat. 91:363.

Deanesly, R. 1930a. The corpora lutea of the mouse with special reference to fat accumulat ion during the oestrous cycle. Prec. Roy. Soc. B 106:578.

Deanesly, R. 1930b. The development and vascularization of the corpus lu teum in the mouse and rabbit. Prec. Roy. Soc. B 107:60.

Deanesly, R. 1938. The reproductive cycle of the golden hamster . Cricetus auratuso Prec. Zool. Soc. London A 108:31.

Deanesly, R. 1960a. Endocrine activity of the early placenta of the guinea-pig. J. Endocr. 21:235.

Deanesly, R. 1960b. Implanta t ion and early pregnancy in ovariectemized guinea pigs. J. Reprod. Fertil. 1:242.

Deanesly, R. 1966. The effects of purified sheep luteinizing hormone on the guinea-pig ovary. J. Reprod. Fertil. 11 : 303.

Deanesly, R. and W. H. Newton. 194l. The infuence of the placenta on the corpus lu teum of pregnancy in the mouse. J. Endocr. 2:317.

Dempsey, E. W. and U. U. Uotilla. 1940. The effect of pi tui tary stalk section upon reproductive phenomena in the female rat. Endocrinology 27:573.

Desclin, L. 1948. Action de la prolactine sur la structure de l 'ovaire du rat normal et hypopbysectomise. C.R. Soc. Biol. 142:1436.

Desclin, L. 1949a. Observations sur la structure des ovaires chez des rats soumis a l'influence de la prolactine. Ann. d 'Endocrin. 10:1.

Desclin, L. 1949b. A p r o p o s de l 'action combinre de la prolactine et des oestrog~nes sur la structure de l 'ovaire chez le rat. C.R. Soc. Biol. 143:I154.

Desclin, L. 1950. A p r o p o s du mecanisme d'action des oestrog~nes sur le lobe anterieure de l 'hypophyse chez le rat. Ann. d 'Endocrin. 11:656.

Desclin, L. and L. Koulischer. 1960. Action d 'un t rai tement oestrogrnique sur la teneur en prolactine de l 'hyp6physe greff~e chez le rat. C.R. Soc. Biol. 154:1515.

Dey, F. L. 1941. Changes in ovaries and uteri in guinea pigs with hypothalamic lesions. Am. J. Anat. 69:61.

Dominic, C. J. 1966a. The inhibition of the olfactory block to pregnancy in mice by the adminis t ra t ion of exogenous progesterone. Naturwissenschaf ten 53:310.

Dominic, C. J . 1966b. Reserpine: inhibition of olfactory blockage of pregnancy in mice. Science 152 : 1764.

Donaldson, L. and W. Hansel. 1965. Histological s tudy of bovine corpora lutea. J. Dairy Sci. 48:905.

Dresel, I. 1935. The effect of prolactin on the estrus cycle of nonparous mice. Science 82:173.

Dubreuil, G. 1962. Corps progestatifs et gestatifs leurs varietes et leurs modes d 'evolution chez les ver- tebres vivipares. Ann. d 'Endocrin. 23:1.

Eckstein, P. and S. Zuckerman. 1956. The oestrous cycle in the mammal ia . In A. S. Parkes [ed.] Mar - shall's Physiology of Reproduction. (3rd ed.). Longman, Green, London, Vol. I, Par t 1.

Enders, A. C. and W. R. Lyons. 1964. Observations on the fine structure of lutein cells. II. The effects of hypophysec tomy and mammot roph ie hormone in the rat. J. Cell Biol. 22:127.

Engle, E. T. and P. E. Smith. 1929. The origin of the corpus lu teum in the rat as indicated by s tud- ies upon the luteinization of the cystic follicle. Anat. Rec. 43:239.


Eto, T., H. Masuda, Y. Suzuki and T. Hosi. 1962. Progesterone and pregn-4-ene-20-a-ol-3-one in rat ovarian venous blood at different stages in reproductive cycle. Japan J. of An. Reprod. 8:34.

Evans, H. M., M. E. Simpson, W. R. Lyons and K. Turpeinen. 1941. Anterior pituitary hormones which favor the production of traumatic uterine placen- toma. Endocrinology 28:933.

Everett, J. W. 1944. Evidence suggesting a role of the lactogenic hormone in the estrous cycle of the albino rat. Endocrinology 35:502.

