2015 年 36 巻 5 号 p. 331-341
The renal glomeruli in lower vertebrates display mesangium-like cells and matrices interposed between the capillary endothelium and the basement membrane, while those in mammals reportedly lack such interpositions except in pathological conditions. By combined scanning and transmission electron microscopic observations, the pericapillary mesangial tissues were comparatively analyzed in four vertebrate classes: mammals (rats and rabbits), reptiles (green iguanas), amphibians (bullfrogs), and teleosts (carps). The observations discriminated three types of pericapillary interposition. The first, acellular interpositions, occurred universally, with mammalians displaying rudimental ones. This tissue type corresponded with extracellular matrices held in subendothelial grooves which were supported by fine endothelial projections anchored to the basement membrane. In lower vertebrates these grooves constituted an anastomosed system of subendothelial channels that communicated with the mesangial region, to favor cleaning of the glomerular filter. The second, compound type was specific to reptiles and amphibians, affecting the entire capillary circumference in the latter. In this tissue type, fine mesangial processes—which accompanied considerable amounts of fibrillar matrices—were loosely associated with the endothelial bases, indicating their possible nature as a kind of myofibroblast. Occurrence of the third, cellular interpositions was confined to small incidental loci in mammalian and teleost glomeruli. This tissue type was mostly occupied by thick processes or main bodies of the mesangial cells that tightly interlocked their short marginal microvilli with corresponding indentations on the endothelial bases.