Embryology of bladder exstrophy.

A hypothesis in respect to the teratogenesis of bladder exstrophy and its variants is offered. The central feature of this hypothesis is the abnormal persistence of the caudal position of the insertion of the body stalk on the embryo. As a consequence of this, the normal advance and interposition of mesenchymal tissue to the midline becomes impossible. The cloaca cannot be translocated backwards into the body cavity, and the cranial end of the cloacal membrane remains in contact with the inferior aspect of the low-set body stalk. This, in contrast to the previously proposed abnormal rostral extension of the cloacal membrane, causes a wedge-effect resulting in the lateralization of the abdominal wall structures and also in the prevention of the midline fusion of the genital hillocks (labioscrotal or genital folds). A cloacal membrane normally is an unstable structure lacking mesoderm, and it retains these characteristics in the superficial and infraumbilical position to be described. It has a strong tendency to disintegrate. It may rupture at variable times and to a variable extent. The consequence of such an embryonic event is either a typical bladder exstrophy or one of the variants of the exstrophy malformation. Three different variants are presented that the proposed embryologic hypothesis can readily explain.