THE EMERGENCE OF AN ADULT MUSCLE PHENOTYPE IN URODELAN AMPHIBIANS - AN IMMUNOHISTOCHEMICAL STUDY

Electrophoretic techniques adapted for the analysis of muscles of lowe r invertebrates reveal four myosin heavy chain isoforms in the dorsali s trunci of Pleurodeles waltlii: two fast (MHC-IIA, MHC-IIB), and one slow (MHC-I) in the adult and one isoform (MHC-La) in the larvae. Poly clonal antibodies were prepared against the larval (anti-MHC-La) and o ne of the fast myosin (MHC-IIA) isoforms and their specificity was con firmed by western blot analysis. An immunohistochemical analysis was t hen carried out on frozen sections of the dorsalis trunci of P. waltli i at different stages of development. From stage 44 it was possible to demonstrate the presence of MHC-IIA in the small diameter fibers at t he periphery of the muscle; the number and diameter of these fibers in creased from stage 44 to stage 56 when anatomical metamorphosis had fi nished. By stage 56 these fibers could also be readily identified usin g standard histochemical techniques as type IIA fibers. We conclude th at fast IIA myosin is expressed well before the final adult muscle phe notype has been established and its expression is therefore independen t of thyroid hormone.