Male Reproductive Physiology

Spermatogenesis - Sperm Boot Camp

• Process: Formation of spermatozoa from spermatogonial stem cells in seminiferous tubules.
• Duration: ~74 days. Temp: 2-3°C < core body temp.
• Key Cells & Hormones:
  • Sertoli cells: Support spermatogenesis (FSH, Testosterone), blood-testis barrier.
  • Leydig cells: Produce Testosterone (LH); crucial for spermatogenesis.
• Phases:
  1. Spermatocytogenesis (Mitosis): Spermatogonia (2n) → Primary spermatocytes (2n).
  2. Meiosis:
     • Meiosis I: Primary spermatocyte (2n) → 2 Secondary spermatocytes (n).
     • Meiosis II: Secondary spermatocyte (n) → 2 Spermatids (n).
  3. Spermiogenesis: Spermatids (n) → Spermatozoa (n) (morphological change).
     • Acrosome/flagellum formation, nuclear condensation, cytoplasm shed. No division.

⭐ One full cycle of spermatogenesis, from spermatogonium to mature spermatozoa, takes approximately 74 days in humans.

Hormonal Regulation - Testosterone Tune-Up

• HPG Axis: Central control.
  • Hypothalamus: Pulsatile Gonadotropin-Releasing Hormone (GnRH).
  • Anterior Pituitary:
    • Luteinizing Hormone (LH) → Leydig cells → Testosterone (T) synthesis.
      • T: Drives spermatogenesis, 2° sex characteristics, libido, anabolic effects.
      • Negative feedback: ↓GnRH pulses (Hypo), ↓LH sensitivity (Ant. Pit.).
    • Follicle-Stimulating Hormone (FSH) → Sertoli cells.
      • Sertoli cells: Produce ABP (concentrates T), Inhibin B (↓FSH), Aromatase ($T \rightarrow E_2$).
• Testosterone (T):
  • Circulation: ~2% free (active), ~65% SHBG-bound, ~33% albumin-bound.
  • Peripheral Conversion:
    • To Dihydrotestosterone (DHT) via 5α-reductase: Potent; for external genitalia, prostate, hair.
    • To Estradiol ($E_2$) via aromatase: Bone maturation, feedback.

⭐ Pulsatile GnRH secretion is crucial; continuous GnRH (e.g., leuprolide) causes receptor downregulation, leading to medical castration.

Semen & Fertilization - The Vital Fluid

Semen: Key Parameters & Sources

• Vol: 2-5 mL; pH: 7.2-7.8 (alkaline)
• Sperm: >16 M/mL; Motility >40% progressive; Morphology >4% normal (WHO 2021)
• Sources:
  • Seminal Vesicles (~65%): Fructose (energy), prostaglandins, coagulants
  • Prostate (~25%): PSA (liquefaction), citrate, zinc, acid phosphatase
  • Bulbourethral Glands (<5%): Alkaline mucus (neutralizes acidity, lubrication)

Fertilization Steps

• Capacitation: Sperm activation in female tract (cholesterol loss, $↑Ca^{2+}$ influx).
• Acrosome Rxn: Enzymes (hyaluronidase, acrosin) released to digest Zona Pellucida (ZP).
• Cortical Rxn: Oocyte releases granules; ZP hardens (prevents polyspermy).
• Syngamy: Fusion of male & female pronuclei → Zygote.

⭐ Capacitation in the female tract is vital for sperm to fertilize; involves cholesterol removal from the sperm membrane and changes in surface glycoproteins, unmasking binding sites.

Male Sexual Act - The Climax Cascade

• Sequence: Erection → Emission → Ejaculation → Resolution.
• Erection (Parasympathetic: S2-S4, Pelvic n.):
  • NO release → ↑cGMP → vasodilation (corpora cavernosa/spongiosum).
  • PDE-5 inhibitors (Sildenafil) maintain erection by ↓cGMP breakdown.
• Emission (Sympathetic: T10-L2, Hypogastric n.):
  • Smooth muscle contraction (vas deferens, seminal vesicles, prostate) → semen to prostatic urethra.
• Ejaculation (Sympathetic & Somatic: Pudendal n.):
  • Rhythmic muscle contractions (bulbospongiosus, ischiocavernosus) → semen expulsion.
  • Bladder neck (internal sphincter) closes (sympathetic) preventing retrograde flow.
• Resolution (Sympathetic): Vasoconstriction → detumescence.

⭐ Emission is the "point of no return"; ejaculation typically follows.

High‑Yield Points - ⚡ Biggest Takeaways

• Spermatogenesis occurs in seminiferous tubules; requires temperature 2-3°C below body.
• Sertoli cells support sperm, form blood-testis barrier, secrete Inhibin B (↓FSH).
• Leydig cells (interstitial), via LH, produce Testosterone.
• Testosterone drives male secondary sexual characteristics, libido, and anabolism.
• FSH → Sertoli cells (spermatogenesis); LH → Leydig cells (testosterone).
• Erection: Parasympathetic (NO); Emission/Ejaculation: Sympathetic.
• Sperm capacitation (female tract) & acrosome reaction vital for fertilization.