Chapter 20

1. Chapter 20 Reproduction

2. Sexual Determination • Each zygote inherits 23 chromosomes from mother and 23 chromosomes from father. • Produce 23 pairs of homologous chromosomes. • 1-22 pairs of chromosomes: autosomal • 23rd pair are sex chromosomes. • Chromosomal gender of zygote determined by fertilizing sperm.

3. Formation of Testes • First 40 days after conception the gonads of males and females are similar in appearance. • Spermatogonia and oogonia migrate from yolk sac to developing embryonic gonads. • TDF (testis-determining factor) promotes the conversion to testes: • Seminiferous tubules: • Germinal cells: sperm. • Nongerminal cells: Sertoli cells. • Leydig cells: • Appear about day 65.

4. Formation of Testes • 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.

5. Formation of Ovaries • Absence of Y chromosome and TDF, female develop ovaries. • Ovarian follicles do not appear until 2nd trimester.

7. Development of Accessory Sex Organs and Genitalia • Presence or absence of testes determines the accessory sex organs and external genitalia. • Male accessory organs derived from wolffian ducts. • Sertoli cells secrete MIF. • Female accessory organs derived from mullerian ducts.

8. Endocrine Regulation of Reproduction • Hypothalamus releases LHRH (GnRH) into hypothalamo-hypophyseal portal vessels. • Anterior pituitary secretes: • LH: luteinizing hormone. • FSH: follicle-stimulating hormone. • Secreted in pulsatile fashion to prevent desensitization and down regulation of receptors.

9. Endocrine Interactions • Primary effects of LH and FSH on gonads: • Stimulation of spermatogenesis and oogenesis. • Stimulation of gonadal hormone secretion. • Maintenance of gonadal structure.

10. Endocrine Regulation • Negative Feedback: • Inhibit GnRH from hypothalamus. • Inhibit anterior pituitary response to GnRH. • Inhibin secretion inhibit anterior pituitary release of FSH. • Female: estrogen and progesterone. • Male: testosterone.

12. Onset of Puberty • FSH and LH high in newborn, falls to low levels in few weeks. • Brain maturation increases GnRH secretion. • Decreased sensitivity of GnRH to negative feedback. • Late puberty, pulsatile secretion of LH and FSH increase during sleep. • Stimulate a rise in sex steroid secretion.

13. Onset of Puberty • Stimulate rise in testosterone and estradiol-17b. • 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.

14. Pineal Gland • Secretes melatonin. • Secretion influenced by light-dark cycles. • Inhibit gonadotropin secretion. • Role in humans not established.

15. 4 Phases of Human Sexual Response Excitation phase: • 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.

16. 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.

17. Control of LH and FSH Secretion • Negative feedback: • Testosterone inhibits LH and GnRH production. • Inhibin inhibits FSH secretion. • Aromatization reaction producing estadiol in the brain is required for the negative feedback effects of testosterone on LH.

19. Testosterone Secretion • Responsible for initiation and maintenance of body changes in puberty. • Stimulate growth of muscles, larynx, and bone growth until sealing of the epiphyseal discs. • Promote hemoglobin synthesis. • Acts in paracrine fashion and is responsible for spermatogenesis.

20. Estrogen Secretion • Sertoli and Leydig cells secrete small amounts of estradiol. • May be responsible for: • Negative feedback in brain. • Sealing of epiphyseal plates. • Regulatory function in fertility.

21. Spermatogenesis • Spermatogonia: • Replicate initially by mitosis. • One of the primary spermatocytes undergoes meiosis: • 2 nuclear divisions. • 2nd meiotic division produce 4 spermatids.

22. Spermiogenesis • Maturation of spermatozoa. • Cytoplasm is pinched off and ingested by the Sertoli cell cytoplasm.

23. Sertoli Cells • Blood-testes barrier: • Prevents autoimmune destruction of sperm. • Produce FAS ligand which binds to the FAS receptor on surface to T lymphocytes, triggering apoptosis. • Secretes inhibin. • Phagocytize residual bodies: • Transmit information molecules from germ cells to Sertoli cells. • Secrete ABP: • Binds to testosterone and concentrates testosterone in the tubules.

