In developing mouse urinary bladder the urothelial cells respond to urine accumulation by cell detachment, i.e. desquamation. To elucidate the steps of urothelial cell detachment during embryonic development and first urine accumulation in bladder lumen, superficial and intermediate cell layers were investigated. Different electron microscopic and cytochemical methods have been used. It was possible to distinguish between an early and late events on the basis of morphology. At the beginning cell detachment involves interruption of tight junctions between neighbouring superficial cells and the formation of numerous endocytotic vesicles. Endocytotic and cup-shaped vesicles then fuse and form large parallel rows of vacuoles and multivesicular bodies in desquamating superficial cells and in the neighbouring superficial and intermediate cells. These observations clearly demonstrate the existence of specific steps during the detachment of embryonic uroepithelial cells.
Apoptosis accompanies cell desquamation. Only predetermined superficial bridge cells die in an apoptotic manner.