The requirement for RNA synthesis in the differentiation of cultured chick embryo chondrocytes

Secondary chick embryo sternal chondrocyte colonies, derived from differentiated clones, were treated with 0.005 μg/ml of actinomycin D for 18 hours. While RNA synthesis was inhibited by 70%, both growth and matrix accumulation were inhibited. The incorporation of 3 H‐uridine into acid precipitable‐base hydrolyzable material, 35 SO 4 into hyaluronidase‐sensitive material, and of 3 H‐proline into collagenase sensitive and insensitive material was measured in high density cultures treated with various doses of actinomycin D for 18 hours. The inhibition of sulfate incorporation was highly correlated with RNA synthesis and was observable within the first six hours after drug addition. Ethidium bromide and lucanthasone had no effect on sulfate incorporation. On the other hand, collagen synthesis was more resistant to actinomycin D treatment and was not inhibited until non‐collagenous protein synthesis was also inhibited. The results are discussed in terms of the independent regulation of the synthesis of two specialized cell products made by the same cell.