CONTRIBUTIONS TO THE EMBRYOLOGY OF THE URINARY BLADDER

Abstract

Serial sections of 42 human embryos (range 5.2 to 45 mm. CR) were studied, 15 of these being reconstructed in wax, others graphically. The Wolffian duct caudal to the ureteric bud is absorbed into the urogenital sinus wall by widening of this segment of the duct and by replacement of its epithelium by ingrowing sinus epithelium. This widening may occur before, during or following the ingrowth of sinus epithelium, but eventually wide sinus cornua extend to the junction of ureter and Wolffian duct. During this process the openings of the Wolffian ducts are moved deep from their original position near the cloacal membrane. Separation of the openings of the Wolffian ducts and ureters is effected by active migration of Wolffian duct openings caudo-medially, due to the formation of a caudo-medial convex loop of the Wolffian duct during the absorption phase. The explanation of the further separation and shifting of the ureteric openings is not as clear. For a short time the ureteric operings are occluded by a membrane of sinus epithelium. At about 35 mm. there is a small ureteric dilatation adjacent to this membrane, presumably due to beginning metanephric secretion.