Larval-to-adult conversion of a myogenic system in the frog, Xenopus laevis, by larval-type myoblast-specific control of cell division, cell differentiation, and programmed cell death by triiodo-L-thyronine

For the clarification of larval-to-adult muscle conversion, the authors est ablished primary culture methods for adult- and larval-type myoblasts in th e frog, Xenopus laevis, and examined the hormonal response in each case. Th e cell types were enzymatically dissociated from adult frog leg and tadpole tail muscles, respectively. The cells became attached to culture plates, p roliferated, and fused with each other to form multinucleated myotubes with in one week. Five significant differences between the two cell types were n oted. (1) Adult cells showed greater proliferation activity than larval cel ls, the former increasing 5.5-fold over 6 days while the latter increase on ly 2.5-fold. (2) Differentiation (fusion) of larval type myoblasts started earlier. Cell fusion began on day 2 or 3 in larval cells and on day 4 in ad ult cells. (3) The metamorphic hormone, triiodo-L-thyronine (T-3) decreased larval cell numbers to 56% of that of control-cultures on day 7 but had no effect on adult cell number. DNA synthetic activity (H-3-thymidine incorpo ration) in larval cells decreased under T-3 (10(-8)M) to 45% of the control level on day 7. (4) Differentiation of adult myoblasts into myotubes was p romoted by T-3, whereas that of larval cells diminished by half. (5) Myotub e death was induced by Tg specifically in larval but not in adult cultures. In addition to the myotube death, double staining with TUNEL (in situ DNA nick end labeling) and anti-desmin antibody indicated that T-3 induces myob last (desmin(+) cell) death specifically in larval but not in adult cells. It is thus evident that the conversion of a larval-type myogenic system dur ing metamorphosis becomes possible through nearly totally specific control of cell division, cell differentiation, and programmed cell death at a prec ursor cell level by T-3.