The expression and distribution of two cell adhesion molecules, N-cadherin and N-CAM, at the surface of cultured leg muscle cells from 11-day-old chicken

Embryonic cells in somites are first committed to become muscle precursor cells. These precursors migrate to invade the future muscle territories while they are continuing to divide. Then, during the process of terminal commitment and differentiation, myoblasts are withdrawn from the pool of dividing cells (Bishoff and Holtzer, 1968). The fusion of these post-mitotic, mononucleated myogenic cells to generate multinucleated syncitia called myotubes is achieved through a very complex series of differentiation steps during which a battery of muscle-specific genes is activated. Myotube formation is preceded by cell alignment (Nameroff and Manor, 1976; Wakelam, 1988), cell recognition and cell adhesion steps (Knudsen and Horwitz, 1977), and is achieved by the union of myoblast lipid bilayers (Kalderon and Gilula, 1979) involving modifications of the lipid content and of the fluidity of the plasma membrane (Prives and Shinitzky, 1977; Wakelam, 1988). Two adhesive systems coexist at the surface of myoblasts, one being Ca2+ dependent and the other Ca2+ independent (Gibraltar and Turner, 1985). At least two cell surface molecules, the neural cell adhesion molecule (NCAM) (Dickson et al., 1990; Knudsen et al., 1990a) and N-cadherin (Knudsen et al., 1990b), are implicated in myogenesis. Both molecules belong to a class of cell surface glycoproteins mediating cell-cell adhesion, currently designated as CAM (cell adhesion molecules, for review see Edelman, 1988; Anderson, 1990). Recently, an integrin, VLA-4, and its receptor, VCAM-1, have been shown to be also implicated in myogenesis (Rosen et al., 1992). N-CAM, one of the best characterized Ca2+ independent cell adhesion molecules, is a polymorphic, transmembranous glycoprotein of the immunoglobulin superfamily mediating cell adhesion by homophilic binding (Cunningham et al., 1987). N-CAM is expressed in somites and in myotomes (Thiery et al., 1982), then on myoblasts, myotubes and muscle fibers until innervation (Covault and Sanes 1985; Rieger et al., 1985). Blockade of N-CAMmediated adhesion of myoblasts in culture decreases fusion rate (Knudsen et al., 1990a) and conversely, overexpression of N-CAM in the C2 muscle cell line increases fusion rate (Dickson et al., 1990). N-cadherin (Hatta et al., 1988), a molecule similar or identical to the adherens junction-specific CAM (A-CAM; Volk and Geiger, 1986) and to the molecule Ncal-CAM (Crittenden et al., 1987) belongs to the cadherin family, a class of structurally related CAMs mediating Ca2+ depen897 Journal of Cell Science 103, 897-906 (1992) Printed in Great Britain © The Company of Biologists Limited 1992