Reproductive System

1. Reproductive System • Primary sex organs (gonads) • testes in males • ovaries in females • Gonads produce sex cells called gametes and secrete sex hormones • Accessory reproductive organs – ducts, glands, and external genitalia • Sex hormones • Androgens (males), • Estrogens and progesterone (females)

2. Male Reproductive System • The male gonads (testes) produce sperm and lie within the scrotum • Sperms are delivered to the exterior through a system of ducts: epididymis, ductus deferens, ejaculatory duct, and the urethra • Accessory sex glands: • Empty their secretions into the ducts during ejaculation • Include the seminal vesicles, prostate gland, and bulbourethral glands

3. The Scrotum and associated structures • Sac of skin and superficial fascia that hangs outside the abdominopelvic cavity at the root of the penis • Contains paired testicles separated by a midline septum • Its external positioning keeps the testes 3C lower than core body temperature (needed for sperm production) • Each testis is surrounded by two tunics: • The tunica vaginalis, derived from peritoneum • The tunica albuginea, the fibrous capsule of the testis

4. Testicular Thermoregulation • Sperms are not produced at core body temperature • In the scrotum, the testes are kept 2-3°C cooler than in the pelvic cavity. This is essential for sperm production. • Cooling mechanisms • The cremaster muscle contains strips of the internal abdominal oblique muscle around the spermatic cord. • It can elevate or lower the testes. • The dartos muscle is a subcutaneous layer of smooth muscle that wrinkles skin reducing surface area of scrotum. Can lifts testis upwards • The pampiniform plexus is an extensive network of veins that surround the testicular artery in the spermatic cord, keeping the testes cooler countercurrent heat exchange that cools arterial blood entering testis

5. The Testes • Septa divide the testis into 250-300 lobules, each containing 1-4 seminiferous tubules • Seminiferous tubules: • Produce the sperm • Converge to form the tubulus rectus • The straight tubulus rectus conveys sperm to the rete testis

6. Cells in the testes • Between the seminiferous tubules are clusters of interstitial (Leydig) cells, the source of testosterone. • A seminiferous tubule • lumen lined with germinal epithelium consisting of several layers of germ cells in the process of becoming sperm • Sustentacular (Sertoli) cells.

7. The functions of Sertoli cells and BTB • Sustentacular cells protect the germ cells and promote their development. Tight junctions between adjacent sustentacular cells form a blood-testis barrier (BTB) • The fluid inside the tubules contains high levels of androgens, potassium and amino acids • The BTB prevents the immune system from attacking the developing spermatozoa (contain specific Ag that are not found on any other cell) • Sertoli cell supply nutrients to the developing sperms • Phagocytize cytoplasm shed by spermatids • Secrete inhibin (we will talk about this hormone later) • Secrete androgen-binding-protein (ABP) – binds testosterone inside tubules to maintain high levels

9. Spermatic Ducts from rete testis to urethra • 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/vas 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 • Spermatic cord – encloses PNS and SNS nerve fibers, blood vessels, and lymphatics that supply the testes

10. Epididymis • The epididymis functions: • The duct of the epididymis has stereocilia that: • Absorb testicular fluid • Pass nutrients to the sperm • Recycling center for damaged spermatozoa • Storage place for spermatozoa • Facilitate functional maturation preparing them for capacitaion. • The spermatozoa mature in the epididymis but are not motile yet

11. Ductus Deferens and Ejaculatory Duct • Runs from the epididymis through the inguinal canal into the pelvic cavity • Its terminus expands to form the ampulla and then joins the duct of the seminal vesicle to form the ejaculatory duct • Propels sperm from the epididymis to the urethra • Vasectomy – cutting and ligating the ductus deferens, which is a nearly 100% effective form of birth control

12. Urethra • Conveys both urine and semen • Consists of three regions • Prostatic – portion surrounded by the prostate • Membranous – lies in the urogenital diaphragm • Spongy, or penile – runs through the penis and opens to the outside at the external urethral orifice

13. The Penis • A copulatory organ designed to deliver sperm into the female reproductive tract • Consists of an attached root and a free shaft that ends in the glans penis • Foreskin – cuff of skin covering the distal end of the penis • Erectile tissue – spongy network of connective tissue and smooth muscle riddled with vascular spaces • Corpus spongiosum – surrounds the urethra and expands to form the glans and bulb of the penis • Corpora cavernosa – paired dorsal erectile bodies bound by fibrous tunica albuginea • Erection – during sexual excitement, the erectile tissue fills with blood causing the penis to enlarge and become rigid

