HIGH-RESOLUTION ULTRASTRUCTURAL-STUDY OF THE RAT GLOMERULAR-BASEMENT-MEMBRANE IN LONG-TERM EXPERIMENTAL DIABETES

The ultrastructure of the glomerular basement membrane of the long-ter m diabetic and age-matched control rats was studied with the applicati on of advanced high-resolution microscopy. By using the freeze substit ution method for the preparation of the renal tissue, it was possible to observe that the glomerular basement membrane in control and diabet ic animals is composed of only a single lamina densa without the prese nce of a lamina lucida interna or externa. High-resolution electron mi croscopy of the diabetic glomerular basement membrane showed significa nt alterations in its morphology and ultrastructure. First, the baseme nt membrane in diabetic condition appeared to be split into two halves , endothelial and epithelial. In the epithelial half of the membrane, the network of distinct strands referred to as cords, which were clear ly present in the glomerular basement membrane of age-matched control animals, became less distinct and showed a diffused appearance being e venly replaced by thin filaments. The openings of the network were fil led with a granular material. In the endothelial half of the membrane, on the other hand, the cord network was variably lost in diabetic con dition and, within the resulting vacant spaces, bundles of fibrils 12 nm in width, identified as basotubules, were deposited. Immunolabeling for type IV collagen was found to be enriched in the endothelial half of the basement membrane being associated with the bundles of basotub ules. The ultrastructural changes reported by high-resolution microsco py could be related to the molecular alterations of the basement membr ane components and to the loss in permselectivity occurring during dia betes.