Purpose: Although a dual origin of the vagina has been popularized, other theories support a müllerian or wolffian duct origin or various combinations of these structures and the urogenital sinus. Uroplakins are specialized membrane proteins of the urothelial plaque, constituting the asymmetrical unit membrane of the bladder, and represent specific molecular markers of urothelial differentiation. We hypothesize that the epithelium of the dorsal wall of the urogenital sinus is involved in the formation of the sinovaginal bulbs and will express uroplakins. In addition, localization of the androgen receptor and its temporal expression during development may in part explain the varied effects of androgens on the lower female genital tract in congenital adrenal hyperplasia.
Materials and methods: Lower genitourinary tracts from 4 human female fetuses (9 to 18 weeks) were serially sagittally sectioned. Representative sections were stained with hematoxylin and eosin, rabbit antibodies against panuroplakin and antibodies to the androgen receptor.
Results: At 9 weeks of gestation the urogenital sinus showed evidence of evagination and the formation of the sinovaginal bulbs. The urothelium of the entire urogenital sinus expressed uroplakins including the region of the dorsal wall involved in evagination and formation of the sinovaginal bulbs. The müllerian ducts were in direct contact with the area of urogenital sinus evagination but were not in continuity with the sinus. Androgen receptors were expressed in the epithelium and the stroma of the urogenital sinus, sinovaginal bulbs, and müllerian and wolffian ducts. By 14 weeks androgen receptor expression was almost absent in the urothelium of the urogenital sinus, and the epithelium and surrounding stroma of the lower vagina and müllerian ducts.
Conclusions: The area of evagination of the urogenital sinus expresses uroplakins, is involved in the formation of the sinovaginal bulbs and further substantiates the urogenital sinus origin of the lower vagina. Since testosterone inhibits formation of the lower vagina, the timing of exposure to systemic testosterone in congenital adrenal hyperplasia will determine the phenotypic appearance of the external genitalia and effect of testosterone on the development of the lower genital tract. If exposure to testosterone occurs after 12 weeks only clitoromegaly occurs. Androgen receptor is absent in the urogenital sinus urothelium, vaginal epithelium and müllerian ducts by 14 weeks, suggesting that these tissues become androgen insensitive and vaginal development will proceed normally after that critical time.