14. Accessory Glands: Seminal Vesicles • Lie on the posterior wall of the bladder and secrete 60% of the volume of semen • Join the ductus deferens to form the ejaculatory duct • The composition of the gland secretions include: • High concentration of fructose that is easily metabolized by spermatozoa • Prostaglandins that stimulate smooth muscle contractions in both male and female reproductive tract • Fibrinogen that forms temporary clot within the vagina after ejaculation • When the inactive sperms mix with the seminal fluid they undergo the first step of activation and the flagella start moving (in some book this is done by the help of the prostate) • The secretion of the seminal gland are discharged into the ejaculatory duct under the control of the sympathetic system

15. Accessory Glands: Prostate Gland • Doughnut-shaped gland that encircles part of the urethra inferior to the bladder • Produces 20-30% of the semen volume • Contains citrate (nutrient source) and enzymes • Fluid enters the prostatic urethra during ejaculation

16. Accessory Glands: Bulbourethral Glands (Cowper’s Glands) • Pea-sized glands inferior to the prostate • Produce thick, clear mucus prior to ejaculation that neutralizes traces of acidic urine in the urethra

17. semen • The combination of sperm and the fluids secreted by the accessory glands. • The ejaculate (fluid produced during ejaculation) contains 20-100 million spermatozoa/ml • The volume is about 2-5 ml

18. Ejaculation • The propulsion of semen from the male duct system • At ejaculation, sympathetic nerves serving the genital organs cause: • Reproductive ducts and accessory organs to contract and empty their contents • The bladder sphincter muscle to constrict, preventing the expulsion of urine • Propulsion of semen from the urethra (peristaltic contractions)

19. Meiosis • The sequence of events that produces sperm in the seminiferous tubules of the testes • Each cell has two sets of chromosomes (one maternal, one paternal) and is said to be diploid (2n chromosomal number) • Humans have 23 pairs of homologous chromosomes • Gametes only have 23 chromosomes and are said to be haploid (n chromosomal number) • Gamete formation is by meiosis, in which the number of chromosomes is halved (from 2n to n)

20. Chromosomes in Mitosis and Meiosis

21. Spermatogenesis • Cells making up the walls of seminiferous tubules are in various stages of cell division • These spermatogenic cells give rise to sperm in a series of events • Mitosis of spermatogonia, forming spermatocytes • Meiosis forms spermatids from spermatocytes • Spermiogenesis – spermatids form sperm

22. Mitosis of Spermatogonia • Spermatogonia – outermost cells in contact with the epithelial basal lamina • Spermatogenesis begins at puberty as each mitotic division of spermatogonia results in type A or type B daughter cells • Type A cells remain at the basement membrane and maintain the germ line • Type B cells move toward the lumen and become primary spermatocytes

23. Spermatocytes to Spermatids • Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes • Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids • Spermatids are small round cells seen close to the lumen of the tubule

24. Spermiogenesis: Spermatids to Sperm • Late in spermatogenesis, spermatids are haploid but nonmotile • Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming sperm

25. spermiogenesis spermatogenesis http://distance.stcc.edu/AandP/AP/AP2pages/reprod/spermato.htm

26. Spermiogenesis: Spermatids to Sperm • Sperm have three major regions • Head – contains DNA and has a helmetlike acrosome containing hydrolytic enzymes that allow the sperm to penetrate and enter the egg • Midpiece – contains mitochondria spiraled around the tail filaments • Tail – a typical flagellum produced by a centriole

27. Brain-Testicular Axis • Hormonal regulation of spermatogenesis and testicular androgen production involving the hypothalamus, anterior pituitary gland, and the testes • Testicular regulation involves three sets of hormones: • GnRH, which indirectly stimulates the testes through: • Follicle stimulating hormone (FSH) • Luteinizing hormone (LH) • Gonadotropins, which directly stimulate the testes • Testicular hormones, which exert negative feedback controls

28. Hormonal Regulation of Testicular Function • The hypothalamus releases gonadotropin-releasing hormone (GnRH) • GnRH stimulates the anterior pituitary to secrete FSH and LH • FSH causes sustentacular cells to release androgen-binding protein (ABP) • LH stimulates interstitial cells to release testosterone • ABP binding of testosterone enhances spermatogenesis

29. Testosterone • The principal androgen (male sex hormone) is testosterone. • This steroid is manufactured by the interstitial (Leydig) cells of the testes. • Secretion of testosterone increases sharply at puberty and is responsible for the development of the secondary sexual characteristics of men. • Testosterone is also essential for the production of sperm.

30. Male Secondary Sex Characteristics • Male hormones make their appearance at puberty and induce changes in nonreproductive organs, including • Appearance of pubic, axillary, and facial hair • Enhanced growth of the chest and deepening of the voice • Skin thickens and becomes oily • Bones grow and increase in density • Skeletal muscles increase in size and mass • Testosterone is the basis of libido in both males and females