Male Reproductive System & Spermatogenesis

• The male reproductive system's function is spermatogenesis, the synthesis of hormones, and enabling coitus.

Male Reproductive System Structure

• Testes contain convoluted seminiferous tubules for spermatogenesis.
• Interstitial cells of Leydig secrete testosterone.
• The head of the epididymis is a network of ducts connected to the ends of tubules.
• The body and tail of the epididymis are convoluted canals that lead to the vas deferens, seminal vesicles, and ejaculatory ducts through the prostate gland, then to the urethra.

Spermatogenesis

• Spermatogenesis occurs during active sexual life within the seminiferous tubules among Sertoli cells.
• The blood-testis barrier is formed by Sertoli cells and the basal lamina.
• Spermatogenesis begins with primordial germ cells, which develop into spermatogonium.
• Spermatogonium (diploid) divide via mitosis into primary spermatocytes (diploid).
• Primary spermatocytes undergo meiotic division I to become secondary spermatocytes (haploid).
• Secondary spermatocytes undergo meiotic division II to become spermatids (haploid).
• Spermatids differentiate into mature spermatozoa (haploid).
• Spermatogenesis and maturation take approximately 70 days.
• One spermatogonium yields 512 spermatids.
• Daily sperm production can reach up to 120 million.
• Sperm leaving the testis are nonmotile but they gain motility in the epididymis.
• Sperm are stored in the vas deferens and epididymis for weeks and requires a temperature of 32° C.

Spermatozoon

• The head contains a condensed nucleus and an acrosome with proteolytic enzymes and hyaluronidase.
• The body contains many mitochondria.
• The tail has microtubules which enables flagellar movement at a rate of 1-4 mm/min, fueled by ATP.

Factors Necessary for Spermatogenesis

• Sertoli cells phagocytize spermatid cytoplasm, form inhibin and androgen-binding protein (ABP), nourish developing cells and convert testosterone to estrogen via aromatase.
• Testosterone promotes the growth and division of testicular germ cells.
• LH stimulates Leydig cells to produce testosterone, which is converted to estrogen by aromatase.
• FSH stimulates Sertoli cells, leading to spermatogenesis.
• Estrogens produced by Sertoli cells stimulate spermatogenesis.
• GH stimulates division of spermatogonia and metabolic effects.
• Temperature is necessary for spermatogenesis.

Erection

• Erection is a sign of male arousal.
• Penile arterioles dilate, filling erectile tissue with blood and compressing veins.
• Parasympathetic fibers in the pelvic nerve release acetylcholine, which inhibits noradrenaline vasoconstriction, and VIP, which causes vasodilation.
• Nonadrenergic, noncholinergic fibers release nitric oxide (NO), causing vasodilation.
• Termination of erection: sympathetic vasoconstrictor impulses.

Nitric Oxide and Erection

• Nitric oxide (NO) activates soluble guanylyl cyclase to convert GTP to cGMP.
• cGMP causes relaxation of smooth muscle, leading to vasodilation.
• Phosphodiesterase breaks down cGMP.
• Sildenafil inhibits phosphodiesterase, prolonging vasodilation.

Ejaculation

• Ejaculation is a diphasic spinal reflex and the culmination of male sexual act.
• Emission involves movement of semen into the urethra (prostatic part).
• Touch receptors of glans penis (pudendal nn.) stimulate upper lumbar segments, activating sympathetic nerves (hypogastric nn.) to contract smooth muscle in the vas deferens, prostate, and seminal vesicles, moving sperm.
• Ejaculation (proper) is the propulsion of semen out of the urethra.
• Filling of the urethra (pudendal nn.) stimulates lower lumbar and sacral segments, activating motor nerves (pudendal) that contract the bulbocavernosus muscle.
• Compression of penile erectile tissue moves sperm out of the urethra.

Semen

• Semen volume is approximately 3 ml.
• The sperm count is at least 15 million per milliliter.
• Seminal vesicles contribute 60% of the secretion, containing fructose, citric acid, fibrinogen, and prostaglandins which promote fertilization.
• The prostate gland contributes 30% of the secretion, which is alkaline to neutralize acidity and improve sperm motility, with a pH of 7.5.
• Mucus is secreted from the bulbourethral glands.

Endocrine Function of Testes: Testosterone

• Leydig cells produce testosterone, a C19 steroid derived from cholesterol.
• 17α-hydroxylase is present, while 21- and 11β-hydroxylases are absent.
• LH regulates testosterone production by binding to receptors, activating G proteins, adenylyl cyclase, and increasing cAMP.
• PKA then converts cholesterol esters to cholesterol and then to pregnenolone.
• 98% of testosterone is bound to protein, mainly gonadal steroid-binding globulin (GBG), and less to albumin.
• Testosterone is converted to androsterone in the liver and excreted in urine.
• Testosterone acts by binding to intracellular receptors, which then translocate to the nucleus and transcribe genes.
• In some tissues (except muscles), testosterone is converted to dihydrotestosterone.

Testosterone Functions

• Supports the development of male sexual organs during fetal development.
• Facilitates the descent of testes, stimulated by placental choriogonadotropin.
• Facilitate secondary sex characteristics during puberty, including body hair distribution, baldness, deeper voice, acne, and muscle development.
• Testosterone further helps in spermatogenesis, bone growth, calcium retention, basal metabolism, red blood cell production, electrolyte balance, water balance, sex drive, and aggression.

Regulation of Testicular Function

• FSH (via cAMP) stimulates Sertoli cells, maintaining spermatogenesis (with androgens) and secretion of inhibin.
• LH (via cAMP) stimulates Leydig cells, promoting testosterone secretion.
• Inhibin inhibits FSH secretion.

Plasma Testosterone

• Circulating testosterone and androstenedione are aromatized (80%) in testicular production of estrogens.
• Testosterone is aromatized in Sertoli and Leydig cells.

Abnormalities

• Cryptorchidism is the failure of testes to descend, leading to tumors, affected spermatogenesis, and testes degeneration.
• Hypogonadism is characterized by abnormal development of male sex organs during embryonic life.
• Before puberty, hypogonadism results in eunuchoidism, where there is increases in body height, female body composition, small genitalia, and high-pitched voice.
• In adulthood, there is a slow regression of sex characteristics, decreased libido, passive behavior, and muscle mass loss.