Intrinsic fibrillar components of human glomerular basement membranes: a TEM analysis following proteolytic...

At the level of ultrastructure, basement membranes (BMs) are usually described as thin layers of extracellular matrix comprised of an interwoven mat of fine 3-4 nm fibrils embedded in a granular matrix. In order to improve the resolution of the fibrillar components, we have carried out TEM studies on human glomerular BMs (GBMs) made acellular by sequential detergent solubilization. Some GBMs were pretreated with pronase, trypsin, or pepsin for 30 min to 72 h prior to preparation for microscopy. Our study shows that irrespective of which enzyme is employed, background granular matrix is first solubilized leaving a three dimensional fibrillar network comprised of 3-8 nm fibrils. Larger 7-8 nm fibrils are concentrated near subepithelial portions of the GBM and are most resistant to proteolysis. Smaller 3-4 nm fibrils are located primarily subjacent to endothelium and mesangial cells and are more protease-sensitive. An unexpected finding in pepsinized samples was a quasi-hexagonal fibrillocrystalline structure associated with mesangial matrix and subendothelial portions of the GBM. These data suggest that intrinsic fibrillar components of human GBMs are heterogeneously distributed throughout the thickness of their laminae densae. We speculate that the network consists of type IV collagen and that the hexagonal crystals may represent type VI or some previously unreported BM collagen type.