Everett, J. W. 1945. The microsopically demonstra- able lipids of cyclic corpora lutea in the rat. Am. J. Anat. 77:293.

Everett, J. W. 1947. Hormonal factors responsible for deposition of cholesterol in the corpus luteum of the rat. Endocrinology 41:364.

Everett, J. W. 1948. Progesterone and estrogen in the experimental control of ovulation time and other features of the estrous cycle in the rat. En- docrinology 43 : 389.

Everett, J. W. 1954. Luteotrophic action of autografts of the rat hypophysis. Endocrinology 54:685.

Everett, J. W. 1956. Functional corpora lutea maintained for months by autografts of rat hypophyses. Endocrinology 58: 786.

Everett, J. W. 1961. The mammalian reproductive cycle and its controlling mechanisms. In W. C. Young led.] Sex and Internal Secretions. (3rd ed.). Williams and Wilkins, Baltimore.

Everett, J. W. 1964. Central neural control of reproductive functions of the adenohypophysis. Physiol. Rev. 44:373.

Fajer, A. B. and C. A. Barraclough. 1967. Ovarian secretion of progesterone and 20-a-hydroxy-pregn- 4-en-3-one during pseudopregnancy and pregnancy in rats. Endocrinology: (In press.).

Falek, B. 1959. Site of production of oestrogen in rat ovary as studied in microtransplants. Acta Physiol. Scand. Vol. 47. Suppl. 163.

Firor, W. M. 1933. Hypophysectomy in pregnant rabbits. Am. J. Physiol. 103:209.

Flament-Durand, J. and L. Desclin. 1964. Observa- tions concerning the hypothalamic control of pituitary luteotrophin secretion in the rat. Endocrinol- ogy 75:22.

Flerko, B. and V. Bardos. 1966. Prolongation of dioestrus in rats with diencephalic mesencephalic lesions. Exp. Brain Res. 1:299.

Forbes, T. R. 1953. Pre-ovulatory progesterone in the peripheral blood of the rabbit. Endocrinology 53:79.

Forbes, T. R. and C. W. Hooker. 1957. Plasma levels of progestin during pregnancy in the mouse. En- docrinology 61:281.

Foster, M. A, R. C. Foster and F. L. Hisaw. 1937. The interrelationship of the pituitary sex hormones in ovulation, corpus luteum formation and corpus luteum secretion in the hypophysectomized rabbit. Endocrinology 21 : 249.

Gardner, W. U. and E. Allen. 1942. Effects of hypophysectomy at midpregnancy in the mouse. Anat. Rec. 83:75.

Gospodarowicz, D. 1965a. La production "in vitro" d'androgenes par des corps jaunes de lapine. Bio- chim. and Biephys. Acta 100:618.

Gospodarowicz, D. 1965b. The effect of human chorionic gonadotropin and of follicle-stimulating hormone on the incorporation of [I-I4C] acetate into steroids by the rabit corpus luteum in vitro. Biochim. and Biophys. Acta 107:363.

Greenwalcl, G. S. 1958a. A histological study of

the reproductive tract of the lactating mouse. J. Endocr. 17:17.

Greenwald, G. S. 1958b. Formation of deciduomata in the lactating mouse. J. Endocr. 17:24.

Greenwald, G. S. 1963. Coital block to superovulation in the hamster. J. Reprod. Ferdl. 5:217.

Greenwald, G. S. 1964. Ovarian follicular development in the pregnant hamster. Anat. Rec. 148:605.

Greenwald, G. S. 1965a. Luteolytic effect of estrogen on the corpora lutea of pregnancy of the hamster. Endocrinology 76 : 1213.

Greenwald, G. S. 1965b. Histologic transformation of the ovary of the lactating hamster. Endocrinol- ogy 77:641.

Greenwald, G. S. 1966. Ovarian follicular development and pituitary FSH and LH content in the pregnant rat. Endocrinology 79:572.

Greenwald, G. S. 1967a. Luteotropic complex of the hamster. Endocrinology 80:118.

Greenwald, G. S. 1967b. Species differences in egg transport in response to exogenous estrogen. Anat. Rec. 157:163.

Greenwald, G. S. 1967c. Induction of ovulation in the pregnant hamster. Am. J. Anat.: In press.

Greenwald, G. S., J. E. Keever and K. L. Grady. 1967. Ovarian morphology and pituitary FSH and LH concentration in the pregnant and lactating hamster. Endocrinology 80:851.

