Basement membranes: structural and biosynthetic considerations.

Basement membranes are extracellular matrices synthesized by a variety of cells including the basal cells of the epidermis; the respiratory, gastrointestinal, and glandular epithelium; the capillary endothelium; the epithelial cells of the glomerulus, the renal tubule, and the lens capsule; and the endothelium of Descemet's membrane. Basement membranes in the mature animal are free of lipids, DNA, and proteoglycans and are composed of dissimilar protein subunits. One of these is a procollagen-like molecule associated with a noncollagenous matrix glycoprotein. The proportion of the latter component varies among basement membranes. These various subunits are stabilized by hydrogen bonds, disulfide bonds, and aldehyde-derived cross-links which are so extensive that they render the basement membranes highly insoluble. Immunochemical studies indicate three distinct antigenic components which correspond to the collagenous moiety, its nonhelical extension, and the matrix glycoprotein. The collagen component of basement membranes, free of the nonhelical extension, is composed of three identical alpha-chains. It is highly rich in hydroxylysine, 3- and 4-hydroxyproline and contains 4 to 8 residues of half-cystine. It contains 38 residues of glucosyl-galactosyl-hydroxylysine per chain and minimal amounts of mannose, glucosamine, and fucose. Newly synthesized basement membrane collagen is secreted in the extracellular space as the precursor molecule "procollagen." This molecule does not undergo conversion to collagen but interacts with the matrix glycoprotein to give rise to the appropriate structure.