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AN46.1-5 | Male external genitalia — Self-Directed Learning

Part 1: The Testis — Coverings, Descent, and Applied Anatomy (AN46.1)

Structure of the testis:
The testis is a paired, oval organ (~5×3×3 cm) enclosed in fibrous and fascial coverings.

Internal structure:
- Tunica albuginea — dense fibrous capsule. Sends inward partitions (septa testis) dividing the testis into ~250 lobules, each containing 1–4 seminiferous tubules (where spermatogenesis occurs, at 35°C).
- Each lobule's seminiferous tubules converge → rete testis (in the mediastinum testis, a thickening of the posterior tunica albuginea) → efferent ductules → epididymis.
- Leydig (interstitial) cells between the tubules produce testosterone.

Coverings (from inside out, corresponding to abdominal wall layers):
1. Visceral layer of tunica vaginalis — reflected serous layer (peritoneum) directly on the testis
2. Parietal layer of tunica vaginalis — forms a sac; the cavity between visceral and parietal layers = potential space for hydrocele fluid
3. Internal spermatic fascia (from transversalis fascia)
4. Cremasteric muscle and fascia (from internal oblique; function: temperature regulation by raising/lowering testis)
5. External spermatic fascia (from external oblique aponeurosis)
6. Dartos muscle (smooth muscle in scrotal skin; reflex contraction in cold)
7. Scrotal skin (rugose, thin, with sebaceous glands)

Side determination (how to tell left from right testis in a cadaveric specimen):
- The epididymis is always posterior (posteromedial surface)
- The testicular artery approaches on the lateral side above
- The vas deferens ascends on the posteromedial surface → into the tail of epididymis

Testicular descent:
- Testis develops retroperitoneally near L1–L2 level (below the kidney)
- Descends from ~7th week of gestation, guided by the gubernaculum testis (a mesenchymal cord from testis to scrotum)
- Passes through the inguinal canal during the 7th–9th months of gestation (dragging its coverings)
- Normally reaches the scrotum by birth (or shortly after in premature infants)

Undescended testis (cryptorchidism): Failure to descend — most commonly arrested in the inguinal canal. If not corrected by 2 years of age → failure of spermatogenesis (higher temperature of inguinal position destroys sperm-producing cells) + 15× increased risk of testicular malignancy.

Blood supply:
- Testicular artery — directly from the abdominal aorta at L2 (because the testis developed in the abdomen). This is the embryological reason testicular vessels do NOT come from the iliac vessels.
- Venous drainage: pampiniform plexus → testicular vein → right to IVC; left to left renal vein (at a right angle, creating higher venous pressure on the left — explains left-sided varicocele predominance).

Nerve supply: Testicular plexus (T10) — visceral afferents travel with the testicular artery → pain referred to periumbilical region (T10 dermatome) in orchitis, torsion, and trauma.

Lymphatic drainage: Para-aortic (lumbar) lymph nodes at L2 level — NOT the inguinal nodes. This reflects the testis's abdominal origin. Testicular cancer metastases appear in para-aortic nodes on CT/MRI — NOT the inguinal nodes. Scrotal skin does drain to inguinal nodes — so a testicular tumour that has eroded through scrotal skin CAN eventually reach inguinal nodes.*

Part 2: The Epididymis — Structure and Function (AN46.2)

The epididymis is a coiled comma-shaped structure (~7 m of coiled duct) lying along the posterolateral surface of the testis. It is the site of sperm maturation and storage.

Parts (head to tail):
1. Head (caput) epididymis — receives the efferent ductules from the rete testis; located at the upper pole of the testis; most prominent palpable part
2. Body (corpus) epididymis — middle portion along the posterior surface
3. Tail (cauda) epididymis — at the lower pole; becomes the vas deferens (ductus deferens)

Functions:
- Sperm maturation: Spermatozoa leaving the testis are immotile. Transit through the epididymis (~12 days) achieves full motility and fertilisation capacity.
- Storage: Mature spermatozoa stored in the tail of the epididymis await ejaculation.
- Fluid reabsorption: Reabsorbs most of the fluid from the rete testis.

Applied anatomy:
- Epididymo-orchitis: Infection of the epididymis (most common bacterial: Chlamydia trachomatis in young men, E. coli in older men). Presents as painful swelling of the epididymis/testis. The cremaster reflex is preserved (unlike torsion) — crucial differentiating sign.
- Appendix epididymis: A small vestigial structure at the head that can undergo torsion → localised head-of-epididymis tenderness; not an emergency.
- Vasectomy is performed on the vas deferens (not the epididymis) — the vas is palpable through the scrotal skin at its lower pole.

