Basal lamina, a thin but tough and flexible membrane that is not made from cells purely. Basal lamina composed of laminin, type IV, XVIII collagen, nidogen, perlecan and fibronectin. They are normally found in bone, tendon and the dermal layer of the skin. It not only surrounds epithelial cells it also surrounds nonepithelial cells. Its primary function is to separate cells from the surrounding connective tissue. Besides having a simple structural function it also has filtering roles in the kidney glomerulus. They are able to determine cell polarity, regulating cell metabolism, protein organization in adjacent plasma membranes, cell survival and etc..