Segregation of Ferritin in Glomerular Protein Absorption Droplets

Ferritin was used as a tracer to study the mechanism by which proteins are segregated into droplets by the visceral epithelium of glomerular capillaries. In glomeruli from both normal and aminonucleoside-nephrotic rats ferritin molecules introduced into the general circulation penetrated the endothelial openings and were seen at various levels in the basement membrane. Striking differences between nephrotic and controls were seen only in the amount of ferritin incorporated into the epithelium. In normal animals, a few ferritin molecules were seen in small invaginations of the cell membrane limiting the foot processes, within minute vesicles in the epithelium, or within occasional large vacuoles and dense bodies. In nephrotics, epithelial pinocytosis was marked, and numerous ferritin molecules were seen within membrane invaginations and in small cytoplasmic vesicles at all time points. After longer intervals, the concentration of ferritin increased in vacuoles and particularly within the dense bodies or within structures with a morphology intermediate between that of vacuoles and dense bodies. In nephrotic animals cleft-like cavities or sinuses were frequently encountered along the epithelial cell surface facing the urinary spaces. Some of these sinuses contained material resembling that filling the dense bodies except that it appeared less compact. The findings suggest that ferritin molecules—and presumably other proteins which penetrate the basement membrane—are picked up by the epithelium in pinocytotic vesicles and transported via the small vesicles to larger vacuoles which are subsequently transformed into dense bodies by progressive condensation. The content of the dense bodies may then undergo partial digestion and be extruded into the urinary spaces where it disperses. The activity of the glomerular epithelium in the incorporation and segregation of protein is similar in normal and nephrotic animals, except that the rate is considerably higher in nephrosis where the permeability of the glomerular basement membrane is greatly increased.