In amphibia, as in many other animals with free-swimming larvae, muscle is one of the first differentiated cell types to be formed in early development. In Xenopus, the first contractions of axial body muscle take place about 30 h after fertilization, but genes required to form muscle are activated long before this, during gastrulation. Muscle actin proteins are first seen to be synthesized at the early neurula stage (Sturgess et al. 1980). More recently Mohun et al. (1984), using cloned cDNA probes, have found that cardiac actin, the type of muscle actin characteristic of adult heart, is a major component of the larval axial muscle. Xenopus cardiac actin gene transcripts are detected by S1 nuclease and Northern analysis at the early neurula stage (Mohun et al. 1984), and the use of SP6 probes on poly(A)+ RNA enables cardiac actin transcripts to be seen as early as the midgastrula stage (Cascio & Gurdon, 1986).

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