Architecture of structures in the urogenital triangle of young adult males; comparison with females

Abstract

The fibro‐muscular architecture of the urogenital triangle remains contentious. Reasons are small size of the constituting structures and poor visibility with most imaging methods. We reinvestigated the area in serial sections of three males (21–38 years old) of the American and Chinese Visible Human Projects and two 26‐week‐old male fetuses, and compared the findings with earlier observations in females. The mass of the levator ani muscle was approximately twofold smaller and its funnel shape steeper in males than females. In the levator hiatus, a strand of the smooth longitudinal muscle layer of the rectum, the ‘rectourethral (RU) muscle’, extended anteriorly from the anorectal bend to the penile bulb. Fibrous tissue that formed in the inferior reach of the fetal RU muscle identified the location of the developing perineal body (PB) and divided the muscle into posterior ‘rectoperineal’ and anterior ‘deep perineal’ portions. In males, the PB remained small and bipartite, so that the RU muscle presented as an undivided midline structure. The well‐developed female PB, instead, intertwined with the deep perineal muscle and both structures passed the vagina bilaterally to form the perineal membrane in the posterior portion of the urogenital triangle. The urethral rhabdosphincter extended in the anterior portion of the urogenital triangle between the penile bulb inferiorly and the bladder neck superiorly, and consisted of a well‐developed circular ‘membranous’ portion with bilateral posteroinferior ‘wings’ and a thinner ‘prostatic’ portion on the prostate anterior side. In men, muscles occupy the urogenital triangle, but additional tightening of the locally fibrous adipose tissue by the superficial transverse perineal muscle appears necessary to generate functional support in women. An interactive 3D pdf file with these anatomical details (available online) should allow more accurate interpretation of ultrasound, computed tomography and magnetic resonance images.

Interactive three‐dimensional teaching models of the female and male pelvic floor

“…Thus, Akita and colleagues argue that description of the perineal body as a node is a too delimited description and should, instead, be considered a region (Muro et al, ). However, our description of the perineal body as a fibromuscular structure in the wedge‐shaped space between the lower portion of the rectum and the vagina or urethra (“region”) that penetrates with many antenna‐like extensions between the fibers of the rectourethral muscle (“shape”) (Wu et al, ; Wu et al, ; Wu et al, ) is not very different from the impression that emanates from their trichrome‐stained illustrations of the rectourethral muscle (Muro et al, ). We, therefore, conclude that, not uniquely, both groups describe the same observation with different terminologies.…”

“…Consecutive sections of five female (CVH2, −4, −5, CVO and VHF (VHP female)) and five male specimens (CVH1, −3, VHM (VHP male), and two 26‐week fetuses (Leiden S2289 and S2600)) were studied in detail (Wu et al, ; Wu et al, ; Wu et al, ). We focused on the pelvic floor.…”

“…A). On its anterior side, the EAS passed the superficial transverse perineal and bulbospongious muscles superficially in the female (Wu et al, ), while a midline spur extended even further anteriorly in the male (Arakawa et al, ; Wu et al, ). We cannot exclude that some muscle fibers connect EAS and bulbospongious muscle, as reported recently in a diffusion tensor‐imaging study of perineal muscles (Zifan et al, ).…”

“…The developmental anatomy of the rectourethral muscle and perineal body can account for much of the controversies that surround the deep perineal muscle (Table ). In fetuses a well‐developed and well‐delineated anterior slip of the longitudinal smooth muscle of the rectum extended toward the urethral rhabdosphincter as the so‐called rectourethral muscle (Wu et al, ). When the (superficial portion of the) perineal body began to form prenatally inside this smooth muscle as a jagged fibrous structure, the part posterior to the perineal body became the rectoperineal muscle and the part anterior to it the deep perineal muscle (Table ).…”

“…We recently studied the pelvic floor of the Chinese Visible Human and presented our findings as interactive 3D‐PDFs of almost 50 structures (Wu et al, ; Wu et al, ; Wu et al, ). The main challenges of the “Visible Human” approach are the limited spatial resolution in the “section” and the dependence of segmentation on differences in structure and color that are present in the tissue block before sectioning.…”