Molecular sieve in renal glomerular and tubular basement membranes as revealed by electron microscopy

Bovine glomerular basement membrane (GBM) was isolated and purified according to a modification of Spiro's method. Rat and bovine tubular basement membranes (TBM) were isolated and purified by sonic disruption or by the method of Carlson et al. Electron microscopic studies on the ultrastructure of GBM and TBM were performed after negative staining with 1% phosphotungstic acid solution, pH 7.3. When negatively stained, GBM and TBM were seen as fragments varying in size. The surface of the membranes showed a characteristic felt-like or spongy appearance. At higher magnification, GBM and TBM showed a fine meshwork composed of strands and pores which three-dimensionally resembled a crystal lattice. Pores were fairly uniform in size and shape. They were round, oval or polygonal in shape. Some of the pores were elongated to form short straight or bent channels. Strands were also uniform in diameter and surrounded a pore or channel. For an average of 50 pores, the long dimension was 3.1 +/- 0.6 nm and the short dimension 2.5 +/- 0.3 nm in bovine GBM, 3.8 +/- 1.2 and 2.5 +/- 0.7 nm in bovine TBM, and 4.9 +/- 1.5 and 2.8 +/- 0.6 nm in rat TBM, respectively. The strand was 1.8 +/- 0.3 nm in diameter in bovine GBM, 2.5 +/- 0.6 nm in bovine TBM and 3.7 +/- 0.7 nm in rat TBM for an average of 50 strands. The diameters of the pores were less than or close to the short axis of an albumin molecule. It was concluded that renal GBM and TBM were molecular sieves composed of pores and strands.