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( Basement membrane)
The basement membrane is a structure that supports overlying epithelial or endothelial cells. The basement membrane consists of an electron-dense membrane called the lamina densa, about 30–70 nanometers in thickness, and an underlying network of reticular collagen (type III) fibrils (its precursor is fibroblasts) which average 30 nanometers in diameter and 0.1–2 micrometers in thickness. This type III collagen is of the reticular type, in contrast to the fibrillar collagen found in the interstitial matrix.[1][2] In addition to collagen, this supportive matrix contains intrinsic macromolecular components. The Lamina Densa (which is made up of type IV collagen fibers; perlecan (a heparan sulfate proteoglycan)[3] coats these fibers and they are high in heparan sulfate) and the Lamina Lucida (made up of laminin, integrins, entactins, and dystroglycans) together make up the basal lamina. Lamina Reticularis attached to basal lamina with anchoring fibrils (type VII collagen fibers) and microfibrils (fibrilin) is collectively known as the basement membrane.[4] The primary function of the basement membrane is to anchor down the epithelium to its loose connective tissue underneath. This is achieved by cell-matrix adhesions through cell adhesion molecules (CAMs).
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