Glomerular Basement Membrane

The glomerular basement membrane (GBM) is formed at the boundary of the basal laminas of the podocytes of the glomerulus and the endothelial cells of renal capillaries. It is an acellular, extracellular matrix layer organized into three sublayers — the lamina densa, lamina rara interna, and lamina rara externa.The lamina densa is a relatively homogeneous extracellular matrix layer, whereas the laminae rarae is less dense and contains anionic sites. The GBM is covered by podocytes, epithelial cells whose extending foot processes interdigitate to create the slit diaphragm, a unique type of tight junction. The GBM and the interdigitating podocytes separate the capillary bed from the urinary space. The slit diaphragm maintains a specific pore size such that only molecules smaller than approximately 5000 daltons can traverse the filtration barrier and enter the urinary space.