Glomerular basement membrane outpockets and glomerular growth in the postnatal development of the rat kidney.

Distribution of glomerular basement membrane (GBM) outpockets and dimensional growth of glomeruli were studied in the maturing stage of rat glomeruli after completion of nephrogenesis. We observed the postnatal rat glomeruli from 5 to 60 days of age by transmission electron microscopy and estimated the structural development of glomeruli by computerized morphometry. On day 5, the GBM was double in structure, possessing an epithelial and endothelial lamina densa. After day 10, the lamina densa of the GBM was single and sent branches toward the epithelial side making outpockets. There is no change in the distributional pattern of the outpockets, at least from day 10 to day 60, although they decreased considerably in number between days 20 and 40. They were found almost exclusively on the peripheral surface of the glomerulus. The rat glomeruli increased in volume constantly in this period, and the capillary volume increased more significantly than the mesangial volume. The GBM surface area increased in parallel with the glomerular tuft volume. The growing mode of capillaries was different before and after day 40; namely before day 40 elongation was predominant, whereas after day 40 widening was more pronounced. These results indicate that if the outpockets are the other site of GBM assembly after fusion of double basement membranes, the GBM must be redistributed from the peripheral to the paramesangial site to enable elongation and branch formation of capillaries during the growth of glomeruli.