Morphological observations on the epithelium of the developing urinary bladder of the mouse and rat.

Bladders from fetal and neonatal BALB/cStCrlfC3H/Nctr mice and Sprague-Dawley rats were studied to establish the sequence of events in their morphological development by using scanning electron, transmission electron and light microscopy. On fetal day 18 or 19 the epithelium from the mouse and the rat displayed 2 or 3 distinct cell layers. With transmission electron microscopy a star-like contraction of the cell surface of the mouse bladder occurred which was not seen in the developing rat bladder. In both the mouse and the rat, some of the superficial cells sloughed between fetal day 18 or 19 and the day of birth. On the day of birth, the epithelium was composed of only 2 layers. The nuclei of both the superficial and basal layers contained prominent euchromatin, and mitotic figures were often present in the basal layer. By the 5th postnatal day, some of the superficial cells contained autophagic vacuoles, and the epithelium was still 2 cell layers thick. One week after birth the epithelium consisted of 2 to 3 cell layers. Three weeks after birth the epithelium was 3 cell layers thick and appeared as the adult pattern with both transmission and scanning electron microscopy. The study demonstrated that the fetal and neonatal mouse and rat urinary bladders undergo a series of rapid developmental changes and suggest that the fetal and neonatal urinary bladder may be a target organ susceptible to toxic insult.