Tenascin and thrombospondin belong to the growing

Tenascin and thrombospondin are multimeric glycoproteins with restricted distributions in the embryonic extracellular matrix (for review see Chiquet-Ehrismann, 1991; Erickson and Bourdon, 1989; Frazier, 1991; Sage and Bornstein, 1991). Immunohistochemistry has revealed that tenascin and thrombospondin are particularly abundant in the developing central and peripheral nervous systems as well as in areas of connective tissue morphogenesis (Grumet et al., 1985; Mackie et al., 1987; Mackie et al., 1988; O’Shea and Dixit, 1988). In vitro studies have demonstrated that these glycoproteins provide a favorable substratum for promoting neurite outgrowth and the motility of a variety of cell types (Chuong et al., 1987; Halfter et al., 1989; Wehrle and Chiquet, 1990; O’Shea et al., 1991; Neugebauer et al., 1991). As some cells fail to form focal adhesions on tenascin and thrombospondin, and the addition of these glycoproteins can cause cells that have already spread on adhesive glycoproteins to lose their strong attachments (ChiquetEhrismann et al., 1988; Spring et al., 1989; Halfter et al., 1989; Lotz et al., 1989; Murphy and Höök, 1989; MurphyUllrich et al., 1991), it is generally believed that both tenascin and thrombospondin may promote cell motility by acting as anti-adhesive extracellular matrix components. Three alternative splice variants of tenascin have been identified in the chick. The largest (Mr 230×103) splice variant has 11 fibronectin-type III repeats, whereas the smaller forms (Mr 200×103 and 190×103) have 9 or 8. Immunohistochemistry with splice variant-specific antibodies has shown that the high Mr form of tenascin is found in a subset of the matrix stained with tenascin antibodies that recognize all tenascin forms (Matsuoka et al., 1990; Kaplony et al., 1991), and high Mr tenascin was shown to be less resistant to proteolysis than the low Mr variants (Chiquet et al., 1991). These data indicate that different tenascin variants may have distinct functions and turnover rates in the embryonic extracellular matrix. In contrast to tenascin, which is encoded by a single gene, different forms of thrombospondin are encoded by two or 347 Development 117, 347-358 (1993) Printed in Great Britain © The Company of Biologists Limited 1993