Molecular sieve in rat glomerular basement membrane as revealed by negative staining

Human and bovine glomerular basement membranes (GBM) were previously shown to be a three-dimensional molecular sieve composed of pores and strands by negative staining and electron microscopy. In this study, rat GBM were isolated under several different conditions to rule out morphological changes due to isolation procedures. Rat GBM isolated under different conditions all showed the same morphological features as bovine and human GBM. The strands forming the molecular sieve were almost equal in width, measuring approximately 3.1 +/- 0.8 nm. Pores were oval or polygonal. The size of pores varied a little averaging 4.4 +/- 1.0 nm in the long dimension and 3.0 +/- 0.6 nm in the short dimension. The average density of the pores was 16 +/- 2/1,000 nm2. Negative staining demonstrated pores in isolated and unfixed GBM, indicating that the function of GBM is mechanical filtration of macromolecules on the basis of size.