Greep, R. O. 1938. The effect of gonadotropic hormones on the persisting corpora lutea in hypophysectomized rats. Endocrinology 23:154.

Greep, R. O., H. B. van Dyke and B. F. Chow. 1942. Gonadotropin of the swine pituitary. I. Various biologic effects of purified thylakentrin (FSH) and pure metakentrin (ICSH). Endocrinology 30:635.

Greep, R. O. and R. J. Barnett. 1951. The effect of pituitary stalk-section on the reproductive organs of female rats. Endocrinology 49:172.

Greta, L. J. and K. B. Eik-Nes. 1967. Plasma progesterone concentrations during pregnancy and lactation in the rat. J. Reprod. Fertil. 13:83.

Guraya, S. S. and G. S. Greenwald. 1964. Histochemi- cal studies on the interstitial gland in the rabbit ovary. Am. J. Anat. 114:495.

Guraya, S. S. and G. S. Greenwald. 1965. A histochemical study of the hamster ovary. Am. J. Anat. 116:257.

Guttenberg, I. 1961. Plasma levels of "free" progestin during the estrous cycle in the mouse. Endocrinol- ogy 68 : 1006.

Hagen, E. O. and H. E. Rawlinson. 1963. The duration of the effect of isologous pituitary implants on the estrous cycles in the intact mouse. Canad. J. Biochem. and Physiol. 41:101.

Hammond, J. and F. H. A. Marshall. 1925. Repro- duction in the rabbit. Oliver and Boyd, Edinburgh.

Hammond, J., Jr. and J. M. Robson. 1951. Local maintenance of the rabbit corpus luteum with oestrogen. Endocrinology 49: 384.

Harper, M. J. K. 1963. Ovulation in the rabbit: the time of follicular rupture and expulsion of the eggs, in relation to injection of tuteinizing hormone. J. Endocr. 26:307.

Haterius, H. O. 1935. Oophorectomy and mainte- nace of pregnancy in the rat. Prec. Soc. Exp. Biol. and Med. 33 : 101.

Haterius, H. O. 1936. Reduction of litter size and maintenance of pregnancy in the oophorectomized rat: evidence concerning the endocrine role of the placenta. Am. J. Physiol. 114:399.

Heap, R. B., J. S. Perry and I. W. Rowlands. 1965. The effect of hypophysectomy on the corpus luteum

160 G R E E N W A L D A N D R O T H C H I L D

of the non-pregnant guinea-pig. Acta Endoor. Suppl. 100:76.

Heap, R. B. and R. Deanesly. 1966. Progresterone in systemic blood and p!acentae of intact and ovariectomized pregnant guinea pigs. J. Endocr. 34:417.

Hermreck, A. S. and G. S, Greenwald. 1964. The effects of unilateral ovariectomy on follicular matu- ration in the guinea pig. Anat. Rec. 148:171.

Hertz, R. 1960, Gonadotropin and adrenocorticotro- pin from rat pituitary homografts as manifested by host response to chorionie gonadotrophin and amphenone. Endocrinology 66:842.

Hilliard, J., D. Archibald and C. H. Sawyer. 1963. Gonadotropic activation of preovulatory synthesis and release of progestin in the rabbit. EndocrinoI- o~y 72 : 59.

Hilliard, J., J. N. Hayward and C. H. Sawyer. 1964. Postcoital patterns of secretion of pituitary gonadotropin and ovarian progestin in the rabbit. En- docrinology 75 : 957.

Hughes, R. L. and K. Myers. 1966. Behavioural cydes during pseudopregnancy in confined populations of domestic rabbits and their relation to the histology of the female reproductive tract. Australian J. of Zeol. 14:173.

Jaitly, K. D., J. M. Robson, F. M. Sullivan and C. Wilson. 1966. Hormonal requirements for the maintenance of ~estation in hypophysectomized mice. J. Endocr. 34:iv.

Kent, G. C., Jr. and G. Atkins. 1959. Duration of Dseudopregnancy in normal and uterine-traumat- ized hamsters. Prec. Soc. Exp. Biol. and Med. 101 : 106.

KidweU, W. R., K. Balo~h, Jr. and W. G. Wiest. 1966. Effects of luteinizin~ hormones on glucose-6-phos- phate and 20a-hydroxysteroid dehydroeenase activities in superovulated rat ovaries. Endocrinology 79:352.

