Chapter 28: The Reproductive System

1. Chapter 28: The Reproductive System BIO 211 Lecture Instructor: Dr. Gollwitzer

2. Today in class we will: • Compare the male and female reproductive systems • Discuss cell division • Compare and contrast mitosis and meiosis • Discuss gametogenesis (spermatogenesis) in the male • Spermiogenesis • Spermiation • Capacitation • Anatomy of a spermatozoon • Structures involved in spermiogenesis and their roles • Describe the composition of seminal fluid • Identify the glands whose secretions contribute to the production of seminal fluid • Discuss male reproductive endocrinology • Endocrine structures and hormones that regulate male reproductive function

3. Reproductive System • Only organ system not essential to life • Ensures continued existence of human species • Produces, stores, nourishes, and transports male and female reproductive cells (gametes) • Produces reproductive hormones

4. Male and Female Reproductive Systems • Functionally very different • Female produces 1 gamete/month • Retains and nurtures zygote • Male produces large quantities of gametes • 500M/day! • Begins at puberty and continues past age 70

5. Male • Testes (male gonads) • Produce male gametes (spermatozoa, sperm) • Produce hormones • Male sex hormones (androgens, primarily testosterone) • Inhibin • Emission • Movement of mature spermatozoa move through male duct system, are mixed with secretions of accessory glands • Semen • Sperm mixed with accessory gland secretions

6. Female • Ovaries (female gonads) • Release 1 immature gamete (oocyte) each month • Produce hormones • Female sex hormones (estrogens, progestins) • Inhibin • Uterine tube carries oocyte to uterus • If sperm reaches oocyte in uterine tube: • Fertilization is initiated • Oocyte matures into ovum

7. Reproduction • During sexual intercourse, ejaculation introduces semen into vagina • Spermatozoa ascend female reproductive tract • Seek out oocyte (generates heat, attracts sperm like heat-seeking missile) • If fertilization occurs in uterine tube: • sperm + ovum  zygote • Zygote travels to uterus • Uterus encloses/supports developing embryo • Embryo grows into fetus and prepares for birth

8. Gametogenesis • Involves mitosis and meiosis • Mitosis • Process of somatic cell division • Produces 2 diploid daughter cells • Have same number of (paired) chromosomes as parent cell, i.e., 46 (23 x 2) • Meiosis = reduction division • Special cell division involved in gamete production • Produces 2 haploid daughter cells • Have one-half (unpaired) the number of chromosomes in the parent cell, i.e., 23

9. Chromosomes in Mitosis and Meiosis Figure 28–6

10. Gametogenesis • Meiosis • Involves two cycles of cell division • Chromosomes (each with two chromatids) pair up = tetrad • During first division, tetrads split • During second division, chromatids split • Produces gametes with one-half the number of chromosomes, i.e., 23 • Fusion of male gamete (sperm) and female gamete (oocyte) produces cell with correct number of chromosomes (diploid), i.e., 46 (23 from each parent)

11. Gametogenesis

12. Spermatogenesis • Occurs in seminiferous tubules (ST) in testes • 3 integrated processes • Mitosis • Meiosis • Spermiogenesis

13. Spermatogenesis • Mitosis • Spermatogonium (stem cell)  spermatogonium + primary spermatocyte • Primary spermatocyte pushed toward lumen of ST • On-going throughout lifetime • Meiosis • Primary spermatocyte  first division  secondary spermatocytes  second division  spermatids = undifferentiated male gametes • Each primary spermatocyte  4 spermatids

14. Spermatogenesis • Spermiogenesis • Last stage of spermatogenesis • Begins with spermatids • Small, relatively unspecialized cells • Physical maturation of spermatids • Involves major structural changes • Differentiate into mature spermatozoa • Highly specialized cells

15. Figure 28-7, 7th edition

16. Spermiation • When spermatozoa: • Detach from Sertoli cells • Enter lumen of ST • From spermatogonium to spermiation: • 9 weeks

