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Physiology Of

Menstruation

Dr Anusha Rao PPGY2 (OBG)

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Menstruation is a visible manifestation of cyclic, physiologic

uterine bleeding due to shedding of the endometrium following

invisible interplay of hormones mainly through H-P-O axis.

Normal limits:

Frequency: 24-38 days

Regularity: +/- 2-20 days

Duration: 4-8 days

Volume: 5-80 mL

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+HORMONES FOLLICULAR

PHASE

OVULATION LUTEAL PHASE

DAILY

PRODUCTION

(ug)

OESTRADIOL

PROGESTERONE

50

2 - 3

150-300 100

20 -30

SERUM VALUES

OESTRADIOL

(pg/ml)

PROGESTERONE

(ng/ml)

FSH (mIU/ml)

LH (mIU/ml)

50

<1

10

5

300- 600

15- 20

60

150-200

>5

10

5

DAILY

EXCRETION

TOTAL

OESTROGEN (ug)

PREGNANEDIOL

(mg)

10-25

<1

35- 100 25-75

3-6

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+ Menstrual cycle can be explained in two cycles which occur

concurrently

• The ovarian cycle and

• The uterine cycle

The Ovarian Cycle consists of

The follicular phase and

The luteal phase

The Uterine Cycle consists of

The proliferative phase and

The secretory phase

+THE OVARIAN

CYCLE

+Follicular

phase

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+Primordial follicle

Originate in the Endoderm

Migrate to the genital ridge at 5-6weeks

Maximum at 16 – 20 wks : 6 – 7 million

At Birth : 2 million

At Pubery : 0.3 to 0.5 million

Only 400 – 500 follicles ovulate during a woman’s reproductive

years.

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The primordial follicle is nongrowing and consists of an oocyte,

arrested in the diplotene stage of meiotic prophase, surrounded

by a single layer of spindle-shaped granulosa cells.

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The initial recruitment and growth of the primordial follicles is gonadotropin

independent and affects a cohort over several months

The total duration of time to achieve pre ovulatory status is approximately 85

days

First visible signs of development are

Increase in the size of oocyte

granulosa cells becoming cuboidal

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Gap junctions composed of channels formed by arrangement of

proteins known as Connexins –

up regulated and kept open by FSH and

down regulated and closed by LH

+The Pre antral Follicle

Oocyte enlarges and is surrounded by a membrane, the zona

pellucida.

The granulosa cells undergo a multilayer proliferation as the theca

layer continues to organize from the surrounding stroma.

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The granulosa cells of the preantral follicle synthesizes all 3 classes of

steroids

Estrogens are produced more than androgens or progestins

An aromatase enzyme system converts androgens to estrogens and is a

factor limiting ovarian estrogen production.

Aromatization is induced or activated through the action of FSH.

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FSH both

initiates steroidogenesis (estrogen production) in granulosa

cells and

stimulates granulosa cell growth and proliferation

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The Antral Follicle Under the influence of estrogen and

FSH, there is an increase in the

production of follicular fluid.

Oocyte and the surrounding granulosa

cells are nurtured in this follicular fluid

The granulosa cells surrounding the

oocyte are now designated the

cumulus oophorus

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P45OC17

P450arom

+The Two-Cell,

Two-Gonadotropin System

The aromatase activity of the granulosa cells is more than

thecal cells.

In human preantral and antral follicles,

LH receptors are present only on the theca cells and

FSH receptors only on the granulosa

LH stimulates thecal cells to produce androgens that can then

be converted, through FSH-induced aromatization, to

estrogens in the granulosa cells.

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As the follicle emerges, the theca cells are characterized by their

expression of P450c17, the enzyme step that is rate limiting for the

conversion of 21-carbon substrate to androgens.

Increasing expression of the aromatization system (P450arom) is a

marker of increasing maturity of granulosa cells.

The presence of P450c17 only in theca cells and P450arom only in

granulosa cells is an impressive evidence confirming the two-cell,

two-gonadotropin explanation for estrogen production

+Selection of the Dominant Follicle

The process of conversion of a single follicle to a estrogen

dominant follicle depends on

(1) a local interaction between estrogen and FSH within the follicle,

:-positive feedback

(2) the effect of estrogen on pituitary secretion of FSH:- negative

feedback.

Serves to withdraw gonadotropin support from the other less

developed follicles.

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The first event in the process of atresia is a reduction in FSH

receptors in the granulosa layer

A wave of atresia among the lesser follicles,

is seen to parallel the rise in estrogen.

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Lower GnRH pulse frequencies favor FSH secretion, and

higher GnRH pulse frequencies favor LH secretion.

Low levels of estrogen enhance FSH and LH synthesis and

storage, have little effect on LH secretion, and inhibit FSH

secretion.

