A thin sheet of fibrous proteins that supports the cells of an overlying epithelium or endothelium, separating this from underlying connective tissue. Such membranes also surround muscle cells, Schwann cells, and fat cells, and a thick basement membrane is found in the kidney glomerulus, where it acts as a filter (see ultrafiltration). Basement membranes are components of the extracellular matrix and help to regulate passage of materials between epithelial cells and adjacent blood vessels. Electron microscopy reveals that a basement membrane consists of two principal layers: an electron-dense basal lamina, typically about 30–70 nm thick, which is secreted by the epithelial or endothelial cells and consists largely of the protein laminin; and an underlying reticular lamina, which comprises collagen fibrils and other components and is secreted by connective tissue cells. The laminins bind the basement membrane to neighbouring cells via cell adhesion molecules.