Pathobiochemistry of connective tissue

Abstract

Connective tissue is a complex system consisting of cells, fibers and interstitial mucoproteins and glycoproteins. Among fibers collagen is the most important. Several genetically distinct types of this structural protein are known which are specific for various tissues. Their synthesis includes numerous steps beginning with the formation of procollagen peptide chains which subsequently are modified by hydroxylation and glycosylation and are twisted to triple-helical structures. Removal of terminal globular domains is required for assembly of monomers to fibrils which later on are stabilized by covalent cross-linking. Several inherited diseases like dermatosparaxis or special forms of Ehlers-Danlos syndrom are due to the inability of performing one of these steps. In addition, incomplete processing of collagen can be the result of malnutrition e.g. lack of ascorbic acid, copper deficiency or lathyrogens in the food.

Far more important among connective tissue diseases is an excess of fiber formation in soft tissues. Fibrosis of liver and lung, probably, is not the result of an increased collagen synthesis but of an increased conversion of soluble collagen precursors to fibrils. Unless fibrils are formed the soluble collagen derivatives are removed by proteolytic digestion. Regulation of fibrillogenesis, however, is little understood and, evidently, involves the participation of glycosaminoglycans and glycoproteins. One model assumes that collagen precursors deposit on preformed fibronectin fibrils constituting the pericellular matrix of fibroblasts and other adherent cells. The expression of this pericellular matrix, on the other hand, appears to be regulated by glycosaminoglycans on the cell surface and the near environment. It varies with the cell cycle, the cell density and the proliferation activity of the cells and is influenced by several drugs. Following establishment of collagen fibers the fibronectin guide-lines, evidently, are removed.

The cell environment also influences the type of collagen expressed. Chondrocytes producing collagen type II, under special conditions, convert to fibroblast-like cells synthesizing type I and type III. Simultaneously the kind of glycosaminoglycans produced is changed.