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High levels of estrogen induce the LH surge at midcycle, and

high steady levels of estrogen lead to sustained elevated LH

secretion.

Low levels of progesterone acting at the level of the pituitary

gland enhance the LH response to GnRH and are responsible

for the FSH surge at midcycle.

High levels of progesterone inhibit pituitary secretion of

gonadotropins by inhibiting GnRH pulses at the level of the

hypothalamus.

+Inhibin, Activin, and Follistatin

This family of peptides is synthesized by granulosa cells in

response to FSH and secreted into the follicular fluid and

ovarian venous effluent.

They are expressed in many tissues through out the body as

autocrine-paracrine regulators.

Inhibin is an important inhibitor of FSH secretion.

Activin stimulates FSH release in the pituitary and augments

FSH action in the ovary.

Follistatin suppresses FSH activity by binding to activin.

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Inhibin

Blocks the synthesis and secretion of FSH,

Prevent the up-regulation of GnRH receptors by GnRH,

Reduce the number of GnRH receptors present,

At high concentrations, promote the intracellular degradation of

gonadotropins.

+The Preovulatory Follicle

Granulosa cells in the preovulatory follicle

enlarge and acquire lipid inclusions

And theca becomes vacuolated and richly

vascular, giving the preovulatory follicle a

hyperemic appearance.

The oocyte proceeds in meiosis,

approaching completion of its reduction

division.

Approaching maturity, the preovulatory

follicle produces increasing amounts of

estrogen.

Estrogen peaks approximately 24 to 36

hours prior to ovulation.

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The onset of the LH surge occurs when the peak levels of

estradiol are achieved.

In providing the ovulatory stimulus to the selected follicle, the

LH surge seals the fate of the remaining follicles, with their

lower estrogen and FSH content, by further increasing

androgen superiority.

LH promotes luteinization of the granulosa in the dominant

follicle, resulting in the production of progesterone

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After adequate estrogen priming, progesterone facilitates the

positive feedback response.

And in the presence of subthreshold levels of estradiol can

induce a characteristic LH surge.

When administered before the estrogen stimulus, or in high

doses (achieving a blood level greater than 2 ng/mL),

progesterone blocks the midcycle LH surge.

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Progesterone at midcycle is significantly responsible for the

FSH surge.

Thus ensures completion of FSH action on the follicle,

especially making sure that a full complement of LH receptors

is in place in the granulosa layer.

As products of thecal tissue are androgens, the increase in

stromal tissue in the late follicular phase is associated with a

rise in androgen levels.

- for atresia of lesser follicles and for libido

enhancement

+Ovulation

+

A threshold of LH concentration must be maintained for at least

14 to 27 hours in order for full maturation of the oocyte to occur.

Usually the LH surge lasts 48 to 50 hours

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LH, FSH, Progesterone, growth factors

Plasminogen activator synthesis (granulosa &theca cells)

Plasminogen Plasmin

Collagenase

Disrupts follicular wall

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Estradiol levels plunge as LH reaches its peak. This may be a consequence of LH down-regulation of its own receptors on the follicle.

Due to-

High LH causes supression of steroidogenesis

Low midcycle levels of progesterone exert an inhibitory action on further granulosa cell multiplication, and hence the drop in estrogen

Estrogen can exert an inhibitory effect on P450c17(aromatase enzyme)

+LUTEAL

PHASE

+

Luteinization and the corpus luteum:

granulosa cells increase in size and assume a characteristic

vacuolated appearance associated with the accumulation of a

yellow pigment , lutein.

theca lutein cells may differentiate from the surrounding theca

and stroma to become part of the corpus luteum.

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Angiopoietin-1 binds to the endothelial Tie-2 receptor & inc.

expr. Of VEGF + LH -> Angiogenesis

Angiopoietin-2, leads to vascular breakdown that accompanies

luteolysis.

Vascularization of the granulosa layer is essential to allow LDL-

cholesterol to reach the luteal cells to provide sufficient

substrate for progesterone.

By day 8 or 9 after ovulation, a peak of vascularization is

reached, associated with peak levels of progesterone and

estradiol in the blood.

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The leukocytes in the corpus luteum secrete cytolytic

enzymes, prostaglandins, and growth factors involved in

angiogenesis, steroidogenesis, and luteolysis.

Endothelin-1 is a mediator of luteolysis

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Luteal cell population is composed of two distinct cell types,

large and small cells.

Large cells are derived from granulosa cells and

the small cells from theca cells.

The small cells are the most abundant.

Steroidogenesis takes place in the large cells,

Small cells contain LH and hCG receptors.

LH/hCG receptors are absent on the large cells,

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The corpus luteum rapidly declines 9 to 11 days after ovulation.

