Chapter 20
Reproduction
Reproduction
Reproductive System Not needed for the survival of the individual Species survival
Sexual reproduction Genes from two individual Combine at random Creates new combinations Increases chances of species survival
Sexual Reproduction
Each individual produces gametes Formed in gonads by meiosis
Male: testes produce sperm Female: ovaries produce ova
Gametes unite in process of fertilization Restores diploid number Forms zygote
Sexual Determination Each zygote inherits
23 chromosomes from mother 23 chromosomes from father 23 pairs of homologous chromosomes
Kinds of chromosomes 1-22 pairs of chromosomes: autosomal 23rd pair are sex chromosomes
Male: XY, Female: XX
Chromosomal gender of zygote determined by fertilizing sperm
Male Reproduction System
Testes Seminiferous tubules
Where spermatogenesis occurs Contain receptor proteins for FSH
in Sertoli cells
Leydig cells Secrete testosterone Contain receptor proteins for LH
Formation of Testes
First 40 days after conception: the gonads of males and females are similar in appearance
Cells that will give rise to spermatogonia and oogonia migrate from yolk sac to developing embryonic gonads Gonads could become either
TDF (testis-determining factor) promotes the conversion
to testes gene located SRY (sex determining region of Y chromosome) Found in all mammals
Leydig cells secrete testosterone Begins 8th week and peaks at 12-14th week Masculinizes embryonic structures Testosterone then declines to very low levels until puberty
Testes descend into scrotum shortly before birth
Temp below internal temp (34.4 - 35.5)
Testosterone
Responsible for development of male accessory sex organs
External genitalia identical first 6 weeks, then testosterone stimulates development of penis
Not the active agent in all cells Converted to dihydrotestosterone in some target cells Needed for penis, spongy urethra, scrotum, prostrate
Testosterone directly needed for wolfian derivatives
Epididymis, ductus deferens, ejaculatory duct, seminal vesicles
Formation of Ovaries
Absence of Y chromosome and TDF, female develop ovaries
Ovarian follicles do not appear until 2nd trimester, day 105
Chromosomal sex and development of embryonic gonads
Endocrine Regulation of Reproduction
First trimester Embryonic testes are active endocrine glands
Secrete large amounts of testosterone Embryonic ovaries not mature until third trimester
Time of birth: Gonads in both sexes relatively inactive
Before puberty: Low levels of sex steroids in both
Due to lack of stimulation
Puberty: Increased stimulation from gonadotropic hormones Induce increase in sex steroids
Hypothalamus releases GnRH into hypothalamo-hypophyseal portal vessels
Anterior pituitary secretes LH (luteinizing hormone)
In male: interstitial-cell stimulating hormone (ICSH)
FSH (follicle-stimulating hormone)
Secreted in pulsatile fashion to prevent desensitization and down regulation of receptors
Endocrine Interactions
Primary effects of LH and FSH on gonads Stimulation of spermatogenesis and oogenesis Stimulation of gonadal hormone secretion Maintenance of gonadal structure
Endocrine Regulation
Negative Feedback Inhibit GnRH from hypothalamus Inhibit anterior pituitary response to GnRH Inhibin secretion inhibit anterior pituitary release of FSH
By sertoli cells
Onset of Puberty
FSH and LH high in newborn, falls to low levels in few weeks
Puberty: driven by increased secretion of FSH and LH
FSH and LH: Brain maturation increases GnRH secretion Decreased sensitivity of GnRH to negative feedback
LH: Increased secretion triggers puberty
Late puberty, pulsatile secretion of LH and FSH increase during sleep
Stimulate a rise in sex steroid secretion
Stimulate rise in testosterone and estradiol-17β Produce secondary sexual
characteristics
Age of onset related to the amount of body fat and physical activity in the female
Leptin secretion from adipocytes may be required for puberty
Pineal Gland
Secretes melatonin Secretion influenced by light-dark cycles Inhibit gonadotropin secretion Role in humans not established
4 Phases of Human Sexual Response
Excitation phase (arousal) Myotonia and vasocongestion Engorgement of a sexual organ with blood Erection of the nipples
Plateau phase: Clitoris becomes partially hidden
Erected nipples become partially hidden by swelling of areolae
Orgasm: Uterus and orgasmic platform of vagina contract Contractions accompanying ejaculation
Resolution phase: Body return to preexcitation conditions
Refractory period
In males Erection possible, but not ejaculation
Control of LH and FSH Secretion
Negative feedback Testosterone inhibits LH and GnRH production Inhibin inhibits FSH secretion
Spermatogenesis
Spermatogonia Replicate initially by mitosis
One of the 2 primary spermatocytes undergoes
meiosis 2 nuclear divisions 1st meiotic division produces 2 secondary spermatocytes 2nd meiotic division produces 4 spermatids
