Metamorphic Changes in Anuran Muscles Following Thyroxine Treatment

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  • Metamorphic Changes in Anuran Muscles F

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The effects of thyroxine o n the abductor muscles of the mouth during metamorphosis were investigated in Bufo vulgaris japonicus larvae. Normal differentiation of the muscles is compared in detail with that following 1-thyroxine treatment, using one of levatores hyoidei muscles, namely, m. orbitohyoideus as a structural indicator. Within this muscle, new myoblasts have been shown to differentiate into the adult muscle, m. depressor mandibulae. while all the larval muscles disappear through a degenerative process. Although concentrations below 0.005 ppm had no effect on muscle differentiation, rather interesting developmental changes occurred at higher doses. Treatment with thyroxine above 0.005 ppm induced the accelerated appearance of newly formed adult muscle and retarded the degeneration of larval muscles. This simultaneous action produced an array of anomalies. The concentration of thyroxine required for normal metamorphosis was lower than is generally held, namely less than 1.5 times that of the thyroid hormone concentration in normal tissues. The findings are discussed in terms of current theories of t h yroid action. The authors suggest that metamorphosis is biphasic process involving both adult neogenesis and larval degeneration. The results indicate that thyroxine accelerates adult neogenesis to a greater extent than the degeneration of the larval tissues.

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