Mills Ch35 Prostate

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SECTION IX : GenitourinaryTract

Middle One-Third of the Prostate (Mid Gland) McNeal’s studies revealed a 35-degree angulation of the prostatic urethra at mid gland, dividing the urethra into proximal (toward the base) and distal (toward the apex) segments (Fig. 35.9A–B). The key anatomic landmark in the mid gland is the verumontanum, an exaggerated area of glandular–stromal tissue, located subjacent to the posterior urethral wall, into which the ejaculatory ducts insert and from which the glandular zones arise (16). Histologically, the verumontanum consists of a crowded collection of pros- tatic glands, lined by secretory epithelium and often with abundant intraluminal corpora amylacea, directly underly- ing the urothelium of the prostatic urethra. When promi- nent, the term verumontanum gland mucosal hyperplasia is applied (Fig. 35.10) (17). At mid gland, the transition zone becomes evident as bilateral lobes in the anteromedial region of the gland. The ducts of the transition zone appear to arise from the posterior boundary of the periurethral space and course anterolaterally to serve as a boundary between transition and peripheral zones. A stromal bound- ary between these two zones has also been described (7), but may be difficult to identify in individual cases. In the normal mid gland, the peripheral zone still composes the posterior, lateral, and the majority of anterolateral tissues (Fig. 35.9A). In prostates with BPH, this tissue may be sig- nificantly compressed toward the lateral most portions of the gland (Fig. 35.9B). The anterior fibromuscular stroma in this region may be less evident owing to the increased

immediately surrounded by a thin layer of stroma and a variable number of periurethral glands. The latter intermingle anteromedially with a semicircular band of medium-sized, compactly arranged and vertically ori- ented muscle fibers. This band is incomplete posteriorly, appearing consistently as a densely eosinophilic, aglan- dular muscular column which extends posteriorly from the urethra (Fig. 35.7) and is distinct from the glandular verumontanum of the mid gland. This compact morpho- logic appearance led some authors to designate this as the “striated sphincter” (5,10,15), yet careful histopatho- logic evaluation of the region shows that it is composed purely of smooth muscles. This impression is confirmed by the lack of immunohistochemical staining for sarco- meric actin, a marker of mature skeletal muscle, in the muscle cells of this region (14). Anterior to the semicircular muscle, fibers of the anterior fibromuscular stroma traverse horizontally and laterally as they extend to the anterior- and apical-most aspects of the prostate. While heightened intraprostatic pressure may cause bulging of hypertrophic transition zone acini, no normal transition zone tissue is located in the prostatic apex. The bilateral peripheral zone, which composes essentially all of the glandular tissue at the apex, occupies the posterior, lateral, and anterolateral prostates, abutting the anterior fibromuscular stroma medially and forming a nearly complete ring in histologic sections.

A

B

FIGURE 35.9  A: Whole mount section from mid prostate at the level of the verumontanum ( V ). Note the bilobed transition zone ( TZ ) arising from elongated ducts ( D ) which course anterolaterally. The peripheral zone ( PZ ) still occupies the posterior, lateral, and anterolateral portions of the gland, with a cancer nodule ( CA ) evident in the right anterior peripheral zone. In the mid prostate, the anterior fibromuscular stroma ( AFMS ) is much condensed and the anterior extraprostatic space ( AEPS ) largely retains its apical consistency. B: Whole mount section from mid prostate at the level of the verumontanum ( V ) in a gland with extensive benign prostatic hypertrophy ( BPH ). Anterolateral “horns” of the peripheral zone ( PZ ) are compressed laterally by the expanded transition zone tissue and the AFMS is diminished in extent.