Analysis of the three dimensional structure of the kidney glomerulus capillary network

Abstract

The capillary network of the kidney glomerulus filters small molecules from the blood. The glomerular 3D structure should help to understand its function, but it is poorly characterized. We therefore devised a new approach in which an automated tape collecting microtome(ATUM) was used to collect 0.5 micron thick serial sections from fixed mouse kidneys. The sections were imaged by scanning electron microscopy at [~]50 nm / pixel resolution. With this approach, 12 glomeruli were reconstructed at an x-y-z resolution [~]10x higher than that of paraffin sections. We found a no-cross zone between afferent and efferent branches on the vascular pole side; connections here could allow blood to exit without being adequately filtered. Network analysis indicates that the glomerular network does not form by repetitive longitudinal splitting of capillaries. It also suggests that capillaries vary their diameter to make flow more efficient. The shortest path (minimum number of branches to travel from afferent to efferent arterioles) is relatively independent of glomerular size and is present primarily on the vascular pole size. This suggests that the shortest path is established on the vascular pole side, after which new branches and longer paths form on the urinary pole side. Thus the 3D structure of the glomerular capillary network provides useful information with which to understand glomerular function. Other tissue structures in the body may benefit from this new three dimensional approach.