Mills Ch35 Prostate

970

SECTION IX : GenitourinaryTract

also variable (7,21) and the proportion and arrangement of collagenous tissue is inconsistent. Consequently, the pros- tate capsule cannot be regarded as a well-defined anatomic structure with constant features. While the capsule envelopes most of the external sur- face of the prostate, there is a defect of variable caliber at the prostatic apex anteriorly and anterolaterally such that the most distal (apical) fibers of the anterior fibromuscular stroma often mingle with the prostatic glandular tissue ante- rior and lateral to the urethra. Hence, if carcinoma is pres- ent in the apical third of the prostate anteriorly, it may be quite difficult to determine whether it has invaded beyond the boundary of the gland. Similar difficulty is encountered at the most proximal portion of the urethra in the bladder neck section, in which no clear capsule is evident. The anterior fibromuscular stroma is an apron of tissue that extends downward from the bladder neck over the anteromedial surface of the prostate, narrowing to join the urethra at the prostate apex (Fig. 35.3) (7). Its lateral mar- gins blend with the prostate capsule along the line where the capsule covers the most anteriorly projecting border of the peripheral zone. Its deep surface is in contact with the preprostatic sphincter and the transition zone proximally (toward the base) and with the “striated” or the semicircu- lar sphincter distally (toward the apex). It is composed of large bundles of smooth muscle cells that may be separated by bands of dense fibrous tissue, and are more randomly oriented than those of the bladder neck and blend with the latter at its proximal (basal) extent. Unlike the posterolateral prostate, in histopathologic sec- tions the anterior-most region of the gland does not exhibit a distinct “capsule” (19). Rather, as one proceeds from apex to base, the anterior fibromuscular stroma is variably inter- twined with skeletal muscle fibers emanating from the uro- genital diaphragm (apical prostate) (Fig. 35.15) or levator ani muscles (mid prostate) and may fuse with detrusor smooth

FIGURE 35.13  Ejaculatory duct encircled by a fibroconnective tissue sheath containing numerous lymphovascular spaces.

tissues and their arrangement vary considerably from region to region (19,20). At the inner capsular border, transverse smooth muscle blends with periglandular prostatic smooth muscle, and a clear separation between them cannot be iden- tified microscopically (19). The distance from terminal acini of the peripheral and central zones to the prostate surface is

FIGURE 35.14  The prostate capsule consists of a layer of mainly trans- verse smooth muscle bundles ( red ), which is of variable thickness and blends with periacinar smooth muscle bundles at the capsule’s poorly defined inner aspect ( left ). Collagen fibers ( blue ) are always present and usually concentrated in a thin compact membrane at the external capsular border ( right ) ( trichrome stain ).

FIGURE 35.15  Anterior fibromuscular stroma showing admixture of smooth muscle bundles with skeletal muscle fibers from the urogenital diaphragm at the apex of the prostate.