Kidwell, W. R. and W. G. Wiest. 1967. 20a-hydroxysteroid dehydrogenase (20-SDH), progesterone (P), and 20a-hydroxypreg-4-en-3-one (20a-OH) in rats during pregnancy. Federation Proceedings 26: Abstr. No. 1271.

Kilpatrick, R., D. T. Armstrong and R. O. Greep. 1964. Maintenance of the corpus luteum by gonadotrophins in the hypophysectomized rabbit. Endocrinology 74:453.

Klein, M. 1933. Sur l'ablation des embryons chez la ]apine gravide et sur los facteurs qui determinent le maintien du corps iaune pendant la deuxibme partie de la grossesse. C.R. Soc. Biol. 113:441.

Klein, M. 1935. Sur le role du placenta darts I'arr& du cycle ovarien au cours de la grossesse. C.R. Soc. Biol. 119:579.

Klein, M. 1938. Action de l'uterus gravide sur l'ovaire gestatif chez le Hamster dord. C.R. Soc. Biol. 127: 1298.

Klein, M. 1939. Action du placenta sur le corps jaune gravidique et sur le cycle vaginal chez le cobaye. C.R. Sop. Biol. 130:1393.

Kovacic, N. 1964. Biological characteristics of pituitary and placental hormones. J. Reprod. Fertil. 8: 165.

Kovaeic, N. 1965. Effects of prolactin, luteinizing hormone, human chorionic gonadotrophin or pregnant mares' serum gonadotrophin on corpora lutea of adult normal or hypophysectomized mice. J. En- door. 34:1.

Lahr, E. L. and O. Riddle. 1936. Temporary suppression of estreus cycle in the rat by prolactin. Proc, See. Exp. Biol. and Med. 34:880.

Levy, H., H. W. Dearie and B. L. Rubin. 1959. Vis- ualization of steroid-3B-OL-dehydrogenase activity in tissues of intact and hypophysectmnized rats. Endocrinology 65:932.

Leblond, C. P. and W. O. Nelson. 1937. Etude his- tologioue des organes de la souris sans hypophyse. Bu!. d'Histologie Appliquee a la Physiologic e t a Ia Pathologic r}o. 181-206.

Lobel, B., R. M. Rosenbaum and H. W. Deane. 1961. Enzymic correJates of physiological regression of follicles and corpora lutea in ovaries of normal rats. Endocrinology 68 : 232.

Loeb, L. 1906, The formation of the corpus luteum in the guinea pig. J. Am. Med. Assn. 46:416.

Loeb, L. 1911. The cyclic changes in the ovary of the guinea pig. J. Morph. 22:37.

Long, J. A. and H. M. Evans. 1922. The oestrous cycle in the rat and its associated phenomena. Memoirs Univ. Calif. Vol. 6.

Lyon, R. A. and W. M. Allen. 1938. The duration of sensitivity of the endometrium during lactation in the rat. Anat. Rec. 122:624.

Lyons, W. R., M. E. Simpson and H. M. Evans. 1943. Hormonal requirements for pregnancy and mammary development in hypophysectomized rats. Prec. Soc. Exp. Biol. and Med. 52:134.

MacDonald, G. J., D. T. Armstrong and R, O. Greep. 1966. Stimu!ation of estrogen secretion from normal rat corpora lutea by luteinizing hormone. Endocri- nology 79:289.

Maekawa, K. 1956. Activation of corpora lutea by estrogen. J. Faculty of Sci., Univ. of Tokyo, Sec. IV, Vol. 7. Part 4.

Malone, T. E. 1957. The cellular origin of the corpus luteum as revealed histochemically by the neotetra- zolium reaction in the albino rat. J. Morph. 100:1.

Malone, T. E. 1960. Observations on the histochemical localization of alkaline glycerophosphatase in deve!oDing corpora lutea of albino rats. Am. J. Anat. 106:41.

Malven, P. V. and C. H. Sawyer. 1966. A luteolytic action of prolactin in bypophysectomized rats. En- docrinology 79:268.

Mayer, G. and M. Klein. 1946. Production d'une nou- velle gestation au cours d'une gravidit6 chez la latrine. C. R. Soc. Biol. 140:1011.

McCann, S. M. and H. M. Friedman. 1960. The effect of hypothalamic lesions on the secretion of luteotrophin. Endocrinolo~g 67:597.