The regression of luteal cells is induced by the estradiol

produced by the corpus luteum.

This action of estrogen is mediated by nitric oxide.

The final signal for luteolysis, however, is prostaglandin F2

alpha, produced within the ovary in response to the locally

synthesized luteal estrogen.

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Prostaglandin F2Alpha stimulates the synthesis of endothelin

Endothelin-1

inhibits luteal steroidogenesis,

stimulates prostaglandin production in luteal cells.

stimulates the release of TNF Alpha,which induces apoptosis.

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The Luteal-

Follicular

Transition

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The demise of the corpus luteum results in a nadir in the circulating levels of estradiol, progesterone, and inhibin.

The decrease in inhibin-A removes a suppressing influence on FSH secretion in the pituitary.

The decrease in estradiol and progesterone allows a progressive and rapid increase in the frequency of GnRH pulsatile secretion and a removal of the pituitary from negative feedback suppression.

The removal of inhibin-A and estradiol and increasing GnRH pulses combine to allow greater secretion of FSH compared with LH, with an increase in the frequency of the episodic secretion.

The increase in FSH is instrumental in rescuing approximately a 70-day-old group of ready follicles from atresia, allowing a dominant follicle to begin its emergence.

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+HORMONES FOLLICULAR

PHASE

OVULATION LUTEAL PHASE

DAILY

PRODUCTION

(ug)

OESTRADIOL

PROGESTERONE

50

2 - 3

150-300 100

20 -30

SERUM VALUES

OESTRADIOL

(pg/ml)

PROGESTERONE

(ng/ml)

FSH (mIU/ml)

LH (mIU/ml)

50

<1

10

5

300- 600

15- 20

60

150-200

>5

10

5

DAILY

EXCRETION

TOTAL

OESTROGEN (ug)

PREGNANEDIOL

(mg)

10-25

<1

35- 100 25-75

3-6

+

From the midluteal peak to menses,

there is a 4.5-fold increase in LH pulse frequency

FSH pulse frequency increases 3.5-fold

The increase in FSH is, as noted, greater than that of LH.

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UTERINE CYCLE

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The changes in the endometrium will be discussed in five

phases:

(1) The menstrual endometrium

(2) The proliferative phase

(3) The secretory phase

(4) Preparation for implantation, and finally

(5) The phase of endometrial breakdown.

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+The Proliferative Phase

The glands :

narrow and tubular, lined by low columnar epithelium cells.

Mitoses

Pseudostratification

A continuous epithelial lining facing the endometrial cavity is

formed.

+ THE

PROLIFERATIVE

PHASE

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All of the tissue components demonstrate proliferation, which peaks on days

8-10 of the cycle, corresponding to peak estradiol levels in the circulation and

maximal estrogen receptor concentration in the endometrium

Changes are most intense in the functionalis layer in the upper two-thirds of

the uterus, the usual site of blastocyst implantation.

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The endometrium grows from approximately 0.5 mm to 3.5 to

5.0 mm in height

Restoration of tissue constituents has been achieved by

estrogen-induced new growth as well as incorporation of ions,

water, and amino acids.

An important feature of this estrogen-dominant phase of

endometrial growth is the increase in ciliated and microvillous

cells

+THE

SECRETOR

Y PHASE

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The endometrium now demonstrates a combined reaction to estrogen and progesterone activity.

Epithelial proliferation ceases 3 days after ovulation.

Total endometrial height is fixed at roughly its preovulatory extent (5-6 mm) despite continued availability of estrogen. This limitation is due to :

Progesterone interference with estrogen receptor expression

stimulation of 17beta-hydroxysteroid dehydrogenase and sulfotransferase, which convert estradiol to estrone sulfate (which is rapidly excreted from the cell)

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Tissue components continue to display growth, but confinement in a fixed structure leads to progressive tortuosity of glands and intensified coiling of the spiral vessels.

The first histologic sign that ovulation has occurred is the appearance of subnuclear intracytoplasmic glycogen vacuoles in the glandular epithelium on cycle days 17-18.

These structural alterations are soon followed by

- active secretion of glycoproteins and peptides into the

endometrial cavity

-Transudation of plasma

-immunoglobulins obtained from the circulation

The peak secretory level is reached 7 days after the midcycle gonadotropin surge, coinciding with the time of blastocyst implantation

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IMPLANTATION

PHASE

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By 13 days postovulation, the endometrium has differentiated into three

distinct zones.

1/4th of the tissue is the unchanged basalis, straight vessels and spindle-

shaped stroma.

