Changes in the Distribution of Collagen Types During Embryonic Development

The diversity in physiological and mechanical properties of connective tissues may in part be explained by the genetic heterogeneity in their constituting macromolecules which are predominantly proteoglycans, collagens, elastin and glycoproteins. Different combinations of these molecules can give rise to a variety of structural organizations and chemical features of the extracellular matrices. At present five genetically distinct types of collagen are chemically sufficiently characterized to allow their unequivocal identification in tissues or cell cultures. ’*’ In addition, data have been reported which indicate the existence of at least four other collagen Collagen types 1, I1 and 111, the socalled “interstitial collagens”, have in common a triple helical domain of about 300 nm, which requires a glycine residue in every third position in the amino acid sequence and a high content of proline and hydroxyproline. They differ by substitution of amino acids in approximately 30% of the residue^.^.' Type IV collagen or basement membrane collagen refers to collagenous proteins extracted from basement membrane containing tissues, which are rich in hydroxylysine and hydroxyproline and low in alanine.’ They differ from interstitial collagens by the sensitivity to pepsin treatment at various sites in the molecule, giving rise to discrete fragments in the range of MW 50 OOO, 70 OOO, 95 OOO, 140 OOO and 160 0002 and by the inability to form fibrils. So far two different subunits of type IV collagen have been described (cxl(1V) and cx2(IV) or C and D chain), however, the stoichiometry of the type IV collagen molecule is still a matter of discussion. Type V collagen resembles basement membrane collagens in its amino acid composition and has been localized in basal laminae,’ although its ability to form fibrils,”.” its physicochemical properties’’ and occurrence in mesenchymal tissue^'^ suggest a close relationship to interstitial collagens. Although the particular functions of the different collagen types are still unknown, the effort of many laboratories has been directed towards the identification and localization of collagens in the organism with biochemical and immunological tools. Our attention has been attracted by the dramatic changes in the distribution of collagen types which occur during embryonic development of many tissues and organs. A crucial role for collagen has been assumed in cell-matrix interactions during embryonic development. Based on experiments on the influence of collagen substrates on myoblast fusion in vitro, Hauschka and K0nigsbe1-g’~ proposed an active role for collagen in differentiation; similarly, Meier and Hay’’ reported the stimulation of corneal differentiation in vitro by collagenous substrates. In these cases a particular role of distinct collagen type could not