Male Pelvis Anatomy – Prostate, Seminal Vesicles, Scrotal & Penile Sonography

Anatomy of Male Pelvis Structures

Overview of the Male Reproductive System

  • Principal organs reviewed in this lecture:
    • Penis
    • Prostate gland
    • Seminal vesicles
    • Testicles & their appendages (epididymis, scrotal coverings)
  • Collective function: production, maturation, storage, and emission of sperm; secretion of seminal fluid; urinary continence and micturition via the urethra.
  • Imaging modality emphasised: diagnostic ultrasound (transabdominal, transrectal & high-frequency linear probes for scrotum & penis).
  • Cross-sectional‐anatomy viewpoint aids correlation with CT/MRI used in advanced assessment, oncologic staging, or postsurgical follow-up.

Prostate Gland

Location & Size

  • Sits immediately inferior to the urinary bladder & anterior to the rectum.
  • Surrounds proximal urethra (⇒ “prostatic urethra” occupying central core).
  • Approximate dimensions of a normal adult prostate:
    • Length: 3.8\ \text{cm}
    • Anteroposterior (height): 3.0\ \text{cm}
    • Width: 4.0\ \text{cm}
  • Typical volumetric comparison: “walnut-sized.”

Internal (Zonal) Anatomy

  • Cone-shaped organ divided by histology & ductal distribution into 4 zones:
    • Peripheral Zone (PZ) – largest; posterolateral to distal urethra; primary site of adenocarcinoma.
    • Central Zone (CZ) – superior base, abutting bladder & seminal vesicles; traversed by ejaculatory ducts.
    • Transition Zone (TZ) – surrounds proximal urethra; common seat of benign prostatic hypertrophy (BPH).
    • Peri-urethral Glandular Zone – minute mucosal glands immediately adjacent to urethra.
  • Covered anteriorly by non-glandular fibromuscular stroma.

Functional Pathway

  • Produces slightly alkaline prostatic fluid → conveyed through numerous minute ducts emptying into prostatic urethra.
  • Mixed with seminal-vesicle secretion & sperm (from vas deferens) to constitute semen, which exits via distal urethra and external meatus.

Indications for Sonographic Examination

  • Assess size (calculate volume ≈ L\times W\times H\times0.52) & echotexture for BPH.
  • Evaluate prostatitis (vascular hyperemia on Doppler).
  • Detect solid/cystic masses; guide targeted biopsies.
  • Correlate suspicious lesions with elevated serum PSA (normal <4.0\,\text{ng·mL}^{-1}).
  • Follow-up post-TURP cavity or post-prostatectomy bed.

Sonographic Appearance

  • Heterogeneous organ with medium-level echoes; symmetric outline.
  • PZ often slightly hyperechoic relative to central tissues on transrectal imaging.
  • Visualised via:
    • Transabdominal route (full bladder as window).
    • Transrectal route (high-frequency endocavitary probe ⇒ higher resolution, optimal for cancer detection/biopsy).

Seminal Vesicles

Location, Size & Gross Anatomy

  • Paired, convoluted, pouch-like glands positioned posteroinferior to bladder & superior-posterior to prostate.
  • Combine with terminal vas deferens (ampulla) to form ejaculatory ducts that traverse the prostate.
  • Normal dimensions:
    • Length: 5.0\ \text{cm}
    • Max diameter (AP): <1.0\ \text{cm}

Sonographic Indications

  • Assess size/symmetry, echotexture.
  • Exclude cysts, calculi, inflammation, congenital agenesis or duplication (may accompany renal agenesis).

Sonographic Appearance

  • Ovoid, low-level echo structures, superior to prostate, encapsulated by thin echogenic wall.
  • Normally symmetric; loss of symmetry may suggest obstruction, hemorrhage, or neoplasm.

