CHAPTER 81 Michael S. Baggish The female urethra is about 4 cm long and averages 6 mm in diameter. Its lumen is slightly curved as it passes from its internal position in the retropubic space, perforates the perineal membrane, and ends by opening into the vestibule directly above the vaginal opening. Throughout its length, the posterior urethra is embedded in the anterior vaginal wall. The epithelium of the urethra is continuous externally with that of the vulva and internally with that of the bladder. It consists primarily of stratified squamous epithelium that becomes transitional near the bladder. The epithelium is supported by a layer of loose fibroelastic connective tissue—the lamina propria. The lamina propria contains many bundles of collagen fibrils and fibrocytes, as well as an abundance of elastic fibers oriented both longitudinally and circularly around the urethra. Numerous thin-walled veins are another characteristic feature. This rich vascular supply is thought to contribute to urethral resistance. Cross-sections of the urethra below the urethrovesical junction at 6 to 9 mm (distal) clearly show the cavernous vascularity that contributes more than 50% of the volume of tissue constituting the anterior and lateral walls of the urethra (Fig. 81–1A through D). The smooth muscles of the urethra are composed primarily of oblique and longitudinal muscle fibers with a few circularly oriented outer fibers. This smooth muscle, along with the detrusor muscle in the bladder base, forms what can be called the intrinsic urethral sphincter mechanism. Longitudinally directed muscle probably shortens and widens the urethral lumen during micturition, whereas circular smooth muscle contributes to urethral resistance to outflow at rest. Historically, striated muscle termed the striated urogenital sphincter has been divided into three muscles: the sphincter urethrae, which is described as a striated band of muscle that surrounds the proximal two thirds of the urethra, and the compressor urethrae and urethrovaginal sphincter, which consist of two straplike bands of striated muscle that arch over the ventral surface of the distal third of the urethra. The authors’ recent dissections on multiple female cadavers with gross and microscopic examination of this area revealed no separate or distinct striated musculature of the urethra. The authors were unable to identify any striated musculature in the periurethral area that was not an extension of the levator ani muscle (Fig. 81–2A through G). In a series of 12 cadaveric dissections, the levator ani muscle was thought to extend over the anterior surface of the urethra. The authors were thus unable to identify the previously described separate and distinct striated urogenital sphincter. Figs. 81–3 through 81–6 are gross and microscopic sections showing the levator muscle over the urethra. In performing histologic sections throughout the length of the urethra, the authors also observed that most of the vascular contribution of the urethra stemmed from the bulb of the vestibule. The vascularity created an umbrella-type effect over the anterior and lateral walls of the urethra (Figs. 81–1B and 81–7).
Anatomy of the Urethra
Mickey M. Karram
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