Paracrine control of myoblast proliferation and differentiation by fibroblasts

Double-muscling (DM) is a hereditary (apparently single-gene) skeletal muscle hyperplasia which occurs in beef cattle. In order to investigate the cellular basis of this phenotype, cell cultures from developing muscle tissue of normal and DM fetal calves were studied. In cultures composed of both myogenic cells and nonmyogenic, fibroblast-like cells, DM myoblasts exhibited a prolonged proliferative phase. This resulted in delayed, but increased production of fused myotubes in the DM cultures. "Conditioned" media experiments indicated that the fibroblast-like cells in the cultures produced soluble myoblast growth factor activity. Both normal and DM fibroblast-like cells produced the growth factor activity, but the mutant fibroblast-like cells produced a greater level of such activity. The conditioned media failed to increase proliferation of bovine muscle fibroblasts and did not stimulate quiescent Swiss 3T3 cells to divide, indicating that the myoblast trophic activity is distinct from bFGF or PDGF. Also, the myotrophic activity present in the conditioned media acted in an additive fashion with saturating doses of bFGF and of IGF-1, suggesting that the activity is not due to either of these known myogenic growth factors. Both normal and DM fibroblast-like cells produced myoblast trophic activity when the cells were proliferating, but did not produce myotrophic activity when the fibroblasts were mitotically quiescent. These findings indicate that the proliferative state of the connective tissue cells in muscle may have a controlling influence on myoblast proliferation and differentiation during development.