A topographical (SEM) analysis of acellular glomerular mesangial matrix in situ

Normal kidneys from human and New Zealand white rabbits were made acellular by vascular perfusion with detergents. Cortical regions were dissociated from the central renal mass and further minced to 2 mm3 and fixed for TEM and SEM analyses. In an effort to visualize the internal histoarchitecture of glomerular basement membranes (GBM) and associated mesangial matrix (MM), some of the fixed samples were cryofractured prior to preparation for SEM observations. By this technique, the in situ MM exhibits a lacy network of fenestrated plates (septa) that separate and support peripheral glomerular channels. It seems possible that these structures may be intrinsically rigid and that their shape-preserving properties may be transmitted to the entire GBM. The matrix is not restricted to centrolobular zones, but extends throughout the glomerulus via a loose inner (endothelial-mesangial) layer of GBM. This filamentous layer is similar to MM in texture and surface characteristics and is often thrown into folds or trabeculae. In contrast, the outer (epithelial) surface of GBMs is more compact and smoothly contoured. Our SEM and correlative TEM studies indicate that the inner layer is continuous with MM septa distally and extends proximally to the glomerular vascular pole via arteriolar BMs where it reaches the extraglomerular interstitium. This establishes an extracellular morphological pathway from centrolobular zones to the polar cushion.