Abstract
The ultrastructure of the glomerular filtration barrier was observed after cold dehydration in the adult and developing rat. In the adult, thin cytoplasm of the endothelial cells possessed fenestrations without a diaphragm. The glomerular basement membrane was composed of a single lamina densa with lamina rara externa and interna on both sides. The foot processes were connected with each other by filtration slits, and with the lamina densa by connecting fibers. The cell membrane of foot processes at the filtration slits and connecting fibers was counected with electron-dense materials on the cytoplasmic side. On day 5 after birth, the structure of the filtration barrier was immature, consisting of unfenestrated endothelium, 2 layers of laminae densae in the basement membrane, and undifferentiated podocytes without foot processes. By day 15, the filtration barrier had matured to the adult type. The maturing process began at the basement membrane. Outpockets of the glomerular basement membrane, which represented expanding sites of the basement membrane, were numerous on days 10, 15 and 20, then decreased in number on days 40 and 60. Of the two types of outpockets, the loop type was more numerous than the columnar type.
