Ultrastructural characteristics of the fetal and neonatal rat urinary bladder.

The embryologic and neonatal development of the normal rat urinary bladder was investigated in Sprague-Dawley rats by light, transmission, and scanning electron microscopy from day 11 of gestation through 21 days of age. The epithelium at day 11 of gestation is composed of small, loosely-connected, rounded cells with occasional short microvilli on their surfaces. The large polygonal cells characteristic of the adult bladder begin to appear by day 15, but the microridges are not apparent until day 17. By day 20, the epithelium appears morphologically similar to the adult bladder. Several morphological features are observed at different times of gestation which are not seen in the normal adult bladder, but they have been found in bladder tumors. During days 12-15 of gestation, most of the luminal lining cells of the bladder epithelium have a single central cilium. Cilia are also occasionally seen at days 11, 16, and 17 of gestation. Occasional cells with long tentacles are present from days 13-16 of gestation. Cells that appear to form bridges between cells are also seen from day 14 of gestation and continue to be observed through day 11 after birth. No cells with distinctive pleomorphic microvilli, a feature of rapidly proliferating bladder epithelial cells in the hyperplastic or tumorous epithelium of the adult, were seen at any time during gestation or after birth. Small foci of superficial layer sloughing occurred at the time of birth, but were rapidly replaced by one day after birth. It is apparent from this study that the bladder epithelium is a rapidly changing, proliferating tissue in utero and continuing for a brief period after birth.