Scrotum

External & Internal Landmarks

  • Continuation of lower abdominal skin, suspended posterior to penis & anterior to anus.
  • Median Raphe: external midline ridge; internally corresponds to the scrotal septum (Tunica Dartos) separating right & left hemiscrotum.
  • Tunica Dartos – smooth-muscle/fibrous layer conferring wrinkled appearance & modest thermoregulatory role.

Cremaster Muscle

  • Skeletal muscle fibers investing each testis & spermatic cord.
  • Reflex contraction elevates testes to regulate intratesticular temperature (optimal spermatogenesis at \approx34°\text{C}).

Tunica Vaginalis

  • Serous sac derived from peritoneum; two layers:
    • Parietal (lines scrotal wall).
    • Visceral (covers testis & epididymis) leaving a small posterior “bare area” anchoring testis & deterring torsion.
  • Potential space between layers accommodates physiologic trace fluid; excess ⇒ hydrocele.

Sonographic Indications

  • Evaluate testicular size, echogenicity.
  • Diagnose inflammatory conditions (orchitis, epididymitis).
  • Detect masses (intra- vs extra-testicular).
  • Identify fluid collections (hydroceles, hemato- or pyo-celes).
  • Doppler interrogation for torsion (arterial flow absence/reduction).
  • Localise undescended testes (cryptorchidism) in inguinal canal/abdomen.
  • Assess blunt/penetrating trauma, scrotal pain.

Testes

Parenchymal Architecture

  • Enveloped by dense fibrous Tunica Albuginea; posterior thickening forms Mediastinum Testis (linear echogenic stripe on US).
  • Septa radiating from mediastinum divide parenchyma into 200\text{–}300 lobules, each containing 1\text{–}3 convoluted Seminiferous Tubules (site of spermatogenesis).
  • Straight Tubules → Rete Testis (network within mediastinum) → Efferent Ductules → Epididymal Head.

Sonographic Appearance

  • Homogeneous, medium-level echotexture.
  • Mediastinum testis: hyperechoic linear focus.
  • Low-resistance arterial waveforms (intrinsic testicular artery branches).

Normal Measurements

  • Adult: Length 3\text{–}5\ \text{cm}; Height 2\text{–}3\ \text{cm}; Width 2\text{–}3\ \text{cm}.

Epididymis

Anatomy & Function

  • Tightly coiled duct (~6\ \text{m} uncoiled length) lying postero-lateral to testis.
  • Subdivisions:
    • Head (caput) – superior pole; receives efferent ductules.
    • Body (corpus).
    • Tail (cauda) – inferior pole, continuous with Vas Deferens.
  • Stores, matures, and propels sperm.

Sonographic Appearance & Size

  • Iso- to mildly hypoechoic versus testicular parenchyma; coarser texture.
  • Adult dimensions:
    • Head: 10\text{–}12\ \text{mm} AP, 5\text{–}12\ \text{mm} length.
    • Body: 2\text{–}4\ \text{mm} AP; Tail: 2\text{–}5\ \text{mm} AP.

Spermatic Cord

  • Two cords traverse inguinal canals, suspending testes.
  • Contents (mnemonic “VANLDC”):
    • Vas (Ductus) deferens
    • Arteries: Testicular, Cremasteric, Deferential
    • Nerves (autonomic)
    • Lymphatics
    • Pampiniform venous plexus
    • Cremaster muscle fibers

Ductus (Vas) Deferens

  • Muscular conduit (~45\ \text{cm}) originating from epididymal tail → ascends within spermatic cord → passes through inguinal canal → loops over ureter/bladder → widens at ampulla → joins seminal vesicle duct to form ejaculatory duct.
  • Function: rapid transport of mature sperm during emission.

