Chimeras vs X inactivation mosaics: significance of differences in pigment distribution

Cell cultures of cardiac, pectoral, and thigh muscle of chick embryos synthesized myoglobin, as measured by incorporation of radioactive lysine detected by radioimmunoprecipitation. Liver and skin cultures, although active in protein synthesis, failed to demonstrate myoglobin synthesis. Puromycin inhibited myoglobin synthesis by the cell cultures. The electrophoretic characteristics of the myoglobin antigen synthesized by thigh and pectoral muscle were identical. Myglobin synthesizing progenitor cells attached to plastic dishes in 1 hr, but not completely in 0.5 hr. Cells, unattached at 0.5 hr, were enriched in myoglobin synthesizing cells. Incorporation of lysine-U-¹⁴C into myoglobin was maximal in confluent cultures and its increase paralleled the increase of cell fusion in the cultures. The ability of pectoral, white muscle to synthesize myoglobin in a manner equivalent to that of cardiac tissue was unexpected because of its failure to synthesize myoglobin in vivo and may indicate that factors in the whole organism may regulate the expression of this muscle cell's capabilities.