Male Reproductive System

Overall goals
- Produce functional gametes (sperm) – spermatogenesis.
- Effectively deliver sperm to the female reproductive tract for possible fertilisation.
Anatomical structures grouped by role
- Testes – site of production of both sperm and testosterone.
- Tubing (duct system) – delivery
- Epididymis → Vas (ductus) deferens → Ejaculatory duct → Urethra.
- Accessory glands – semen formation & optimisation
- Seminal vesicles.
- Prostate gland.
- Bulbourethral (Cowper’s) glands.

Testes
- Paired gonads housed externally in the scrotum.
- Dual endocrine-exocrine function:
- Exocrine – sperm production.
- Endocrine – testosterone secretion.
Scrotum
- Thin skin + superficial fascia → housing testes outside the core body.
- Temperature regulation
- Optimal spermatogenesis occurs at ≈ 2!\text{–}!3^{\circ}\text{C} below core body temperature.
- Two specialised muscles adjust position/size:
  - Cremaster muscle (skeletal) – elevates/lowers testes.
  - Dartos muscle (smooth) – wrinkles skin, altering surface area for heat loss.

Cell types (from basal lamina → lumen of seminiferous tubule)
- Spermatogonia (stem cells: 2n) – mitotic pool.
- Primary spermatocytes → secondary spermatocytes – meiosis I.
- Spermatids – post-meiosis II haploid (1n).
- Spermatozoa – mature, flagellated cells released into lumen.
Supporting/Sustaining cells
- Sertoli (sustentacular) cells
- Form blood-testis barrier – isolates germ cells from immune attack.
- Secrete androgen-binding protein (ABP), inhibin, growth factors.
- Phagocytose residual cytoplasm of spermatids.
Tubular pathway inside testis
- Seminiferous tubules → Straight tubules (tubuli recti) → Rete testis → Efferent ductules → Epididymis.

Mitosis of spermatogonia on basal lamina maintains stem-cell line + produces primary spermatocytes.
Meiosis I & II cut chromosome number in half ⇒ four haploid spermatids per primary spermatocyte.
Spermiogenesis (morphological maturation) within seminiferous tubule:
- Condensation of nucleus, formation of acrosome.
- Development of flagellum + mitochondrial mid-piece.
Spermiation – release of non-motile spermatozoa into tubule lumen.
Epididymal phase (≈ 2 weeks)
- Sperm acquire motility & capacity to recognise/penetrate ovum.
- Long-term storage in tail of epididymis until ejaculation.

Volume per ejaculation ≈ 2!\text{–}!5\,\text{mL}; sperm count typically >15\times10^{6}\,\text{mL}^{-1}.
Contributions
- Seminal vesicles (~60 %)
- Fructose – energy substrate for mitochondrial ATP production.
- Prostaglandins – stimulate female tract motility.
- Prostate (~30 %)
- Clotting proteins + fibrinolysin → initial coagulation then liquefaction.
- Citrate & zinc.
- Alkaline fluids (raise pH ≈ 7.2!\text{–}!8.0 to counter vaginal acidity).
- Bulbourethral glands (~5 %)
- “Pre-ejaculate”: mucous secretion that lubricates urethra & neutralises acidic urine residue.
- Testicular & epididymal fluid (~5 %) containing mature spermatozoa.

Seminiferous tubules.
Straight tubules → Rete testis.
Efferent ductules → Epididymis (head → body → tail).
Vas deferens (passes through spermatic cord & inguinal canal).
Ejaculatory duct (junction of vas deferens & seminal vesicle duct) traverses prostate.
Prostatic urethra → Membranous urethra → Penile (spongy) urethra → External urethral orifice.

Structural components
- Paired corpora cavernosa (dorsal).
- Single corpus spongiosum (ventral, surrounds urethra; distally forms glans).
- Prepuce (foreskin) present unless removed by circumcision.
Erection mechanism (neurovascular reflex)
- Parasympathetic neurons release nitric oxide (NO).
- NO → \uparrow intracellular cGMP → relaxation of smooth muscle in arterial walls.
- Arteriolar dilation fills cavernous spaces with blood.
- Venous outflow compressed by distension of tunica albuginea – maintains rigidity.
- Terminated by phosphodiesterase-5 (PDE-5) degradation of cGMP (therapeutic target of sildenafil).

GnRH (gonadotropin-releasing hormone) pulsatile release from hypothalamus.
Anterior pituitary secretes
- LH – binds Leydig (interstitial) cells ⇒ testosterone production.
- FSH – targets Sertoli cells ⇒ androgen-binding protein (ABP) synthesis; maintains spermatogenesis.
Testosterone functions
- Stimulates spermatogenesis (with ABP concentrating T around developing germ cells).
- Promotes growth of skeletal muscle & bone; laryngeal enlargement; thickened skin.
- Development/maintenance of male secondary sex characteristics (facial hair, deep voice, body pattern fat distribution).
- Libido in both sexes.
- Critical periods
- First & second trimester fetal masculinisation.
- Pubertal surge (≈ 12!\text{–}!15 years) → continues throughout life, mild decline after 40!\text{–}!50 years (andropause).
Feedback loops
- Testosterone exerts negative feedback on both hypothalamus & pituitary.
- Sertoli-derived inhibin selectively inhibits FSH.

Infertility evaluation – sperm count, motility, morphology, semen pH.
Cryptorchidism – undescended testes; higher temperature → impaired spermatogenesis.
Vasectomy – surgical ligation of vas deferens; sperm production continues but reabsorbed.
Benign prostatic hyperplasia (BPH) – obstructs prostatic urethra; affects urine & semen flow.
Erectile dysfunction – vascular/neurological /psychogenic; PDE-5 inhibitors enhance NO-cGMP pathway.
Endocrine disruptors – environmental estrogens may alter HPG axis & sperm quality.

Optimal testicular temperature: T{testis} = T{core} - (2!\text{–}!3)^{\circ}C.
Spermatogenic yield: 1\,\text{primary spermatocyte} \rightarrow 4\,\text{spermatozoa}.
Typical semen: Volume 2!\text{–}!5\,\text{mL}; pH 7.2!\text{–}!8.0; sperm concentration >15\times10^{6}\,\text{mL}^{-1}.