The basement membrane is form of extracellular matrix that underlies all epithelia. It provides structural support to epithelia and forms a mechanical connection between epithelia and underlying connective tissue. The basement membrane also regulates the metabolism, proliferation, survival and differentiation of epithelial cells. The basement membrane functions as filter and because epithelia lack their own blood supply, all small molecules and gases derived from the blood must diffuse across the basement membrane. The basement membrane also prevents epithelial cells from invading into connective tissue.