A thin extracellular layer upon which an epithelial rests is called a basement membrane.
Basement membrane is a specialised form of extracellular matrix on which a layer of epithelial cells rests . The basement membrane is made up of a mixture of collagens, and glycoproteins such as perlecan, heparan sulphate and entactin. The basement membrane supports the overlying epithelium by preventing any association between the epithelial cells and the other cells present within the tissue. The basement membrane also functions as a molecular filter as it permit only water and tiny molecules to move through.
Although all cells have the same DNA only some parts of it are turned on depending on what the cell needs to do, so each cell turns on different parts of itself depending on what its job is.
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