(a) : Basement membrane (basal lamina) is a thin sheet of fibrous proteins that underlies and supports the cells of an epithelium, separating this from underlying tissue. Basement membranes are components of the extracellular matrix (= the viscous watery fluid that surrounds cells in animal tissue) and help to regulate passage of materials between epithelial cells and adjacent blood vessels. Each consists of a framework of collagen fibrils within which are glycosaminoglycans (mucopolysacc harides) and laminins, which are proteins that bind the basement membrane to neighbouring cells via cell adhesion molecules.