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reproductive system

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reproductive system

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  1. Chapter 20 Reproduction

  2. 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

  3. Sexual Reproduction • Each individual produces gametes • Formed in gonads by meiosis • Male: testes produce: • Sperm • Testosterone • Female: ovaries produce: • Ova • Estrogens, Progesterone • Gametes unite in process of fertilization • Restores diploid number • Forms zygote

  4. Sexual Determination • Each zygote inherits • 23 chromosomes from mother • 23 chromosomes from father. • 23 pairs of homologous chromosomes. • alleles • 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.

  5. Formation of Testes • First 40 days after conception the gonads of males and females are similar in appearance. • During this time: • Spermatogonia and oogonia migrate from yolk sac to developing embryonic gonads • Gonads could become either. • TDF (testis-determining factor): • hypothetical • promotes the conversion to testes: • gene located on short arm of Y, called SRY (sex determining region of Y chromosome) • Found in all mammals

  6. Formation of Testes • Structures in the testes: • Seminiferous tubules: 43 to 50 days post conception • Germinal cells: sperm. • Nongerminal cells: Sertoli cells (sustentacular cells). • Leydig cells (interstitial cells): • Appear about day 65. • Endocrine function: secrete androgens • Main: Testosterone

  7. 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. • Decline occurs by end of second trimester • Testes descend into scrotum shortly before birth. • Temp about 3 degrees below internal temp • 35 degrees C

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

  9. 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.(mesonephric) • Sertoli cells secrete MIF(mullerian inhibition factor). • Female accessory organs derived from mullerian ducts. (paramesonephritic)

  10. Development of Accessory Sex Organs and Genitalia • Both duct systems in both sexes between days 25 and 50 • Regression of mullarian ducts begins about day 60 • 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 (DHT) in some target cells • Needed for penis, spongy urethra, scrotum, prostrate • Testosterone directly needed for wolfian derivatives: • Epididymis, ductus deferens, ejaculatory duct, SV

  11. Development of Accessory Sex Organs and Genitalia

  12. 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

  13. Endocrine Regulation of Reproduction • Hypothalamus releases LHRH (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.

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

  15. 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 • Female: estrogen and progesterone. • Male: testosterone.

  16. 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

  17. Onset of Puberty • 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.

  18. 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.

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

  20. 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

  21. 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.

  22. 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. • Brain is a target organ for testosterone • Converted to derivatives

  23. 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.

  24. Testosterone Secretion • Negative feedback of testosterone and inhibin • Keep relatively constant levels of gonadotropins • Results in relatively constant levels • Different in female • At menopause: no more sex steroids • In males, gradual decrease

  25. Endocrine function: testes • Testosterone: main androgen • Sertoli and Leydig cells secrete small amounts of estradiol. • Have receptors for estradiol (as do other male structures) • May be needed for spermatogenesis • Estradiol may be responsible for: • Negative feedback in brain. • Sealing of epiphyseal plates. • Regulatory function in fertility.

  26. Spermatogenesis • Spermatogonia: • Replicate initially by mitosis. • Produce two cells • One becomes a primary spermatocytes undergoes meiosis: • 2 nuclear divisions. • 2nd meiotic division produce 4 spermatids.

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

  28. 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 (androgen-binding protein): • Binds to testosterone and concentrates testosterone in the tubules.

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

  30. 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.

  31. 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

  32. 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.

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

  34. 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.

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

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

  37. 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.

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

  39. 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.

  40. 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.

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

  42. 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.

  43. 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.

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

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