The midportion (approx 50% of the total) is the lace like stratum

spongiosum,loose edematous stroma with tightly coiled spiral vessels and

dilated glandular ribbons.

the superficial layer of the endometrium (about 25% of the height) called the

stratum compactum, which has become large and polyhedral stromal cell,

forming a compact, structurally sturdy layer.

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The subepithelial capillaries and spiral vessels are engorged

At the time of implantation, on days 21-22 of the cycle, the predominant

morphologic feature is edema of the endometrial stroma , due to inc in

permeability under the influence of steroids

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ENDOMETRIAL

BREAKDOWN

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In the absence of fertilization, implantation, and the consequent

lack of hCG from the trophoblast, the fixed lifespan of the corpus

luteum is completed, and estrogen and progesterone levels

wane.

The most prominent immediate effect of this hormone withdrawal

is a modest shrinking of the tissue height and spiral arteriole

vasomotor responses.

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The following vascular sequence occurs

With shrinkage of height, blood flow within the spiral vessels

diminishes, venous drainage is decreased, and vasodilation ensues.

Thereafter, the spiral arterioles undergo rhythmic vasoconstriction and

relaxation.

Each successive spasm is more prolonged and profound, leading

eventually to endometrial blanching.

Within the 24 hours immediately preceding menstruation, these

reactions lead to endometrial ischemia and stasis.

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White cells migrate through capillary walls, extending throughout the

stroma.

During arteriolar vasomotor changes, red blood cells escape into the

interstitial space. Thrombin-platelet plugs also appear in superficial

vessels.

The prostaglandin content (PGF2 alpha and PGE2) in the secretory

endometrium reaches its highest levels at the time of menstruation.

The vasoconstriction and myometrial contractions associated with the

menstrual events are mediated by prostaglandins from perivascular

cells and the potent vasoconstrictor endothelin-1, derived from stromal

decidual cells.

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In the first half of the secretory phase, acid phosphatase and

potent lytic enzymes are confined to lysosomes, stabilized by

progesterone, which are released with waning of it’s level.

These active enzymes will digest their cellular constraints,

leading to the release of prostaglandins, extravasation of red

blood cells, tissue necrosis, and vascular thrombosis

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Endometrial tissue breakdown also involves a family of

enzymes, matrix metalloproteinases

The metalloproteinases include

collagenases that degrade interstitial and basement membrane

collagens;

gelatinases that further degrade collagens;

and stromelysins that degrade fibronectin, laminin, and

glycoproteins

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Progesterone withdrawal from endometrial cells induces matrix

metalloproteinase secretion.

In a nonpregnant cycle, metalloproteinase expression is

suppressed after menses by increasing estrogen levels.

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Progesterone withdrawal is associated with an increase in

VEGF receptor concentrations in the stromal cells.

Although the VEGF system is usually involved with

angiogenesis, in this case these factors are involved in the

preparation for menstrual bleeding, perhaps influencing the

expression of matrix metalloproteinases.

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Eventually,Leakage occurs as a result of diapedesis, and finally, interstitial hemorrhage occurs due to breaks in superficial arterioles and capillaries.

As ischemia and weakening progress, the continuous binding membrane is fragmented, and intercellular blood is extruded into the endometrial cavity.

New thrombin-platelet plugs form intravascularly upstream at the shedding surface, limiting blood loss.

Increased blood loss is a consequence of reduced platelet numbers and inadequate hemostatic plug formation.

Menstrual bleeding is influenced by activation of clotting and fibrinolysis

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PAI-1 exerts an important restraining action on fibrinolysis and

proteolytic activity.

Blood loss is also controlled by constriction of the spiral

arteries, mediated by the perivascular cells, myofibroblasts that

surround the spiral arteries.

Myofibroblasts respond to progesterone withdrawal by

expressing prostaglandins and cytokines, causing cycling

vasoconstriction and vasodilation

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Thrombin generation in the basal endometrium in response to

extravasation of blood is essential for hemostasis.

The basalis endometrium remains during menses, and repair

takes place from this layer.

This endometrium is protected from the lytic enzymes in the

menstrual fluid by a mucinous layer of carbohydrate products

that are discharged from the glandular and stromal cells.

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A natural cleavage point exists between basalis and

spongiosum, and, once breached, the loose, vascular,

edematous stroma of the spongiosum desquamates and

collapses.

The process is initiated in the fundus and extends throughout

the uterus.

In the end, the typical deflated, shallow, dense, menstrual

endometrium results.

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Within 13 hours, the endometrial height shrinks from 4 mm to

1.25 mm.

Menstrual flow stops as a result of the combined effects of

Prolonged vasoconstriction of the radial arteries and the spiral

arteries in the basalis,

Tissue collapse,

Vascular stasis,

Estrogen-induced healing

In contrast to postpartum bleeding, myometrial contractions are

not important for control of menstrual bleeding.

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