[Arterial subendothelial structures: anatomy, biochemistry, functions]

This review summarizes the main structural and biochemical features of the fibrillar constituents of the subendothelial layers of the arterial wall. Several constituents are directly identified by various histochemical methods and electron-microscopic studies. (1) The microfibrils (MF), stained by tannic acid, cationic stains such as ruthenium red, and various peroxidase-labeled lectins are mostly found in association with elastin within the internal elastic lamina (IEL). They have been characterized by chemical analysis as acidic glycoproteins, hydrolyzed by a variety of proteases, but resistant to collagenases. The endothelial cells seem to participate in their biosynthesis. (2) Elastin (El), which is the main constituent of the IEL, forms a wide, concentric, electron-lucent, tannic-acid-stainable zone. Fibrous El results from the association of tropoelastin (or proelastin) molecules by intermolecular cross-linkage. During the elastigenesis, this cross-linkage occurs directly between tropoelastin molecules which have been previously sterically oriented by the MF probably synthetized by the same cells (smooth muscle cells and possibly endothelial cells). (3) Interstitial collagen forms sparse fibers characterized by their cross-striation (with a periodicity of 640 A). They are relatively resistant to most proteolytic enzymes, except collagenases. They result from the intermolecular cross-linkage of rigid molecules, resulting themselves from the intramolecular cross-linkage of three helical alpha chains as a triple helix. The interstitial subendothelial collagen has been identified by indirect immunofluorescence as a type III collagen. The same technique has also been used to detect type IV collagen and fibronectin. This glycoprotein could play a role in the attachment of the endothelial cells to the fibrillar network of the subendothelium, despite an affinity which is greater toward denatured collagen than toward native collagen. One of the most important functions of the subendothelium is its role in thrombogenesis, in which both MF and collagen are involved. In type III collagen, this property is linked to the preservation of an ordered structure in which a 9-amino acids fragment, localized in the central part of each chain, could bear an adhesion site.