Regulation of specific developmental fates of larval- and adult-type muscles during metamorphosis of the frog Xenopus.

During anuran metamorphosis, larval-type myotubes in both trunk and tail are removed by apoptosis, and only trunk muscles are replaced by newly formed adult-type myotubes. In the present study, we clarified the regulatory mechanisms for specific developmental fates of adult and larval muscles. Two distinct (adult and larval) types of myoblasts were found to exist in the trunk, but no or very few adult myoblasts were found in the tail. Each type of myoblast responded differently to metamorphic trigger, 3,3',5-triiodo-L-thyronine (T(3)) in vitro. T(3)-induced cell death was observed in larval myoblasts but not in adult myoblasts. These results suggest that the fates (life or death) of trunk and tail muscles are determined primarily by the differential distribution of adult myoblasts within the muscles. However, a transplantation study clarified that each larval and adult myoblast was not committed to fuse into particular myotube types, and they could form heterokaryon myotubes in vivo. Cell culture experiments suggested that the following two mechanisms are involved in the specification of myotube fate: (1) Heterokaryon myotubes could escape T(3)-induced death only when the proportion of adult nuclei number was higher than 70% in the myotubes. Apoptosis was not observed in any larval nuclei within the surviving heterokaryon myotubes, suggesting the conversion of larval nuclei fate. (2) Differentiation of adult myoblasts was promoted by the factor(s) released from larval myoblasts in a cell type-specific manner. Taken together, the developmental fate of myotubes is determined by the ratio of nuclei types, and the formation of adult nuclei-rich myotubes was specifically enhanced by larval myoblast factor(s).