McKeown, T. and S. Zuckerman. 1937. Stimulation of the corpora lutea of the rat by means of progesterone and testosterone. Proc. Roy. Soc. Ser. B 124: 362.

Meites, J. and M. C. Shelesnyak. 1957. Effects of prolactin on duration of preonancy, viability of young and lactation in rats, Prec. Soc, Exp. BioL and Med. 94:746.

Meyer. R. K., S. W. Soukup, W. H. McShan and D. C. Biddulph. 1947. Succinic dehydrogenase in rat ovarian tissues during pregnancy and lactation. Endocrinology 41:35.

Mikhail, G., M. W. Noall and W. M. Allen. 1961. Progesterone levels in the rabbit ovarian vein blood throughout pregnancy. Endocrinology 69:504.

Miihlbock, O. and L. M. Boot. 1959. Induction of mammary cancer in mice without the mammary tumor agent by isografts of hypophyses. Cancer Res. 19:402.

Nalbandov, A. V. 1961. Comparative physiology and


endocrinology of domestic animals. Recent Prog. Hormone Res. 17:119.

Nakano, A. 1960. Histological studies of the prenatal and postnatal development of the ovary in the golden hamster (Cricetus auratus). Okajimas Folia Anatomica Japonica 35:183.

Nathanson, I. T., H. L. Fevold and D. B. Jennison. 1937. Inhibition of estrous cyc]e in the rodent with post partum urine and commercial prolactin. Prec. Soc. Exp. Biol. and Med. 36:481.

Nelson, W. W. and R. R. Greene. 1953. The human ovary in pregnancy. Surgery, Gynecol. and Obst., section on Intern. Abstr. of Surgery 97:1.

Newton, W. H. and F. J. Lits. 1938. Criteria of placenta! endocrine activity in the mouse. Anat. Rec. 72:333.

Newton, W. H. and K. C. Richardson. 1940. The secretien of milk in hypophysectomized pregnant mice. J. Endocr. 2:322.

Nikitovitch-Winer, M. B. 1965. Effect of hypophysial sta!k section on luteotropic hormone secretion in the rat. Endocrinology 77:658.

Nikitovitch-Winer, M. and J. W. Everett. 1958. Com- parative study of luteotropin secretion by hypophy- seal autotransplants in the rat. Effects of site and stages of the estrous cycle. Endocrinology 62:522.

Nikitovitch-Winer, M. and J. W. Everett. 1959. His- tocytologic changes in grafts of rat pituitary on the kidney and upon re-transplantation under the dien- cephale n. Endocrinology 65 : 357.

Pederson, E. S. 1951. Histogenesis of lutein tissue of the a!bino rat. Am. J. Anat. 88:397.

Pencharz, R. I. and J. A. Long. 1933. Hypephysec- tomy in the pregnant rat. Am. J. Anat. 53:117.

Pencharz, R. I. and W. R. Lyons. 1934. Hypophysec- tomy in the pregnant guinea pig. Prec. Soc. Exp. Biol. and Med. 31:1131.

Pincus, G. and J. Berkman. 1937. Ascorbic acid during pregnancy in the rabbit. Am. J. Physiol. 119: 455.

Pupkin, M., H. Bratt, J. Weisz, C. W. Lloyd and K. BMogh, Jr. 1965. Dehydrogenases in the rat ovary. I. A histochemieal study of AC3B-and 20a-hydroxysteroid dehydrogenases and enzymes of car- bohydrate oxidation during the estrous cycle. Endocrinology 79:316.

Quilligan, E. J. and I. Rothchild. 1960. The corpus luteum-pituitary relationships: The luteotrophic activity of homotransplanted pituitaries in intact rats. Endocrinology 67:48.

Ray, E. W., S. C. Averill, W. R. Lyons and R. E. Johnson. 1955. Rat placental hormone activities corresponding to those of pituitary mammotropin. Endocrinology 56:359.

Rennels, E. G. 1962. An electron microscope study of pituitary autograft cells in the rat. Endocrinology 71:713.

Rennie, P., J. Davies and E. Friedrich. 1964. Failure of ovine prolactin to show luteotropic or luteolytic effects in the rabbit. Endocrinology 75:622.

Richardson, G. S., A. F. Parlow and L. L. Engel. 1962. Estrogen production by luteinized ovaries of hypophysectomized rats. Program of 44th meeting of the Endocr. Soc., Chicago.

Robson, J. M. 1936. Uterine changes in experimental abortion and their relation to parturition. J. Physiol. 86:171.

