RA regulation of keratin expression and myogenesis suggests different ways of regenerating muscle in adult amphibian limbs.

Formation of a regeneration blastema following limb amputation is believed to occur through a process of dedifferentiation. It has been suggested, however, that the cells contributed to the blastema by the stump muscle are satellite-like cells, rather than cells originated by dedifferentiation. We have previously shown that simple epithelial keratins 8 and 18 are expressed in the mesenchymal progenitor cells of the regenerating amphibian limb and in cultured cells with myogenic potential, and that their expression appears to be causally related to changes in proliferation and differentiation. We show here that retinoic acid (RA) affects the expression of these keratins differently in myogenic cells originated from normal limb and limb blastema. Furthermore, we find that the effects of RA on proliferation, myogenic differentiation and adhesion of these cells also differ. In fact, whereas RA does not affect keratin expression, proliferation or myogenic differentiation in blastemal cells, it does decrease keratin levels and thymidine incorporation and increase myogenesis in cells from normal limb. Conversely, RA increases cell adhesion only in blastemal cells. Significantly, these effects of RA on cultured cells are consistent with those observed in vivo. Overall the results presented here suggest that in the urodele limb there are two distinct cell populations with myogenic potential, one originating from dedifferentiation and one equivalent to the satellite cells of the mammalian muscle, which are likely to be primarily involved in blastema formation and muscle repair, respectively.