Background: Diabetic nephropathy is invariably associated with proteinuria.
Experimental design: To delineate the mechanism(s) of proteinuria in diabetic nephropathy, ultrastructural changes of the glomerular basement membranes (GBMs) were studied by high resolution scanning and immunoelectron microscopy. Acellular glomeruli from diabetic and age-matched control human subjects were prepared by detergent method and subjected to conductive staining, the technique in which the tissues are impregnated with metals rather than surface-coated with metallic alloys for visualization by electron microscopy. Subsequent to conductive staining, the tissues were examined by in-lens field emission scanning electron microscopy.
Results: Thirty glomeruli, each from the control and diabetic groups, were examined by scanning microscopy. In diabetic GBMs, a loose meshwork structure consisting of numerous pores of approximately 8 nm in diameter and distinct strands was observed. In contrast, meshwork structure was not readily discernible in controls and a few pores were observed. Five glomerular capillary loops, each from control and diabetic groups, were examined by immunoelectron microscopy. In controls, heparan sulfate-proteoglycan was localized in the lamina rara interna and externa, and type-IV collagen was distributed throughout the whole width of the GBM. In diabetic GBMs, a relative loss of staining of heparan sulfate-proteoglycan, both in the lamina rara interna and externa of the GBM, was observed. Type IV collagen was distributed in all layers of the thickened GBM, and the absolute number of the immunogold particles was increased. However, immunogold particle density of type IV collagen per unit area was decreased as compared with the control.
Conclusions: These ultrastructural and immunoelectron microscopic changes in the GBM may explain the loss of charge as well as size selectivities of the glomerulus, as observed in diabetic nephropathy associated with proteinuria.