Robson, J. M. 1937a. Maintenance of ovarian and luteal function in the hypophysectomized rabbit by gonadotropic hormones. J. Physiol. 90:125.

Robson, J. M. 1937b. Maintenance of pregnancy and

of the luteal function in the hypophysectomized rabbit. J. Physiol. 90:I45.

Robson, J. M. 1937c. Maintenance by oestrin of the luteal function in hypophysectomized rabbits. J. Physiol. 90:435.

Robson, J. M. 1938. The role of gonadotrophic hormone in the maintenance of luteal function. Quart. J. Exp. Physiol. 28:49.

Rothchild, I. 1960a. The corpus luteum-pituitary relationship: The association between the cause of luteotrophin secretion and the cause of follicular quiescence during lactation; the basis for a tentative theory of the corpus luteum-pituitary relationship in the rat. Endocrinology 67:9.

Rothchild, I. I960b. The corpus luteum-pitiutary relationship: the lack of an inhibiting effect of progesterone on the secretion of pituitary luteotrophin. Endocrinology 67:54.

Rothchild, I. 1964. An explanation for the cause of luteolysis in the rat and its possible application to other species. Prec. of Second Intern. Congr. of Endocr. Series No. 83.

Rothchild, I. 1965a. Interrelations between progesterone and the ovary, pituitary and central nervous system in the control of ovulation and the regulation of progesterone secretion. Vitamins and Hor- mones 23 : 209.

Rothchild, I. 1965b. The corpus luteum-hypophysis relationship. The Iuteolytic effect of LH in the rat. Acta Endocr. 49 : 107.

Rothchild, I. 1966. The nature of the luteotrophic process. J. Reprod. Fertil. Suppl. l:49.

Rothchild, I. 1967. The central nervous system and disorders of ovulation in women. Am. J. Obstet. and Gyn.: (In press.).

Rothchild, I. and R. K. Meyer. 1942. Studies of the pretrauma factors necessary for placentomata formation in the rat. Physiol. Zool. 15:216.

Rothchild, I. and R. Dickey. 1960. The corpus luteum- pituitary relationship: a study of the compensatory hypertrophy of the ovary during pseudopregnancy and lactation in the rat. Endocrinology 67:42.

Rothchild, I. and N. B. Schwartz. 1965. The corpus luteum-hypophysis relationship: The effects of progesterone and oestrogen on the secretion of luteotrophin and luteinizing hormone in the rat. Acta Endocr. 49 : 120.

Rowlands, I. W. 1956. The corpus luteum of the guinea pig. In G. E. W. Wolstenholme [ed.] Ciba Foundation Colloquia on Ageing, Little, Brown and Co., Boston. 2:69.

Rowlands, I. W. 1961. Effect of hysterectomy at different stages in the life cycle of the corpus luteum in the guinea pig. J. Reprod. Fertil. 2:341.

Rowlands, I. W. 1962. The effect of oestrogens, prolactin and hypophysectomy on the corpora lutea and vagina of hysterectomized guinea pigs. J. En- docr. 24:105.

Rowlands, I. W. and R. V. Short. 1959. The progesterone content of guinea pig corpora lutea during the reproductive cycle and after hysterectomy. J. Endocr. 19:81.

Sammelwitz, P. H., J. P. Aldred and A. V. Nalbandov. 1961. Mechanisms of maintenance of corpora lutea in pigs and rats. J. Reprod. Fertil. 2:387.

Schwartz, N. B. 1966. Newer concepts of gonadotrophin and steroid feedback control mechanisms. In J. J. Gold [ed.] Textbook of Gynecologic Endo- crinology. Hoeber, N. Y. C.

Schweizer, M., H. A. Charipper and H. O. Haterius. 1937. Experimental studies 9I the anterior pituitary.

162 G R E E N W A L D A N D R O T H C H I L D

IV. The replacement capacity and the noncyclic behavior of homoplastic anterior pituitary grafts. Endocrinology 21:30.

Selye, H., J. B. Collip and D. L. Thomson. 1933. Ef- fect of hypophysectomy upon pregnancy and lactation in mice. Proc. Soc. Exp. Biol. and Med. 31:82.

Selye, H. and T. McKeown. 1935. Studies on the physiology of the maternal placenta in the rat. Proc. Roy. Soc. Ser. B 119:1.

