Embryonic muscle development permits the study of contractile protein gene regulation during cellular differentiation. To distinguish the appearance of particular actin mRNAs during chicken myogenesis, we have constructed DNA probes from the transcribed 3' noncoding region of the single-copy alpha-skeletal, alpha-cardiac, and beta-cytoplasmic actin genes. Hybridization experiments showed that at day 10 in ovo (stage 36), embryonic hindlimbs contain low levels of actin mRNA, predominantly consisting of the alpha-cardiac and beta-actin isotypes. However, by day 17 in ovo (stage 43), the amount of alpha-skeletal actin mRNA/microgram total RNA increased more than 30-fold and represented approximately 90% of the assayed actin mRNA. Concomitantly, alpha-cardiac and beta-actin mRNAs decreased by 30% and 70%, respectively, from the levels observed at day 10. In primary myoblast cultures, beta-actin mRNA increased sharply during the proliferative phase before fusion and steadily declined thereafter. alpha-Cardiac actin mRNA increased to levels 15-fold greater than alpha-skeletal actin mRNA in prefusion myoblasts (36 h), and remained at elevated levels. In contrast, the alpha-skeletal actin mRNA remained low until fusion had begun (48 h), increased 25-fold over the prefusion level by the completion of fusion, and then decreased at later times in culture. Thus, the sequential accumulation of sarcomeric alpha-actin mRNAs in culture mimics some of the events observed in embryonic limb development. However, maintenance of high levels of alpha-cardiac actin mRNA as well as the transient accumulation of appreciable alpha- skeletal actin mRNA suggests that myoblast cultures lack one or more essential components for phenotypic maturation
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