Why are there two human sexes?

1. Why are there two human sexes? • Reproduction • one property of a living thing • great variety of methods • Sexual reproduction • each offspring has 2 parents and receives genetic material from both • provides genetic diversity • foundation for survival and evolution of species

2. Two Sexes • Male and female gametes (sex cells) combine their genes to form a fertilized egg (zygote) • one gamete has motility (sperm) • parent producing sperm considered male • has Y chromosome in most mammals • other gamete (egg or ovum) contains nutrients for developing zygote • parent producing eggs considered female • in mammals the female also provides shelter for the developing fetus (uterus and placenta)

3. Overview of Reproductive System • Primary sex organs • produce gametes (testes or ovaries) • Secondary sex organs • male - ducts, glands, penis deliver sperm cells • female - uterine tubes, uterus and vagina receive sperm and nourish developing fetus • Secondary sex characteristics • develop at puberty to attract a mate • pubic, axillary and facial hair, scent glands, body morphology and low-pitched voice in males

4. Role of Sex Chromosomes • Our cells contain 23 pairs of chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes (XY males: XX females) • males produce 50% Y carrying sperm and 50% X carrying • all eggs carry the X chromosome • Sex of child determined by type of sperm that fertilizes mother’s egg

5. Hormones and Sex Differentiation • Gonads begin to develop at 6 weeks • The male and female reproductive systems have different embryological origins • mesonephric ducts develop into male reproductive system • paramesonephric ducts (müllerian ducts) develop into female reproductive tract

6. Hormones and Sex Differentiation • SRY gene (Sex-determining Region of Y gene) • in males, codes for a protein that causes development of testes • testes secrete testosterone • testes secrete müllerian-inhibiting factor which degenerates paramesonephric ducts • In absence of hormones, fetus becomes phenotypically ‘female’

7. Development of External Genitalia • External genitals of both sexes begin as a • genital tubercle • becomes glans of penis or • clitoris • pair of urogenital folds • enclose urethra of male or • form labia minora • a pair of labioscrotal folds • scrotum or • labia majora

8. Development of External Genitalia • All 8 week old fetuses have same 3 structures • by end of week 9, begin to show sexual differentiation • distinctly male or female by end of week 12

9. Descent of Testes • Begin development near kidney • gubernaculum (cordlike structure containing muscle) extends from gonad to abdominopelvic floor • it shortens, guides testes to scrotum • Descent begins in weeks 6-10, finished by 28 • 3% born with undescended testes (cryptorchidism) • Location outside pelvic cavity essential for low temperatures needed for sperm production

10. Boundaries of Male Perineum

11. Medical terms • Impotence – Inability to sustain an erection sufficient for sexual intercourse, or inability to ejaculate • Male Sterility – Infertility caused by disorders of the male reproductive system • BPH – Benign Prostatic Hypertrophy • Cryptorchidism – Undescended testicle(s)

12. Scrotum • Pouch holding testes • divided into 2 compartments by median septum • Spermatic cord travels up from scrotum to pass through inguinal canal • contains testicular artery, vein, nerve and lymphatics

13. Male Inguinal and Scrotal Region Circumcision

14. Scrotal thermoregulation • Muscular control • Cremaster muscle • Dartos muscle • Counter-current exchange • Pampiniform plexus

15. Testes • Oval organs, 4 cm long x 2.5 cm in diameter • covered anteriorly by tunica vaginalis • Tunica albuginea • white fibrous capsule on testes • Septa divide testes into compartments containing seminiferous tubules • each tubule lined with a thick germinal epithelium for sperm • interstitial cells between tubules - testosterone • Rete testis • seminiferous tubules drain here

16. Spermatic Ducts • Efferent ductules • 12 small ciliated ducts collecting sperm from rete testes and transporting it to epididymis • Epididymis (head, body and tail) • 6 m long coiled duct adhering to posterior of testis • site of sperm maturation and storage (fertile for 60 days) • Ductus deferens (peristalsis during orgasm) • muscular tube 45 cm long passing up from scrotum through inguinal canal to posterior surface of bladder • Ejaculatory duct • 2 cm duct formed from ductus deferens and seminal vesicle and passing through prostate to empty into urethra Vasectomy

17. Male Urethra • Regions: prostatic, membranous and penile --- totals 20 cm long

18. Accessory Glands • Seminal vesicles - pair • posterior to bladder • empty into ejaculatory duct • Prostate gland - single • below bladder, surrounds urethra and ejaculatory duct • 2 x 4 x 3 cm • Bulbourethral glands - pair • near bulb of penis • empty into penile urethra • lubricating fluid

19. Anatomy of Penis • Internal root, shaft, and glans • external portion 4 in. long when flaccid • The foreskin is termed the prepuce • 3 cylindrical bodies of erectile tissue • corpus spongiosum along ventral side of penis • encloses penile urethra • corpora cavernosa • diverge like arms of a Y • Erection

