Molecular transitions accompanying growth of the axial musculature of Xenopus laevis.

In this paper we examine the expression of several cell adhesion molecules and muscle differentiation markers during larval development and growth of the axial musculature of Xenopus laevis. We have identified a small group of cells, located on the dorsal extremity of the myotome, which express very high levels of N-CAM and early muscle markers such as desmin and muscle myosin, but do not express later-stage markers of muscle development such as sarcomeric actinin. These cells are most probably muscle precursor cells; they may participate in the generation of new fibres in the myotomal muscles of Xenopus. On the basis of these observations we propose a model of myotomal development in Xenopus characterized by a spatial segregation between regions of the myotome where new fibres are generated and regions where myotubes mature. As during muscle differentiation in vivo we observe a sharp change in the profile of expression of cell adhesion molecules, we suggest that different adhesion receptors are involved in the generation of new muscle fibres and in the growth and differentiation of already existing ones.