Blood‐urine barrier formation in mouse urinary bladder development

Formation of the blood‐urine permeability barrier in differentiating mouse transitional urothelium was studied. It was established that the development of superficial cell barrier is a two‐phase process: beginning with formation of the tight junctions, followed by formation of fusiform vesicles and asymmetric apical plasma membranes. Fusiform vesicles differentiate during days 15 and 17 of gestation and fuse with the apical plasmalemma. Thus a thick membrane is formed before the excretion of hypertonic urine into the embryonic bladder. Through some degenerative superficial cells slough between fetal day 17 and the day of birth, the bladder epithelium in mice does not lack an effective permeability barrier. © 1993 Wiley‐Liss, Inc.