24. Hormonal Control of Spermatogenesis • Testosterone required for completion of meiosis and spermatid maturation. • Secrete paracrine regulators: • IGF-1. • Inhibin. • FSH necessary in the later stages of spermatid maturation.

25. 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.

26. Erection, Emission, and Ejaculation • Erection: • Increased vasodilation of arterioles. • NO is the NT. • Blood flow into the erectile tissues of the penis. • Emission: • Movement of semen into the urethra. • Ejaculation: • Forcible expulsion of semen from the urethra out of the penis.

27. 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.

28. Female Reproductive System • Fallopian (uterine) tubes: • Ova drawn into the tube by cilia. • Uterus: • Womb. • Endometrium shed during menstruation. • Vagina: • Cervical mucus plug.

29. 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.

30. Ovarian Cycle • Primary oocytes contained in primary follicles. • FSH stimulates cell growth. • Develop into secondary follicles. • Fusion of its follicles into the antrum. • Mature graafian follicle: • 1st meiotic division completed (secondary oocyte).

31. Ovarian Cycle • Secondary oocyte: • Under FSH stimulation: • Theca cells secrete testosterone. Granulosa cells: contain the enzyme aromatase to convert testosterone into estrogen.

32. Ovulation • Graafian follicle forms bulge on surface of ovary. • Extrudes secondary oocyte into the uterine tube. • Empty follicle becomes corpus luteum and secretes: • Progesterone. • Estrogen. • If not fertilized becomes corpus albicans.

33. Menstrual Cycle • 3 phases: • Follicular Phase • Ovulation • Luteal Phase • Duration approximately 28 days. • Day 1 is the first day of menstruation.

34. Follicular Phase • FSH: • Stimulates production of FSH receptors on granulosa cells. • Follicles grow and become secondary follicle. • Granulosa cells secrete estradiol. • FSH stimulates the production of FSH receptors on the granulosa cells. • Increases sensitivity of FSH receptors. • FSH and estradiol stimulate production of LH receptors in graafian follicle.

35. Follicular Phase • Rapid rise in estradiol: • 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.

36. Follicular Phase • Positive feedback: • LH surge begins 24 hours before ovulation. • Triggers ovulation. • FSH increase stimulates development of new follicles.

37. Ovulation • Under FSH, graafian follicle grows large and thin-walled. • Triggers LH surge. • Wall of graafian follicle ruptures. • Day 14. • Ist meiotic division is completed.

38. Luteal Phase • LH stimulates formation of the empty follicle into corpus luteum. • Corpus luteum secretes: • Progesterone: • Plasma concentration rapidly rises. • Estradiol. • Negative feedback on LH and FSH. • Inhibin: suppress FSH.

39. Luteal Phase • Corpus luteum regresses unless fertilization occurs: • Estradiol decreases. • Progesterone decreases. • Withdrawal of estradiol and progesterone cause menstruation to occur.

42. Endometrial Changes • 3 phases of endometrium changes: • Proliferative phase. • Secretory phase. • Menstrual phase.

43. Proliferative Phase • Ovary is in follicular phase. • Estradiol stimulate growth of endometrium. • Spiral arteries develop. • Stimulate production of receptor proteins for progesterone. • Cornification of vaginal epithelium occurs.

44. Secretory Phase • Ovary is in luteal phase. • Progesterone stimulates development of uterine glands, which become engorged with glycogen. • Endometrium becomes thick, vascular and spongy. • Cervical mucus thickens and becomes sticky.

45. Menstrual Phase • Progesterone cause constriction of spiral arteries. • Necrosis and sloughing of endometrium occurs. • Lasts 1-5 days.

47. 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.

48. 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.

49. Fertilization • As fertilization occurs, secondary oocyte completes 2nd meiotic division. • Sperm enters ovum cytoplasm. • Ovum nuclear membrane disappears, zygote formed.

50. Blastocyct Formation • Cleavage: • 30-36 hrs. after fertilization the zygote divides by mitosis. • Blastocyst develops: • Inner cell mass: fetus. • Surrounding chorion: trophoblasts form placenta.