Molecular Sieve in Renal Glomerular and Tubular Basement Membranes as Revealed by Electron Microscopy

Abstract

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.

Molecular Sieve in Rat Tubular Basement Membrane as Revealed by Negative Staining

“…In our earlier studies [19][20][21], we demon strated the fine meshwork structure in GBM which previously had been postulated by physiologists [1,24] and indirectly inferred from tracer experiments [5,7]. It now seems clear that TBM has a similar meshwork structure which may play an important role as reabsorption barrier to proteins.…”

“…Fragments with larger pore sizes showed more variation within each frag ment. The real size might differ from these measured values because, as discussed be low, the shape of pores seen on a micrograph is a projection of a three-dimensional frame work to a piece of film, and the larger value obtained may correspond to the larger di mension of an obliquely sectioned channel as discussed in the previous paper [21]. An interesting finding is shown in figure 5.…”

“…These findings are almost the same as those for G B M . In earlier reports [19][20][21], electron microscopy after negative staining showed that G B M of human and bovine kidneys is composed of a fine meshwork. In a brief note, we reported recently that rat TBM is also made up of a fine meshwork [15].…”

High-Resolution Ultrastructural Comparison of Renal Glomerular and Tubular Basement Membranes

“…Ultrastructures of proximal and distal TBM are likely to differ, because functions of the tubules are different in each segment. In purified rat TBM, we previously have demonstrated a fine meshwork structure similar to that of the GBM [7,11]. Pores in the meshwork of the TBM were larger than in the GBM.…”

“…Electron microscopic observations of purified unfixed bovine, rat and human GBM by conventional negative staining, revealed that a threedimensional meshwork GBM structure consists of fibrils forming pores with diameters slightly less than those of albumin molecules [5][6][7]. This fine meshwork structure is considered to be a critical size barrier impeding ultrafiltration of serum protein molecules in the glomerulus [8][9][10].…”