20. Brain-Testicular Axis • Hypothalamus produces GnRH • Stimulates anterior pituitary (gonadotrope cells) to secrete • LH • stimulates interstitial cells to produce testosterone • FSH • stimulates sustentacular cells (Sertoli cells) to secrete androgen-binding protein (ABP) that interacts with testosterone to stimulate spermatogenesis

21. Reproductive system remains dormant for years after birth • surge of pituitary gonadotropins begins development • 10-12 in most boys; 8-10 in most girls • Puberty • period from onset of gonadotropin secretion until first menstrual period or first ejaculation of viable sperm • Adolescence • ends when person attains full adult height

22. Other Effects of Testosterone • Enlargement of secondary sexual organs • penis, testes, scrotum, ducts, glands • Development of secondary sexual characteristics • hair, scent and sebaceous glands develop • muscle mass, vocal quality • stimulates erythropoiesis and libido • During adulthood, testosterone sustains libido, spermatogenesis and reproductive tract

23. Other Effects of Testosterone • Enlargement of secondary sexual organs • penis, testes, scrotum, ducts, glands • Development of secondary sexual characteristics • hair, scent and sebaceous glands develop • muscle mass, vocal quality • stimulates erythropoiesis and libido • During adulthood, testosterone sustains libido, spermatogenesis and reproductive tract

24. Mitosis and Meiosis • Mitosis produces two genetically identical daughter cells (for tissue repair, embryonic growth) • Meiosis produces gametes • for sexual reproduction • 2 cell divisions (only one replication of DNA) • meiosis I separates homologous chromosome pairs into 2 haploid cells • meiosis II separates duplicated sister chromatids into 4 haploid cells

25. Spermatogenesis • Blood-testis barrier is formed by tight junctions between and basement membrane under sustentacular cells (Sertoli cells) 1 basal lamina, 2 spermatogonia, 3 spermatocyte 1st order, 4 spermatocyte 2nd order, 5 spermatid, 6 mature spermatid, 7 Sertoli cell, 8 tight junction (blood testis barrier)

26. Spermatogenesis • Spermatogonia produce 2 kinds of daughter cells • type A remain outside blood-testis barrier and produce more daughter cells until death • type B differentiate into primary spermatocytes • cells must pass through BTB to move inward toward lumen - new tight junctions form behind these cells • meiosis I  2 secondary spermatocytes • meiosis II  4 spermatids

27. Spermiogenesis (not a typo) • Changes that transform spermatids into spermatozoa • discarding excess cytoplasm and growing tails

28. Spermatozoon • Head is pear-shaped front end • 4 to 5 microns long structure containing the nucleus, acrosome and basal body of the tail flagella • nucleus contains haploid set of chromosomes • acrosome contains enzymes that penetrate the egg • basal body

29. Spermatozoon • Tail is divided into 3 regions • midpiece contains mitochondria around axoneme of the flagella (produce ATP for flagellar movement) • principal piece is axoneme surrounded by fibers • endpiece is very narrow tip of flagella

30. Semen or Seminal Fluid • 2-5 mL of fluid expelled during orgasm • 60% seminal vesicle fluid, 30% prostatic, 10% sperm • normal sperm count 50-120 million/mL • Other components of semen • fructose - energy for sperm motility • fibrinogen causes clotting • enzymes convert fibrinogen to fibrin • fibrinolysin liquefies semen within 30 minutes • prostaglandins stimulate female peristaltic contractions • spermine is a base stabilizing sperm pH at 7.2 to 7.6

31. Male Sexual Response - Anatomy • Arteries of penis • dorsal and deep arteries (branches of internal pudendal) • deep artery supplies lacunae of corpora cavernosa • dilation fills lacunae causing an erection • normal penile blood supply comes from dorsal artery • Nerves of penis • abundance of tactile, pressure and temperature receptors • dorsal nerve of penis and internal pudendal nerves lead to integrating center in sacral spinal cord • both autonomic and somatic motor fibers carry impulses from integrating center to penis

32. Excitement and Plateau • Excitement is characterized by vasocongestion of genitals, myotonia, and increases in heart rate, blood pressure and pulmonary ventilation • Initiated by many different erotic stimuli • Erection of penis is due to parasympathetic triggering of nitric oxide (NO) secretion • dilation of deep arteries and filling of lacunae with blood • Erection is maintained during plateau phase

33. Orgasm and Ejaculation • Climax (orgasm) is 15 second reaction that typically includes the discharge of semen (ejaculation) • Ejaculation has two stages • emission = sympathetic nervous system propels sperm through ducts as glandular secretions are added • expulsion = semen in urethra activates muscular contractions that lead to expulsion

34. Resolution • Sympathetic signals constrict internal pudendal artery and reduce blood flow to penis • penis becomes soft and flaccid (detumescence) • Cardiovascular and respiratory responses return to normal • Refractory period (10 minutes to few hours)