Chapter 12—Male Reproductive System

1. Chapter 12—Male Reproductive System

2. Said an ovum one night to a sperm, “You’re a very attractive young germ. Come join me, my sweet, let our nuclei meetAnd in nine months we’ll both come to term.”--Isaac Asimov

3. Ch. 12-- Study Guide Critically read: (A)pages pp. 239-247 right before Sexual Differentiation section; (B) Negative feedback regulators (pp. 254-255) right before prepuberty subsection Comprehend Terminology (the text in bold/italic) Study and understand the text and corresponding figures.

4. 12.1. Morphology of the testes— Leydig cells and seminiferous tubules

5. § Testes (Fig. 12.2, x)-A Dimension-- Oval organ; 3 cm (anterior to posterior) x 2.5 cm (wide) x 4 cm (long) Each testis is surrounded by two tunics (C.T.): (From outermost and moving in) • Its anterior and lateral surfaces are covered by tunica vaginalis • Tunica albuginea– testis itself has a white fibrous capsule

6.  

7. § Testicular Thermoregulation • Sperm cannot be produced at core body temperature (too warm; 37 degrees Celsius): • Pampiniform plexus = near testicular artery, a network of veins– forming countercurrent heat exchangerthatcools arterial blood entering testis by 1-2 degree Celsius (Fig. x, y) • When too cold: muscle contraction to: • Pull testes close to body • Contract and scrotum becomes taut • wrinkles skin reducing surface area of scrotum

10. § Testes (Fig. 12.1, 12.4)-B • Septa divide testes into 250-300wedge-shaped lobules containing seminiferous tubules(where sperm are produced); each tubule lined with a thick germinal epithelium: • Several layers of germ cells for sperm production • Tall Sertoli (sustentacular) cells; function? • Between the seminiferous tubules are interstitial (Leydig) cells, the source of testosterone Fig. 27.10 and X

11. See next slide for details of germ and sustentacular cells

12. Stages of sperm maturation:  Lumen of seminiferous tubule Spermatozoon Tight junctions --see next slide Spermatids Secondary spermatocyte Primary spermatocyte Sertoli cell Spermatogonium

13. 12.1. Morphology of the testes— Spermatogenesis

14. § Spermatogenesis--A • Def. of spermatogenesis– The production of sperm cells through a series of mitotic and meiotic cell divisions • Location? • How long does it take? • Microscopic examination– two important cell types (see next slide)

15. § Spermatogenesis--B • Two important cell types in seminiferous tubules • Germ cells– • In various stages of sperm development, such as spermatogonia, primary spermatocytes, secondary spermatocytes • Sustentacular (Sertoli) cells– these cells provide crucial support for spermatogenesis Figure x

17. § Spermatogenesis--C • Three major stages— • Mitotic proliferation— • Spermatogonia located in the outermost layer of the seminiferous tubule, outside the blood-testis barrier (BTB) • One of the daughter cells (Type A spermatogonium) remain at the outer edge of the tubule; importance? • The other daughter cell (Type B spermatogonium) starts moving toward lumen forming 4 identical primary spermatocytes (2N)

18. § Spermatogenesis--D • Meiosis— • Each primary spermatocyte (2N) must pass through BTB (tight junction) and ultimately yield 4 spermatids (1N) • Spermiogenesis— • Spermatids become extremely specialized and motile spermatozoa • Sperm travel lightly Figure w, x, y, z

19. A B Meiosis C

20. Spermatogenesis Stages: Chromosomes: 46; 2n (diploid number; single strands) One daughter cell remains at the outer edge of the seminiferous tubule to maintain the germ cell line Spermatogonia One daughter cell moves toward the lumen to produce spermatozoa Mitosis 46; 2n (diploid number; single strands) Primary spermatocyte 46; 2n (diploid number; doubled strands) First meiotic division Meiosis Secondary spermatocyte 23; n (haploid number; double strands) Second meiotic division 23; n (haploid number; single strands) Spermatids Spermio- genesis 23; n (haploid number; Single strands) Spermatozoa

21. Figure 27.16

22. 12.1. Morphology of the testes— Male reproductive tract

23. § Sperm’s journey (male reproductive tract) • Testes– sperm-producing organs; inside the scrotum (skin-covered sac) • Routes (spermatic ducts) the sperm travel: A-Testes  B-Efferent ductules  C-Epididymis D-Ductus deferens E-Ejaculatory duct  F-Urethra  Exterior Fig. x

24. Anterior/posterior view? Urinary bladder Ureter 2. Prostate gland 1. Seminal vesicle 3. Bulbourethral gland E D F Penis C B A Glans penis

25. 12.2. control of testicular function— Leydig cells

26. § Leydig cells • Principal role of Leydig cells– synthesize testosterone (& estrogen) in response to LH • In the fetus--initially Leydig cells depend on chorionic gonadotropin; later on LH • After birth– Leydig cells regress and die until puberty • Initial steps of testosterone formation– similar to adrenal cortex; from cholesterol to pregnenolone • Four other enzymes convert pregnenolone (21 C) to testosterone (19 C) (Fig. 12.5)

27. Pregnenolone Testosterone

28. § Germinal epithelium (Sertoli cells) • Principal role of germinal epithelium (Sertoli cells)– produce sperm • Sertoli cells– only cells known to express FSH receptors in human males; also have testosterone receptors • FSH, LH, and testosterone all play vital roles in spermatogenesis. How so for LH and testosterone? Fig. 12.6

29. CREB– cyclic AMP response element binding protein FSH LH Sertoli cell Leydig cell

30. 12.3. Testosterone

31. § Secretion and metabolism of T • Testosterone (T) is the principal androgen secreted by the mature testis. In that 5% is from adrenal cortex. • Aging– no sharp drop in T (unlike estrogen in the postmenopausal woman) • In blood 98% of T binds to binding proteins — albumin, sex hormone-binding globulin (SHBG also called TeBG; having higher affinity for T), and ABP (androgen binding protein) in Sertoli cells • The other 2% of T can diffuse out of the blood • Liver is principal site of degradation of T; T also can convert to other steroids (Fig. 12.7)

33. § Mechanism of action by T • Nuclear receptors-- Testosterone (T) often converts to 5alpha-dihydrotestosterone before binding to their nuclear receptors (Fig. 12.8) • T also may bind to membrane receptors-- either directly or through the bind of the sex hormone binding globulin

35. § Effects of testosterone • Testosterone promotes -- growth, differentiation, and function of accessory organs of reproduction. • Maintenance of normal reproductive function in the adult depends on continued T secretion. • T stimulates transport and delivery of sperm • On sexual characteristics– during early adolescence, T stimulates growth of pubic hair, axillary, and facial hair; lowers the pitch of the voice • T stimulates secretion of the hormone erythropoietin; T increases sexual drive (libido) in both men and women .

36. 12.4. Negative feedback regulators

37. § hypothalamus-pituitary-testis • Hypothalamus GnRH– stimulates secretion of both LH and FSH • Testosterone, which is secreted in response to LH, acts as a feedback regulator of LH. • FSH stimulates the Sertoli cells to synthesize and secrete inhibin (glycoprotein), which regulates FSH secretion. Fig. 12. 17

38. Stimulation Inhibition Stimulation