The quantitative development of glomerular capillaries in rats with special reference to unbiased stereological estimates of their number and sizes

The kidneys of 14 female Wistar rats from 5 days to 18 months postnatally have been perfusion-fixed to study the development of glomerular capillaries using design-based stereological techniques. This report uses a topological definition of a capillary structural unit which forms the basis for the estimation of the number of glomerular capillaries using histological sections. The number of mature glomeruli is about 27,500 for all animals older than 5 days, whereas the 5-day-old animals have 21,100 mature glomeruli. Postnatally, the mean glomerular volume increases rapidly for the first 2 months after which it increases at a much slower rate. At first there is a rapid increase in total number, length, and surface area of glomerular capillaries with age; after 2 months the increase is rather moderate. The rapid growth of glomerular capillaries is anisomorphic, and the restrained growth is roughly isomorphic. The mean length of glomerular capillaries is nearly constant at 56 microns in mature and immature rats. The cross-sectional area rises only twofold from the most immature to the oldest rats. Using an approximation of the geometric factor in the law of Poiseuille, a decreasing resistance of glomerular capillaries with age is obtained. This is in agreement with the falling resistance of maturing renal vasculature and the increasing single-nephron glomerular filtration rate. When taking into account the constant mean capillary length and moderate increase in cross-sectional area of glomerular capillaries in growing rats, it appears that the branching of capillaries, forming a complicated network of serial and parallel connections, seems to be the structural basis for normal glomerular capillary growth.