Differential expression and modulation of Thy-1 on myoblast clones.

In the preceding paper [J. S. Schweitzer, M. A. Dichter, and S. J. Kaufman, 1987, Exp. Cell Res. 172, 1] we demonstrated that expression of Thy-1 antigen by rat skeletal muscle myoblasts in culture is modulated by fibroblasts. Growth of myoblasts with myofibroblasts, or in medium in which fibroblasts have been grown, causes a sharp reduction in the percentage of myoblasts that express Thy-1 antigen on their membrane. In this paper we demonstrate that there are two populations of myoblasts that can be distinguished by their capacity to respond to the Thy-1-modulating factor (TMF). The majority of Thy-1-positive or -negative myoblasts grown at clonal densities are responsive to TMF. A second myoblast clonal type is Thy-1 positive and fuses to form contractile myotubes, but is insensitive to TMF and has a more fibroblast-like morphology. These clonal phenotypes are stable upon passage in vitro and may represent distinct subsets of the myogenic lineage.