17. Seminiferous Tubules Figure 28–5a

18. Seminiferous Tubules Figure 28–5d

19. Spermiogenesis and Spermatozoon Structure Figure 28–8

20. Anatomy of a Spermatozoon • Head – nucleus with chromosomes (DNA) • Acrosomal cap – contains enzymes to dissolve oocyte wall • Middle piece – contains mitochondria for energy to move tail • Tail – flagellum (only 1 in human body); provides motility • Loses all other organelles to make light weight • No energy reserves – must use nutrients from surrounding fluid (primarily fructose)

21. Interstitial (Leydig) Cells • Large cells in interstitial spaces between ST • Stimulated by LH  androgens (testosterone, T) • Testosterone • Stimulates spermatogenesis and spermatozoa maturation • Affects CNS, including libido (sexual drive) • Stimulates metabolism, especially protein synthesis, muscle growth • Establishes/maintains secondary sex characteristics, e.g., facial hair • Maintains male accessory glands and organs

22. Seminiferous Tubules Figure 28–5b,c

23. Sustentacular (Sertoli) Cells • “Nurse cells” • Extend between other cells from ST capsule to lumen • Surround developing spermatocytes and spermatids in ST • 6 major functions • Maintain blood-testis barrier • Support mitosis and meiosis • Support spermiogenesis • Produce inhibin • Produce androgen-binding protein (ABP) • Secrete Mullerian-inhibiting factor (MIF)

24. Sustentacular (Sertoli) Cells • Maintain blood-testis barrier • Cells joined by tight junctions • Isolates STs • Support mitosis and meiosis • Cells stimulated by FSH (and presence of T) • Promote spermatogenesis • Support spermiogenesis • Provide nutrients for development • Phagocytize cytoplasm shed by spermatids

25. Sustentacular (Sertoli) Cells • Produce inhibin • Stimulated by factors released by developing spermatozoa • Provides feedback control of spermatogenesis • Inhibits (decreases) production of FSH by AP • Produce androgen-bind protein (ABP) • Stimulated by FSH • Binds T in ST fluid, elevates levels • Produce Mullerian-inhibiting factor (MIF) • Causes regression of fetal ducts that form uterine tubes and uterus

26. Epididymis • Spermatozoa in ST functionally immature • Incapable of fertilization or locomotion • Become functionally mature in epididymis (but, not motile) • Fluid currents (from cilia lining efferent ductules) transport immobile gametes into epididymis • Functions • Monitors and adjusts composition of ST fluid • Recycles damaged spermatozoa • Stores/protects spermatozoa and facilitates functional maturation • Transit time = two weeks

27. Figure 28-9a, b

28. NOTE: • To become motile, spermatozoa must undergo capacitation • Become motile when mixed with seminal vesicle fluid • Capable of successful fertilization when exposed to female reproductive tract

29. Ductus (Vas) Deferens • Transport spermatozoa from epididymis to urethra • Store spermatozoa (several months) • In state of suspended animation • Low metabolic rates

30. Figure 28-10a

31. Seminal Fluid • A mixture of secretions from several glands including: • Seminal vesicles (60%) • Prostate gland (20-30%) • Bulbourethral glands (10-20%)

32. Seminal Vesicles • Secretions contain: • High concentrations of fructose (easily metabolized by spermatozoa) • Prostaglandins – stimulate smooth muscle contractions in male and female reproductive tracts • Fibrinogen – forms temporary clot in vagina after ejaculation (seminal plug) • Secretions make functional spermatozoa motile (flagella begins beating) • Secretions discharged into ejaculatory duct at emission (due to contractions in ductus deferens, SVs, and prostate gland)

33. Prostate Gland • Produces prostatic fluid • Contains seminalplasmin = antibiotic that may help prevent urinary tract infections • Ejected into prostatic urethra

34. Bulbourethral (Cowper’s) Glands • Mucous glands • Secretions • Help neutralize urinary acids remaining in urethra • Lubricate glans (tip of) penis

35. Semen • Ejaculate = 2-5 mL of semen • Contains • Spermatozoa • Sperm count = 20 – 100 million/mL semen (ideally > 60 million/ejaculate) • Seminal fluid = mixture of glandular secretions from: • SV (60%) • Prostate (30%) • Bulbourethral glands (5%) • Sustentacular cells and epididymis (5%) • Enzymes • Protease – helps dissolve vaginal mucous secretions • Seminalplasmin (from prostate) • Prostatic enzyme - converts fibrinogen to fibrin after ejaculation • Fibrinolysin – liquefies clotted semen

