The membranous part becomes the narrowest segment of the urethra. From a very short element at first, it lengthens considerably (f.65-90 mm). It is flanked externally by the bilateral puborectal muscles.
Its crescentic lumen cranially changes into an H-form caudally as the ventral groove is followed caudalward by the temporary ventral ridge, and as the colliculus seminalis on the dorsal side becomes more prominent and continues into a newly formed dorsal ridge that replaces the original dorsal groove (f.50 mm). Thereafter, that H shape becomes obscured by numerous smaller grooves and ridges.
Its originally high (two- to three-layered) ps.str.col. epithelium becomes a two-layered cuboidal type. In some fetuses a few tubular mucous glands develop (f.50 mm).
The lamina propria is a continuation of the lamina in the prostatic urethra, which here gradually changes into the lamina of the spongy urethra, characterized by many longitudinal vessels.
The membranous urethra is surrounded by the internal and external urethral sphincters in the form of a horseshoe, its narrow opening connected to the perineal body. The orientation of the fibers is transverse to oblique transverse with a dorsalcaudalward orientation. Later (f.65 mm), longitudinal bundles appear at the border with the lamina propria. The peripherally developing striated muscle tissue of the external urethral sphincter (f.60 mm) shows the same pattern and extension as the outer layer of the internal sphincter. A great increase in length of this segment (f.65 mm) is associated with a distinct thickening of both components of that sphincter.
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