Vascular Supply & Venous Drainage

  • Testicle:
    • Arterial: Testicular (spermatic) arteries – direct aortic branches.
    • Venous: Pampiniform plexus ⇒
    • Right testicular vein → IVC.
    • Left testicular vein → Left renal vein (↑ incidence of varicocele on left).
  • Vas deferens: Deferential artery (branch of internal iliac).
  • Scrotal wall: Pudendal branches (external & internal).
  • Penis & urethra: Internal Pudendal arteries; Deep Penile (Cavernosal) artery to Corpora Cavernosa; Dorsal & Bulbourethral arteries to Corpus Spongiosum.
  • Venous return via Superficial & Deep Dorsal veins.

Penis

Structural Components

  • Two dorsolateral Corpora Cavernosa (erectile tissue).
  • Single midventral Corpus Spongiosum enclosing spongy (penile) urethra; expands distally as glans penis.
  • Tunica Albuginea: dense fibrous tissue surrounding cavernosa, forming Septum Penis (incompletely partitioned).
  • Buck’s Fascia: superficial fibrous envelope beneath skin.

Blood Supply & Drainage

  • Arterial inflow via paired Internal Pudendal arteries.
    • Deep Penile (Cavernosal) artery → erectile tissue of cavernosa.
    • Dorsal artery & Bulbourethral branches → spongiosum & urethra.
  • Venous outflow: Superficial & Deep Dorsal veins; emissary veins pierce Tunica Albuginea.

Sonographic Indications

  • Detect fibrosis/plaques in Peyronie’s disease.
  • Evaluate tumors, hematoma, peri-urethral pathology.
  • Doppler study for vasculogenic erectile dysfunction (assess peak systolic velocity, end-diastolic velocity, resistive index after pharmacologic erection).

Typical Ultrasound Appearance

  • Corpora Cavernosa: paired, round/oval, homogeneous medium echoes, anterior to spongiosum.
  • Corpus Spongiosum: posterior, homogeneous.
  • Tunica Albuginea: thin, highly echogenic line encasing cavernosa.
  • Septum Penis: echogenic midline extension of tunica.

Laboratory & Ancillary Tests

  • Serum Prostate-Specific Antigen (PSA): normal <4.0 (ng/mL); elevated levels prompt imaging & biopsy.
  • Imaging adjuncts: Multiparametric MRI (prostate cancer staging), MR-guided or US/MRI fusion biopsies.
  • Histologic confirmation: Ultrasound-guided transrectal or transperineal core biopsy.

Ethical, Clinical & Practical Considerations

  • PSA screening remains debated; shared decision-making is essential.
  • Ultrasound provides real-time, radiation-free evaluation & guidance; operator dependence necessitates proper training (addressed within this medical sonography program).
  • Recognition of normal variants & age-related changes prevents over-diagnosis.
  • Immediate Doppler interrogation in acute scrotal pain can salvage a torsed testis (time-critical).
  • Accurate zonal identification in prostate aids targeted biopsy, reducing number of cores & complications.
  • Understanding vascular anatomy mitigates risk of post-biopsy hemorrhage or high-flow priapism after penile trauma.

Cross-Reference & Real-World Relevance

  • Links to Acland’s Video Atlas provide 3-D visual reinforcement; sequence numbers (5.3.x) match curricular modules.
  • Knowledge integrates with prior abdominal lectures (bladder, rectum) for sectional orientation.
  • Clinical rotations: findings correlate with urology consults, oncology staging (Gleason grading), interventional radiology (embolisation for impotence), andrology (fertility assessment).

Quick-Reference Normal Measurements (Consolidated)

  • Prostate: 3.8\times3.0\times4.0\ \text{cm}
  • Seminal Vesicle: Length 5.0\ \text{cm}; AP <1.0\ \text{cm}
  • Adult Testis: 3\text{–}5\ \text{cm}\ (L)\;2\text{–}3\ \text{cm}\ (H\/W)
  • Epididymal Head: 10\text{–}12\ \text{mm} AP.
  • PSA: <4.0\,\text{ng·mL}^{-1}

End of detailed study notes – these points collectively replicate and enrich the source material for comprehensive exam preparation.