The glomerular filtration barrier of the kidney in seven vertebrates classes. Comparative morphological and histochemical observations.

In the mammalian kidney, the glomerular filtration barrier (GFB) is mostly composed of endothelial and epithelial cells with the glomerular basement membrane interposed. In lower vertebrates, extensive regions of the GFB consist of the endothelial and epithelial cells, each lying on its own basement membrane. These two basement membranes border the mesangium, which contains mesangial cells (surrounded by its own discontinuous basement membrane), microfibrils and collagens (cross-striated fibrils, anchoring fibrils, and 5 nm interfibrillar filaments). Occurrence and extension of these mesangial components decrease from fish to mammals. Anionic binding sites associated with the different structures of the GFB of all animal species have been demonstrated by using dyes such as Alcian blue (with or without addition of electrolytes), Ruthenium red or Safranine O. The surface coat of endothelial and epithelial cells, the laminae rarae (interna and externa) of the glomerular basement membrane, the laminae rara and diffusa (wherever distinct endothelial and epithelial basement membranes occurred) as well as the collagen fibrils show particles (with polycationic dyes) or filaments (with monocationic dyes). The mesangial microfibrils are usually well preserved and intensively stained.