Ultrastructural assessment by colloidal iron of the distribution and localization of anionic sites in human...

The distribution of anionic sites within glomerular basement membranes from patients with different glomerulonephritides was examined by the dialyzed colloidal iron (DI) staining technique. The correlation of ultrastructural findings with glomerular disease, renal function, and degree of proteinuria revealed three alterations: 1) speckled DI staining in the lamina densa of patients with decreasing renal function and a proteinuria of greater than 1 g/24 hours; 2) an apparent staining disparity and diminution of DI at the lateral borders of swollen and retracted foot processes with inclination of the foot processes in the direction of the more weakly staining lateral border; and 3) heavy DI reaction on the apical of free surfaces of fused foot processes. Human subjects with a proteinuria of more than 1 g/24 hours display optimal labeling of the endothelial fenestrae, endothelial cell coat, lamina rara interna, and lamina rara externa. The staining observed may be explained either on the basis of direct DI interaction with diffusing plasma proteins secondary to a decrease in the transglomerular charge barrier consequential to a loss of intrinsic anionic sites or on the basis of "unmasking" of anionic moieties within the glomerular basement membrane. Regardless of the mechanism involved, the present study indicates that a threshold proteinuria of 1 g/24 hours is needed to effect staining in the lamina densa.