Shelesnyak, M. C. and P. F. Kraicer. 1963. The role of estrogen in nidation. In A. C. Enders [ed.] De- layed Implantation. University of Chicago Press.

Short, R. V. 1962. Steroids in the follicular fluid and the corpus luteurn of the mare. A 'two-cell type' theory of ovarian steroid synthesis. J. Endocr. 24: 59.

Smith, P. E. 1930. Hypophysectorny and a replacement therapy in the rat. Am. J. Anat. 45:205.

Smith, P. E. and W. E. White. 1931. The effect of hypophysectomy on ovulation and corpus luteum formation in the rabbit. J. Am. Med. Assn. 97:1861.

Snyder, F. F. 1934. The prolongation of pregnancy and complications of parturition in the rabbit following induction of ovulation near term. Bul. Johns Hopkins Hospital 54:1.

Spies, H. G., L. L. Coon and H. T. Gier. 1966. Luteo- lyric effect of LH and HCG on the corpora lutea of pseudopregnant rabbits. Endocrinology 78:67.

Stafford, W. T., R. F. Collins and H. W. Mossman. 1942. The thecal gland in the guinea pig ovary. Anat. Rec. 83:193.

Stafford, R. O., W. H. McShan and R. K. Meyer. 1947. Acid and alkaline phosphatase in ovarian tissues of the rat during pregnancy and lactation. En- docrinology 41:45.

Stormshak, F. and L. E. Casida. 1965. Effects of LH and ovarian hormones on corpora lutea of pseduo- pregnant and pregnant rabbits. Endocrinology 77: 337.

Stormshak, F. and L. E. Casida. 1966. Fetal-placental inhibition of LH-induced luteal regression in rabbits. Endocrinology 78:887.

Telegdy, G. and E. Endr6czi. 1963. The ovarian secretion of progesterone and 20a-hydroxypregn-4- en-3-one in rats during the estrous cycle. Steroids 2:119.

Toyoda, Y. 1961. Changes in corpora lutea in the rat

by means of a simplified inspection method. Japan. J. An. Reprod. 7:111.

Turnbull, J. G. and G. C. Kent, Jr. 1963. The decidual cell response in the golden hamster. Anat. Rec. 145:97.

Turnbull, J. G. and G. C. Kent, Jr. 1966. Prolactin and the luteotropic complex in hamsters. Endocri- nology 79: 716.

von Berswoldt-Wallrabe, I., H. F. Geller and U. Herlyn. 1964. Temporal aspects of decidual cell reaction. I. Induction of decidual cell reaction in lactogenic hormone treated and in pseudopregnant rats. Acta Endocr. 45:349.

Weichert, C. K. and A. W. Schurgast. 1942. Variations in size of corpora lutea in the albino rat under normal and experimental conditions. Anat. Rec. 83: 321.

Westman, A. and D. Jacobsohn. 1937. t~ber Oestrin- wirkungen auf der Corpus Luteum-Funktion. Acta Obstet. Gynecol. Scand. 17:13.

Westman, A. and D. Jacobsohn. 1938. Endokrinolo- gische Untersuchungen an Ratten mit durehtrenn- tern hypophysenstiel. 6. Produktion und Abgabe der gonadotropen Hormone. Act. Pathol. et Micro- biol. Scand. 15:445.

Westman, A. and D. Jacobsohn. 1940. Endokrinol- ogische Untersuehungen an Kaninchen mit durch- trenntem hypophysenstiel. Acta Obstet. and Gyn. Seand. 20:392.

Wiest, W. G. 1959. Progesterone and 4-pregnen-20a- ol-3-one in the tissues of pregnant rats, Endocrinol- ogy 65 : 825.

Wiest, W. G., R. B. Wilcox and T. H. Kirsehbaum. 1963. Rat ovarian 20a-hydroxysteroid dehydrogenase: Effects of estrogens and pituitary gonadotrophins. Endocrinology 73:588.

Yasuda, T. and K. Doi. 1953. Histochemical studies on histogenesis of lutein tissue in the rabbit ovary. Med. J. Osaka Univ. 4:241.

Yochim, J. M. and V. J. De Feo. 1963. Hormonal control of the onset, magnitude and duration of uterine sensitivity in the rat by steroid hormones of the ovary. Endocrinology 72:305.

Zarrow, M. X. and G. M. Neher. 1955. Concentration of progestin in the serum of the rabbit during pregnancy, the puerperium and following castration. Endocrinology 56:1.

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