Mesangial fenestrations, sieving, filtration, and flow.

Small tracers in the circulation enter the rat mesangium rapidly and in large amounts that indicate a sizable plasma flow into the mesangium. Entrance is effected through mesangial fenestrations with a mean width in scanning electron microscopy of 376 A, a size similar to fenestrations in peripheral glomerular capillary walls. This is considerably smaller than the mean size of 678 A found with transmission electron microscopy, but the difference is probably due largely to the anionic surface coat on endothelial cells. Measurements of asymmetric thorium dioxide particles show that smaller ones with a mean length of 315 A enter the mesangium preferentially and that larger particles with a mean length of 405 A are partially restricted, supporting the idea that the measured width in scanning electron microscopy is close to the actual width in vivo. Fluid flow into the mesangium requires fluid flow out. The appearance time and accumulation of tracers suggest the following exit paths of flow from the mesangium: through the overlying epithelium into the urinary space contributing to glomerular filtration and concentrating large tracers beneath the basement membrane in the paramesangial region, into the efferent glomerular capillaries after tracers have been filtered out by the fibrillar matrix, and through the hilus into the juxtaglomerular apparatus (quantitatively small).