Expression of myoblast and myocyte antigens in relation to differentiation and the cell cycle

Cell cycle parameters and expression of myoblast and myocyte antigens were investigated during exponential growth and during the differentiation phase of rat L8( E63 ) myoblasts by an integrated approach involving microspectrophotometry with DNA fluorochromes, [3H]thymidine autoradiography, and immunofluorescent staining with monoclonal antibodies. In addition to the majority of cells which are recruited into myotubes, two distinct populations of mononucleate cells were resolved in cultures of rat myoblasts undergoing differentiation. These mononucleate cells consist of (1) a population of proliferating cells with a prolonged G1 transit time; (2) a population of non-proliferating cells which remain arrested in G1 for more than 72 h. The latter group was examined with respect to the expression of two marker antigens recognized by two monoclonal antibodies: antibody B58 reacts with a macromolecular component present in undifferentiated myoblasts but not in mature myotubes, and antibody XMlb reacts with a muscle-specific isoform of myosin. All four possible combinations of expression of these antigens by single cells were found: B58 +XM1b -, B58 +XM1b +, B58 - XM1b -, and B58 - XMlb +. The implication of these findings with respect to the transition from the proliferative to the differentiative phase of myogenesis is discussed.