Distribution of intravenously injected cationized ferritin within developing glomerular basement membranes of newborn rat kidneys.

To label heparan sulfate proteoglycans and other strong anions within glomerular basement membranes (GBM) during assembly, cationized ferritin (CF), with a narrow isoelectric range of 7.7 to 8.2, was intravenously injected into newborn rats. Kidneys were then fixed and processed for electron microscopy at intervals ranging from 1 to 72 h after CF injection. One hour after injection, CF bound extensively to the lamina rara interna and externa of developing GBM and mesangial matrix and to tubular basement membranes (TBM). In double basement membranes of early stage glomeruli, large amounts of CF were also seen in central areas between the endothelial and epithelial basement membranes. In maturing-stage glomeruli, CF bound throughout interior regions of GBM outpockets projecting into the epithelial side of capillary walls as well as to the laminae rarae. Because in adult rats CF binds only to the laminae rarae, the abundant anionic sites seen here in newborns between double basement membranes and within GBM outpocket interiors may be subsequently neutralized or removed during the GBM assembly process. In addition to basement membranes, CF was also located intracellularly within endocytic vesicles and lysosomes of glomerular endothelial, mesangial, and epithelial cells 1 h postinjection. CF was also present in similar structures within the tubular epithelium. In contrast to these findings, CF was gradually lost from developing GBM 5, 15, and 24 h after injection and was essentially cleared from all GBM, mesangial matrices, and TBM after 48 h. Large CF aggregates were progressively accumulated within mesangial lysosomes, however. The transient binding of CF to GBM anionic sites seen here was most likely due to its endocytic removal by developing glomerular endothelial, mesangial, and epithelial cells. Anions in the circulation probably also competed effectively with the GBM and TBM for bound CF.