Characterization of free surface microprojections on the kidney glomerular epithelium

Scanning and transmission electron microscopy were used to evaluate free surface microprojections on the kidney glomerular epithelium in response to various experimental situations. Normally glomerular epithelial podocytes exhibit a rather sparse population of finger-like or bleb-like projections. Some investigations, however, have suggested that microvillous projections may increase in number during the differentiation of podocytes and in the aging kidney. With the onset of puromycin aminonucleoside induced nephrotic syndrome, podocyte microprojections become very knob-like and irregular in their shapes. In response to the in vitro environment, the number of podocyte microprojections increases dramatically. Neuraminidase removal of the sialic acid component of the glomerular glycocalyx and exposure to either cytochalasin B (25 microgram/ml) or cytochalasin D (2 microgram/ml), prevent the in vitro formation of podocyte microprojections. In vitro incubation in compounds which depolymerize cytoplasmic microtubules (e.g. vinblastine sulfate 10(-5); colchicine 10(-5) M), however, result in the formation of unusually long glomerular epithelial microprojections. Investigations utilizing the cationic ligand polycationized ferritin (PCF) have shown that podocyte microprojections are the most anionic sites on the free surfaces of these cells. During prolonged incubation of PCF treated glomeruli, the tips of podocyte microprojections are the last sites to shed the anionically bound PCF. It is suggested that the highly anionic sialic acid components of the glomerular free surface glycocalyx may play principal roles in the formation, maintenance and shapes of glomerular podocyte microprojections.