The inhibitory effects of normal mouse serum on embryonic chick skeletal muscle cell differentiation, 1987

Normal mouse serum is known to exhibit inhibitory effects on various cell types. It has been shown that the administration of normal mouse serum to embryonic chick skeletal muscle cell cultures causes the accumulation of large amounts of Oil Red 0-positive lipid droplets, as well as the inhibition of normal differentiative processes e.g. cell fusion and spontaneous contractions. Protein synthesis remained linear, but somewhat reduced over a 5-h labelling period with [3H]-leucine following 24 h of in vitro culture. However, following repeated treatments over several days, the cells showed a significant decrease in protein synthesis when compared to differentiated controls. DNA synthesis by the serum-treated cells was significantly reduced by 24 h, prior to the removal of fused myoblasts from the proliferating pool, as seen in control cultures. In addition, there was a significant increase in the content of neutral lipids of treated cells. Studies employing the use of transmission and scanning electron microscopy and light microscopy, further show the extent of cellular alterations, including the aggregation of large numbers of nuclei irregularly distributed within the cytoplasm of treated myotubes, rounded and fibroblast-like cells with partially or totally occluded nuclei and an overall reduction in cell number following treatment. Due to the inhibition of the cell's normal ability to initiate spontaneous contractions, immunofluorescence studies, employing antibodies directed against myosin heavy chains and phalloidin which is specific for polymerized actin, were utilized to demonstrate the effects of mouse serum treatment on muscle- specific proteins. Fusion-inhibited cells showed diminished localization of antibodies of actin and myosin while fused myofibers exhibited an irregular banding pattern not characteristic of non-treated cultures. These observations indicate that the absence or disorganization of muscle-specific proteins in treated cells may be responsible for the lack of terminal differentiation