36. Male Reproductive Endocrinology • Hypothalamus  GnRH  ant pit • Ant pit  • LH (ICSH)  interstitial (Leydig) Cells  testosterone (T) • FSH • Testosterone + FSH  sustentacular cells  • Synthesis of ABP • Stimulation of spermatogenesis and spermiogenesis • Factors released by developing spermatozoa  sustentacular cells  inhibin  inhibits (decreases) FSH production by AP

37. Physiological Effects of Testosterone • Stimulates spermatogenesis (with FSH) • Maintains male accessory glands and organs • Establishes/maintains secondary sex characteristics • Stimulates anabolic metabolism, especially bone and muscle growth, RBC formation • On CNS, including libido (sexual drive)

38. Male Reproductive Endocrinology • In males, GnRH pulse frequency relatively steady  narrow range of plasma FSH, LH, T • T secretion accelerates at puberty  • Sexual maturation • Appearance of secondary sex characteristics • Negative feedback controls T production • Inc T  inhibits release of GnRH  dec LH  dec T

39. Figure 28-12

40. Today in class we will discuss: • The female reproductive system • Gametogenesis (oogenesis) in the female • Compare oogenesis to spermatogenesis • The events and structural changes associated with the ovarian cycle • The structure and histology of the uterus • The events and structural changes associated with the uterine (menstrual) cycle • Discuss female reproductive endocrinology • Endocrine structures and hormones that regulate female reproductive function • Aging and the reproductive system of males and females

41. Female Reproductive System • Produces gametes and reproductive hormones • Protects and supports developing embryo • Nourishes newborn infant

42. Oogenesis • = Ovum production • Begins before birth, accelerates at puberty, ends at menopause • Occurs monthly between puberty and menopause

43. Gametogenesis

44. Oogenesis vs. Spermatogenesis • Before birth • Mitotic divisions complete • Oogonia (stem cells)  primary oocytes (vs. ongoing throughout lifetime in males) • Primary oocytes begin meiosis I • Cytoplasm of oocyte unevenly distributed during 2 meiotic divisions; produces: • One functional ovum (with most of original cytoplasm) • 2-3 polar bodies = nonfunctional cells that later disintegrate (vs. primary spermatocyte  4 spermatozoa) • Ovary releases secondary oocyte instead of mature ovum • Meiosis not completed unless/until fertilization

45. Figure 28-15, 7th edition

46. Primary Oocytes • Are daughter cells of oogonia (stem cells) • Located in ovarian cortex in clusters (egg nests) • Surrounded by follicle cells = primordial follicle • 2 M primary oocytes at birth • Meiosis I (primary oocyte  secondary oocyte) • Begins during fetal development • Stops early in meiosis I (suspended development) • Doesn’t continue until after puberty (numbers reduced to 400,000 due to atresia)

47. Ovarian Cycle • Begins after puberty when groups of primordial follicles develop into primary follicles each month • Process begins due to increased FSH at puberty • Involves 2 phases • Follicular (preovulatory) phase • Luteal (postovulatory) phase

48. Ovarian Cycle Figure 28-16

49. Ovarian Cycle: Follicular Phase • Formation of primary follicles • Every month AP  FSH  some primordial follicles (with primary oocyte)  primary follicles (with primary oocyte) • Zona pellucida = glycoprotein layer around primary oocyte • Granulosa cells = rounded, larger follicle cells outside zona pellucida • Thecal cells = layer of follicle cells adjacent to ovarian stroma • Granulosa + thecal cells work together  estrogen (continues in all follicles)

50. Ovarian Cycle: Follicular Phase • Formation of secondary follicles • Each month, only a few (1-3) primary follicles become secondary follicles • Primary oocyte increases in size • Follicle wall thickens • Granulosa cells secrete follicular fluid • Follicle enlarges rapidly as follicular fluid accumulates