Nephrotic tunnels in glomerular basement membrane as revealed by a new electron microscopic method

To clarify the ultrastructure in situ of the normal human glomerular basement membrane and ultrastructural changes of the glomerular basement membrane in patients with nephrotic syndrome, specimens of normal renal tissue and specimens from patients with membranous nephropathy, lupus nephritis, minimal change nephrotic syndrome, diabetic nephropathy, and Alport's syndrome were obtained. Specimens were examined by transmission electron microscopy by the newly devised "tissue negative staining method." Normal glomerular basement membrane showed a three-dimensional lattice-like meshwork of fibrils measuring 1.9 ± 0.4 nm in diameter that formed numerous uniform, round, oval, or polygonal pores 2.5 ± 0.4 nm in short diameter and 2.8 ± 0.5 nm in long dimension. The nephrotic glomerular basement membrane revealed varying degrees of ultrastructural defects, the most prominent being tunnels and cavities. Tortuous tunnels measuring approximately 15 to 50 nm in diameter penetrated the entire glomerular basement membrane. Cavities of various shapes measuring 15 to 200 nm in diameter were diffusely scattered in the glomerular basement membrane and occasionally aggregated to form a honeycomb structure that occupied the whole thickness of the glomerular basement membrane. These defects appeared to be the pathway for protein leakage.