It has long been appreciated that, for many types of cells, attachment to a substrate is required for their replication. Collagen substrates enhance the growth (64,70,109,238) as well as the differentiation (40,69,85,86,146,161,184), of many cells in culture above that observed with other substrates such as plastic and glass. As discussed in this report, many cultured cells including fibroblasts, myoblasts, hepatocytes, chondrocytes, and certain epithelial cells are thought not to bind directly to the collagen substrate or to the plastic surface of culture dishes (31,73,74,76,77,89,93,113,164,220) . Instead, extracellular glycoproteins bind the cells to the substrate. One of these attachment proteins, fibronectin, has been extensively studied (99,151,166,227,228,244) . Fibronectin is produced by fibroblasts (13,197,246) and endothelial cells (102, 140) as well as some other cell types . It is also present in high concentrations in blood and serum (152) . In culture, serum fibronectin, as well as that produced by the cells, can bind to both the collagen substrates and the tissue culture plastic surface and mediate the attachment of cells (73,113,164,242) . Circulating fibronectin participates in a variety of reactions important to wound healing, including the adhesion and spreading of platelets on collagen (10, 14, 75, 100, 204), binding to the fibrin clot (74,150,198) and to collagen (37,46,94,103,119), promoting opsonization reactions by phagocytic cells (18,94,201), and promoting fibroblast migration (6, 61) . Thus, fibronectin not only functions as an attachment protein, but also may play an important role in repair reactions .In the case of certain differentiated cells, such as chondrocytes (89) and epithelial cells (159, 220), glycoproteins different from fibronectin are active in attachment. The components of these extracellular matrices of fibroblasts, chondrocytes, and epithelial cells differ with the cell type, and they require separate attachment proteins to provide additional specificity to the interaction of the cell with its matrix. Alterations in cellsubstratum interactions are observed in differentiating cells and after spontaneous transformation of cells or exposure to oncogenic agents (99,228) . This review will briefly outline our current knowledge of the types of collagen and their distribution and biosynthesis. Then, the interaction of cells with col-REVIEW
