Development of the glomerular mesangium.

The development of the glomerular mesangium was studied in fetal and newborn rat kidneys by using a widefield electron microscope which can cover a whole glomerulus within one low-power viewfield. A three-dimensional observation of the immature glomeruli was done by performing ultrathin serial sectionings of the specimen for electron microscopy. Scanning electron microscopic observation of the developing glomeruli was also performed. The developmental distribution of contractile protein (actin) in mesangial cells and the main intrinsic component of the extracellular matrix protein (type IV collagen) of the mesangium were examined by immunohistological techniques. The widefield electron micrograph revealed a precise relationship between the mesangium and other components of the glomerulus. The results confirmed that the capillary extends into the S-shaped body from the surrounding vascular system at the initiation of nephronogenesis. The mesangial cells are always continuous to the vascular pericyte-smooth muscle cell system during the whole course of glomerular development and they participate in the subdivision of the capillary network during glomerulogenesis. Morphological findings and the changing distribution of intra- and extracellular proteins of the mesangium during development suggest that the mesangial cell differentiates from the primitive pericyte of the immature capillary.