Male Reproduction: Anatomy, Physiology, Disorders

• These notes cover male reproduction.
• They discuss anatomy, physiology, and clinical endocrinology.
• They discuss disorders of male reproduction.

Spermatogenesis

• Involves germ cells (diploid with 46 chromosomes) becoming sperm (haploid with 23 chromosomes).
• As a process it takes approximately 74 ± 5 days.
• A normal adult male produces 3 x 107 sperm per day.
• Spermatogonium undergo repeated cell division, differentiating into spermatocytes or remaining as spermatogonium.
• A retained spermatogonium allows older men to still fertilize.
• Meiosis occurs between early spermatocytes and later spermatids.

Spermiogenesis

• Is the process of remodeling spermatids into spermatozoa.
• The acrosome, or headpiece of spermatozoa, contains an enzymatic "drill" to help penetrate the egg.
• Mitochondria provides energy to its flagella tail.

Male Reproductive Tract

• Spermatozoa travel through the rete testis and epididymis in 12 days.
• Sperm become more concentrated here in the epididymis.
• Ductus (vas) deferens propel sperm via peristalsis to the ampulla and ejaculatory duct.
• The ejaculatory duct receives 60% of its fluid from the seminal vesicles.
• Other contributors to ejaculate are the prostate (20%) and Cowper's glands.

Testes

• Located within the scrotum.
• Its location in the scrotum maintains testes at 2°C below body temperature.
• Testicular arteries and veins preserve temperature through countercurrent heat transfer.
• They have a lobular architecture, containing 250 pyramidal lobules with seminiferous tubules.

Seminiferous Tubules

• Contain the Sertoli cells lining the basement membrane.
• Tight junctions between Sertoli cells form the "blood-testis barrier."
• The blood-testis barrier prevents transfer of protein and ions between the lumen and interstitium.
• Sertoli cells provide nutrients to developing sperm.
• Additionally, they secrete seminiferous tubule fluid containing androgen-binding protein and phagocytize damaged spermatozoa.
• Spermatogonia are found next to the basal lamina.
• As spermatogonia matures into spermatocytes, they move toward the lumen through division without replication.
• Seminiferous tubules empty into the rete testis.
• The rete testis facilitates outflow from the seminiferous tubules and combines it with neighboring tubules.

Testicular Anatomy

• Sperm are delivered in relatively small volumes.
• Cutting the ductus (vas) deferens (vasectomy) causes sterility because sperm can no longer exit the testes.
• Vasectomy does not affect hormonal or sperm production within the testes.

Sertoli Cells

• Form tight junctions creating a "blood-testis barrier."
• They protect developing sperm and provide nutrients.
• They secrete seminiferous tubule fluid which contains androgen binding protein.
• Phagocytize damaged/fragmented sperm (recycled).

Male Sexual Response

• Erection: Parasympathetic impulses dilate penile arteries which compress veins causing filling of erectile sinuses.
• Mucus secreted from Cowper's gland aids lubrication.
• Emission: Sympathetic impulses move semen into the urethra.
• Ejaculation: Reflex contraction of the bulbocavernosus muscle.
• Resolution: Detumescence when penile veins relax, penile artery contracts, and blood flow decreases.

Hormonal Control of Male Reproduction

• Androgens include hormones that produce masculine characteristics.
• Leydig cells are primary androgen-producing cells.
• They are located in the interstitium of the testes.

Androgen Synthesis

• Major androgen that is produced is testosterone.
• Two pathways to testosterone exist: the delta-5 and delta-4 pathways.
• Humans predominantly use the delta-5 pathway.
• Leydig cells use testosterone to produce ~20% of serum estradiol.

Androgen Clearance

• Clearance occurs via Androstenedione or DHT.

Androgen Metabolism

• The liver is the main site of androgen metabolism.
• Reduction of the A4 double bond [T → DHT] makes the resulting hormone more potent.
• Adipose tissue contains aromatase enzymes which can alter the testosterone to estradiol ratio.

Relative Potency and Concentrations

• Testosterone has a potency rating of 100 and a plasma concentration between 250-1000.
• Dihydrotestosterone has a potency rating of 200-300 but a much lower plasma concentration between 10-45.
• Androstenedione has a potency rating of 15 and a plasma concentration between 40-110.
• DHEA has a potency rating of 8 and a plasma concentration between 200-1000.

Transport of Androgens

• The androgen transport protein, sex steroid binding globulin (SSBG), binds to about 60% of circulating androgens.
• Albumin binds to approximately 38% of circulating androgens, with about 2% circulating freely.

Actions of Androgens

• Reproductive tract development (in utero & at puberty).
• Development of secondary sex characteristics – laryngeal growth, muscular development, and hair growth.
• They help with spermatogenesis and sexual libido.
• They contribute to anabolic effects such as pubertal growth spurt.
• As well as sebaceous gland secretions.

Control of Reproductive Function

• Controlled via the hypothalamic-pituitary-testicular axis.
• GnRH releases in a pulsatile manner from the hypothalamus which becomes more frequent during puberty.
• LH stimulates Leydig cells cells to produce testosterone, has rapid clearance.
• FSH stimulates gametogenesis (Sertoli cells) and has slower clearance.

Male Reproductive Issues

• Testosterone inhibits GnRH release at the hypothalamus.
• Estradiol also inhibits LH release at the pituitary level.
• Sertoli cells that are releasing inhibin, inhibits FSH release without affecting LH.

Reproductive Hypofunction

• Hypogonadism either comes as Primary i.e., a failure at the gonadal level or Secondary i.e., a failure at pituitary or HT level

Reproductive Dysfunction

• The causes for infertility = primary deficits in sperm production
• A normal sperm count should be ~70x106/mL x 2-5mL
• A clinically infertile sperm count would be below ~20x106/mL

Concise notes on the male reproductive system, covering spermatogenesis and spermiogenesis. It details the anatomy, physiology, and clinical endocrinology involved in male reproduction, including potential disorders. Focus is given to the production and journey of sperm cells.