The zonal anatomy of the prostate.

Earlier morphologic studies of the prostate, though often extensive, have never systematically delineated its completed structure. Recent comprehensive analysis of 500 prostates has more precisely defined its anatomic composition, identifying previously undetected features and unsuspected complexities. Using a three-dimensional model, these structures and relationships are demonstrated. Four basic anatomic regions are described. The relationship of each to the urethra provides a central anatomic reference point. 1. The peripheral zone constitutes over 70% of the glandular prostate. It forms a disc of tissue whose ducts radiate laterally from the urethra lateral and distal to the verumontanum. Almost all carcinomas arise here. 2. The central zone constitutes 25% of the glandular prostate. Its ducts arise close to the ejaculatory duct orifices and follow these ducts proximally, branching laterally near the prostate base. Its lateral border fuses with the proximal peripheral zone border, completing in continuity with the peripheral zone, a full disc of secretory tissue oriented in a coronal plane. Marked histologic differences between central and peripheral zones suggest important biologic differences. 3. Preprostatic region. The urethral segment proximal to the verumontanum is kinked anteriorly at a 35-degree angle to the distal segment. No major ducts arise in the proximal segment, but the lateral rows of peripheral zone orifices continue. Duct development is aborted here, producing only a small transition zone and several tinier periurethral ducts. The development of these small ducts is possibly determined and limited by their intimate relationship to a periurethral smooth muscle sphincter that exists only proximal to the verumontanum. These small ducts in a restricted area are the exclusive site of nodular hyperplasia (BPH) origin. 4. The anterior fibromuscular stroma forms the entire anterior surface of the prostate as a thick, nonglandular apron, shielding from view the anterior surface of the three glandular regions. Its inseparable fusion to the glandular prostate has probably delayed recognition of the anatomic features described here.