IMMUNOHISTOCHEMICAL AND ULTRASTRUCTURAL-STUDY OF RHABDOSPHINCTER COMPONENT OF THE PROSTATIC CAPSULE

Purpose: There has been no complete agreement on functional anatomy of muscular components of the urethral sphincteric mechanism, particular ly in the male patient. The prostatic capsule was studied to define it s histological structure and to determine whether its rhabdosphincter component (prostatocapsular rhabdosphincter) consists only of slow twi tch or slow and fast twitch striated myofibers. Materials and Methods: We studied 11 whole prostates, including 1 obtained at autopsy and 10 by radical prostatectomy. Samples of prostatic capsule from 4 operati ve specimens were studied by electron microscopy. Whole mount paraffin sections from transverse slices of the remaining 7 prostates were dou ble labeled with avidin biotin conjugate immunostaining using the prim ary monoclonal antibodies anti-ct smooth muscle actin plus anti-cy sar comeric actin (all striated myofibers) or antiskeletal myosin fast (fa st myofibers only). Tissue components of the prostatic capsule, includ ing smooth muscle and slow versus fast twitch striated myofibers, were quantified by computerized image analysis. Results: The prostatic cap sule consisted of collagen, smooth muscle and striated myofibers. It v aried in thickness and proportion of the 3 components among specimens, and in each in relation to transverse circumferential aspect and cran iocaudal (horizontal) level of the prostate. Collagen and smooth muscl e were equally important components. Striated muscle elements within t he capsule consisted of fast twitch and dominant slow twitch myofibers , and were much more abundant in the caudal (distal, lower) than the c ranial (proximal, upper) half of the capsule, where they were deficien t ventrally (anteriorly) and dorsally (posteriorly). The prostatocapsu lar rhabdosphincter thus had a butterfly-like appearance, with a thick posteriorly open ring at the apex and 2 thinner, divergent leaflets t apering toward the base at the bladder neck. The fast myofiber populat ion decreased progressively from apex to base of prostate. Conclusions : Proof is provided for mixed slow and fast twitch myofiber structure of the prostatocapsular component of human male rhabdosphincter. Susta ined (tonic) contraction of slow myofibers probably reinforces the rol e of urethral smooth muscle in maintaining continence during bladder f illing. Swift contraction of fast myofibers that abound caudally in th e capsule probably supplements urethral closure by the bulkier membran ous urethral part of the rhabdosphincter in preventing leakage of urin e under stress when voiding is imminent or willfully withheld.