The prostate and seminal vesicles are the male accessory sex glands surrounding the ejaculatory ducts. This tab covers prostatic zonal anatomy, the surgically critical fasciae and neurovascular bundle, lymphatic drainage and imaging, and the seminal vesicles and ejaculatory ducts.
Prostate: Structure and Zones
The prostate's transitional and peripheral zones derive from the urogenital sinus and the central zone from the Wolffian duct (it is homologous to the female Skene glands). It is ovoid, with the base at the bladder neck and the apex continuous with the striated urethral sphincter, fixed anteriorly to the pubis by the puboprostatic ligaments.
It is ~30% fibromuscular stroma (continuous with the bladder-neck detrusor) and ~70% glandular, the glandular part divided into three zones:
- Peripheral zone — ~70% of glandular tissue; site of ~70% of prostate cancers.
- Central zone — ~25% of glandular tissue; surrounds the ejaculatory ducts.
- Transitional zone — ~3–5% of glandular tissue; the site of BPH and ~20% of prostate cancers.
Prostate: Fasciae and Neurovascular Supply
Three fascial layers surround the gland: Denonvilliers fascia posteriorly, the prostatic fascia anteriorly/anterolaterally (often mislabelled "capsule" — there is no true histologic capsule), and the endopelvic (levator) fascia laterally.
- Denonvilliers fascia lies between the rectum and prostate, densest near the base and seminal vesicles; because its layers cannot be distinguished microscopically, it is excised completely to obtain a clear posterior margin.
- The cavernous nerves (neurovascular bundle) run between the endopelvic/levator and prostatic fascia, posterolateral to the prostate — the key landmark for nerve-sparing.
Arterial supply enters at the 4 and 8 o'clock positions from the inferior vesical artery, which terminates in urethral vessels (to the bladder neck and periurethral gland) and capsular branches (which run with the pelvic-plexus nerves as the neurovascular bundle). Venous drainage is into the Santorini (periprostatic) plexus, which communicates with the deep dorsal vein of the penis and the internal iliac veins; the deep dorsal vein divides into a superficial branch (over the bladder neck, between the puboprostatic ligaments) and right and left lateral plexuses.
Prostate: Lymphatics and Imaging
- Lymphatic drainage — primarily the obturator and internal iliac nodes, occasionally the presacral or external iliac. Prostatectomy node-dissection limits: superior — common iliac bifurcation; inferior — Cloquet's node; lateral — pelvic sidewall (below the external iliac vein); medial — bladder; posterior — obturator fossa.
- Nerves — the pelvic plexus lies beside the rectum, 5–11 cm from the anal verge; the cavernous nerves carry sympathetic fibres (smooth-muscle contraction) and parasympathetic fibres (secretion) and are most vulnerable at the prostatic apex.
- Imaging — TRUS (6–8 MHz) estimates prostate volume by the ellipsoid formula (π/6 × L × W × H ≈ 0.52 × L × W × H), accurate to within 5%; MRI T2-weighting best shows the zonal anatomy.
Seminal Vesicles and Ejaculatory Ducts
The seminal vesicles lie posterior to the bladder and prostate, lateral to the vas deferens, measuring 1.5 cm wide × 5–7 cm long (3–4 mL); the ureters enter the bladder medial to their tips. Each seminal vesicle joins the vas to form an ejaculatory duct, and the paired ejaculatory ducts open through the verumontanum into the distal prostatic urethra. The epithelium is columnar with goblet cells.
- Vasculature — the seminal vesicles are supplied by the superior and inferior vesical arteries; the ejaculatory ducts by the inferior vesical artery.
- Innervation — parasympathetic from the pelvic plexus and sympathetic from the hypogastric and superior lumbar nerves (the hypogastric nerve bridges the superior hypogastric plexus and the pelvic plexus).
- Imaging — on TRUS the seminal vesicles are hypoechoic, crescent-shaped, paired, and symmetrical; an AP diameter >1.5 cm suggests ejaculatory-duct obstruction (a cause of low-volume azoospermia). A solid mass raises concern for schistosomiasis in endemic areas, and an absent seminal vesicle carries a 79% risk of ipsilateral renal agenesis.
Self-Test
1. From which zone does most prostate cancer arise, and from which does BPH arise? Prostate cancer arises mostly (~70%) from the peripheral zone (with ~20% in the transitional zone); BPH arises from the transitional zone.
2. Which zone surrounds the ejaculatory ducts, and which is the smallest zone? The central zone surrounds the ejaculatory ducts; the transitional zone is the smallest.
3. What are the primary lymphatic drainage sites of the prostate? The obturator and internal iliac nodes.
4. What are the normal dimensions of the seminal vesicles? About 5–7 cm long and 1.5 cm wide.