Part 3: The Penis — Parts, Components, and Blood Supply (AN46.3)

The penis has three cylindrical bodies of erectile tissue:

Three erectile bodies:
1. Two corpora cavernosa — paired, lie side by side dorsally; each surrounded by a tough fibrous tunica albuginea; joined by an incomplete median septum. End blindly at the glans.
2. One corpus spongiosum — single, ventral; surrounds the penile urethra; expands distally to form the glans penis (covered by the prepuce/foreskin); expands proximally as the bulb of the penis (fixed to the perineal membrane).

Parts of the penis:
- Root (fixed part): Two crura (singular: crus) attached to the ischiopubic rami + bulb attached to the perineal membrane + ischiocavernosus (over crura) + bulbospongiosus (over bulb) muscles.
- Body (shaft): Free cylindrical part; skin + subcutaneous tissue (no fat in penile skin) + Buck's (deep penile) fascia + dartos fascia + 3 erectile bodies.
- Glans penis: Expanded distal end of corpus spongiosum; bears the external urethral meatus.

Blood supply:
- Dorsal artery of the penis (branch of internal pudendal artery) — supplies glans and skin
- Deep artery of the penis (branch of internal pudendal) — enters the corpus cavernosum → helicine arteries open into cavernous spaces during erection
- Artery to the bulb — supplies the bulb (corpus spongiosum proximally) and the bulbourethral glands

Erection mechanism: Parasympathetic (S2,3,4 via pelvic splanchnic nerves / nervi erigentes) → relaxation of smooth muscle of helicine arteries → blood fills cavernous spaces → compresses emissary veins against tunica albuginea → veno-occlusion → erection maintained.

Lymphatic drainage:
- Prepuce, scrotal skin, shaft skin → superficial inguinal nodes
- Glans penis → deep inguinal nodes → external iliac nodes
- Carcinoma of the glans presents with inguinal lymphadenopathy.

Dorsal nerve of the penis (S2,3) — sensory to the glans; the nerve blocked in penile nerve block for circumcision.

Part 4: Varicocele and Phimosis — Anatomical Basis (AN46.4, AN46.5)

Varicocele (AN46.4):

A varicocele is an abnormal dilatation of the pampiniform plexus of veins within the spermatic cord — like varicose veins of the scrotum.

Anatomical basis of left-sided predominance (90% of varicoceles are on the left):
- Left testicular vein → left renal vein (joins at a right angle, creating high hydrostatic resistance; column of blood to the renal vein is ~10 cm longer than the right)
- Right testicular vein → IVC directly (at an oblique angle, lower resistance)

The left testicular vein effectively has a higher venous pressure → valves in the left testicular vein fail → retrograde venous flow → dilatation of pampiniform plexus.

Why clinically important:
- Raised scrotal temperature (venous pooling) → impaired spermatogenesis → male factor infertility (found in 40% of infertile men)
- Sudden onset left varicocele in a middle-aged man → rule out a left renal vein tumour thrombus (from renal cell carcinoma) obstructing the left renal vein
- Right-sided varicocele is uncommon and should raise suspicion of a retroperitoneal mass

Examination: "Bag of worms" feel in the left scrotum, more prominent on standing and Valsalva manoeuvre.

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Phimosis and Circumcision (AN46.5):

Phimosis: The prepuce (foreskin) cannot be retracted over the glans because the preputial opening is too tight.
- Physiological phimosis: In boys under 2–3 years, the foreskin is normally non-retractile (preputial adhesions separate naturally with erections). This is NOT pathological and does NOT require treatment.
- Pathological phimosis: Acquired narrowing in older boys/men from scarring (recurrent balanitis, balanitis xerotica obliterans). Causes difficulty with urination, pain during intercourse, and inability to clean under the foreskin → recurrent infections.

Paraphimosis: Retracted foreskin that CANNOT be pulled back forward → constrictive ring cuts off venous/lymphatic return → oedema of glans and prepuce → urological emergency (manual reduction or emergency dorsal slit).

Circumcision: Surgical removal of the prepuce.
- Anatomical basis: The prepuce is a double fold of skin (inner mucosal layer + outer cutaneous layer) covering the glans. Circumcision removes this fold to the level of the corona, exposing the glans permanently.
- Nerve supply of the foreskin: Branches of the dorsal nerve of the penis (S2,3) — blocked with a penile ring block (local anaesthetic infiltrated at the base of the penis, bilaterally, dorsal and ventral) for painless circumcision.
- Complications: Haemorrhage (dorsal artery/vein of the penis), injury to the glans or urethra (rare but serious), meatal stenosis (post-circumcision scarring).