Maturation of spermatozoa
Male Accessory Organs
Epididymis Maturational changes Resistance to pH changes and temperature Storage
Prostate secretes:
Alkaline fluid Citric acid Ca++ Coagulation proteins
Seminal vesicles secrete fructose
Erection, Emission, and Ejaculation
Erection: Increased vasodilation of arterioles NO is the NT Blood flow into the erectile tissues of the penis Parasympathetic
Emission: Movement of semen into the urethra
Sympathetic
Ejaculation: Forcible expulsion of semen from the urethra out of the penis Sympathetic
Female Reproductive System
Ovaries contain large number of follicles which enclose ova Extensions called fimbriae partially cover each ovary At ovulation, secondary oocyte is extruded
Fallopian (uterine) tubes: Ova drawn into the tube by cilia
Uterus: Endometrium shed during menstruation
Vagina: Cervical mucus plug
Ovarian Cycle
5 mo. gestation ovaries contain 6-7 million oogonia
Oogenesis arrested in prophase of 1st meiotic division (primary oocyte)
Apoptosis occurs 2 million primary oocytes at birth 400,000 primary oocytes at puberty
400 oocytes ovulated during the reproductive years
Menstrual Cycle
Cyclic Changes in the Ovaries Follicular Phase Ovulation Luteal Phase
Duration approximately 28 days Day 1 is the first day of menstruation
Follicular Phase FSH stimulates production of FSH receptors on granulosa cells
Follicles grow and become secondary follicle
Granulosa cells secrete estradiol
Increases sensitivity of FSH receptors
FSH and estradiol stimulate production of LH receptors in graafian follicle
Rapid rise in estradiol from granulosa cells. Negative feedback on LH and FSH
Hypothalamus increase frequency of GnRH pulses. Augments the ability of anterior pituitary to respond to GnRH to increase LH secretion
Positive feedback: LH surge begins 24 hours before ovulation Triggers ovulation
FSH increase stimulates development of new follicles
Ovulation
Under FSH, graafian follicle grows large and thin-walled, triggering LH surge
Wall of graafian follicle ruptures at day 14 Ist meiotic division is completed
Graafian follicle forms bulge on surface of ovary,
extruding secondary oocyte into the uterine tube If not fertilized, becomes corpus albicans
Luteal Phase
LH stimulates formation of the empty follicle into corpus luteum Corpus luteum secretes progesterone and estradiol
Progesterone plasma concentration rapidly rises
Negative feedback on LH and FSH Inhibin suppress FSH
Estrogen and progesterone levels fall during the late luteal phase
because the corpus luteum regresses and stops functioning
Withdrawal of estradiol and progesterone cause menstruation to occur
Menopause
Cessation of ovarian activity Age ~ 50 years Ovaries depleted of follicles Estradiol and inhibin withdrawl causes hot flashes,
and atrophy of the vaginal wall LH and FSH increase
Fertilization Ejaculation 300 million sperm, 100
reach (uterine) fallopian tube
Fertilization occurs in the uterine tubes
Acrosomal reaction: Acrosome of sperm contains hyaluronidase, an enzyme that digests a channel through zona pellucida
Sperm fuses with ovum cell membrane
As fertilization occurs, secondary oocyte completes 2nd meiotic division
Sperm enters ovum cytoplasm
Ovum nuclear membrane disappears, zygote formed
Blastocyct Formation
Cleavage: 30-36 hrs after fertilization the zygote divides by mitosis
Blastocyst develops Inner cell mass: fetus Surrounding chorion: trophoblasts form placenta
Implantation
6th day after fertilization, blastocyst attaches to uterine wall
Blastocyst secretes enzymes that allow blastocyst to burrow into endometrium
Trophoblast cells secrete hCG
hCG (Human chorionic gonadotropin)
Trophoblast cells secrete hCG Signals corpus luteum not to
degenerate until placenta secretes adequate [hormone]
Effects similar to LH Basis of pregnancy test
Placenta
Placenta Function
Gas exchange: 02 and C02
Nutrient exchange Waste exchange Synthesis of proteins and enzymes
Placental Hormones
Human placental lactogen (hPL), also called human chorionic somatomammotropin (HCS), is a polypeptide placental hormone
GH effects Diabetic-like effect
Glucose sparing effect Polyuria Lipolysis
Placental Hormones
Fetal-placental unit: Placenta must cooperate with the adrenal cortex in the fetus to produce estrogen
Estrogen stimulates endometrial growth
Inhibit prolactin secretion Growth of mammary ducts Enlargement of mother’s uterus
Progesterone
Suppresses uterine contractions Stimulates uterine growth Suppresses LH and FSH Stimulate development of alveolar tissue of the mammary gland
Parturition
Estrogen in late pregnancy Increases amount of oxytocin stored Stimulate production of oxytocin receptors in myometrium Stimulate prostaglandin production
Uterine contractions: Oxytocin, Prostaglandins
Lactation Hypothalamus releases PRH Anterior pituitary releases prolactin: Stimulate milk
production
Oxytocin needed for milk letdown