Cell commitment and gene expression in the axolotl embryo.

In the axolotl embryo the somitic mesoderm passes through a reversible and then an irreversible phase of commitment with respect to its later differentiation into muscle. We show that the commencement of alpha-actin synthesis and the first appearance of thin myofilaments occur at the the same developmental stage as the transition between these phases. In intact embryos beta-and gamma-actin are made at all stages of development and in all tissues. alpha-actin, however, first appears at the late head process stage and is confined to the somites and tailbud, the regions of the embryo which later form the myotomal muscles. The thin filaments first appear near the myocoel of the somites in association with a novel organelle of mottled appearance and are quite distinct in structure from the epidermal microfilament networks found in embryos of the same stage. Cell fusion occurs somewhat later, although before the appearance of muscle striations in the light microscope. Well formed sarcomeres are not found until some time after the onset of motility. Presumptive somitic mesoderm isolated at a stage before the transition and cultured in a buffered salt solution for six days autonomously begins to synthesize alpha-actin and develop sarcomeres with the normal structure.