CHANGES IN WHOLE-BODY THYROXINE AND TRIIODOTHYRONINE CONCENTRATIONS AND TOTAL CONTENT DURING EARLY DEVELOPMENT AND METAMORPHOSIS OF THE TOAD BUFO-MARINUS

This study describes changes in the thyroid hormones thyroxine (T-4) a nd triiodothyronine (T-3) in the toad Bufo marinus during development from early embryonic stages through metamorphosis. Both the total cont ent and concentrations of T-4 and T-3 were calculated to assess whethe r changes in these hormones might derive from changes in body mass or water content rather than from changes in hormone production or metabo lism. Three clutches of eggs were collected from ponds during the day following fertilization and raised through metamorphosis. Samples of f ive or more individuals were collected daily for thyroid hormone measu rement and body mass determination. Hormones were measured by radioimm unoassay utilizing miniature Sephadex columns after whole-body extract ion. Both T-4 and T-3 were elevated at gastrulation (1 day after ferti lization), but declined to low levels within the first week. This is t he first report that thyroid hormones occur in anuran embryos prior to thyroid differentiation. This suggests that thyroid hormones are depo sited in the anuran egg during oogenesis in a manner similar to that d escribed for teleost fishes and the domestic hen. Levels of T-3 and T- 4 began to rise at about 2 weeks after fertilization (stage 31; staged according to Limbaugh and Volpe, 1957) and peaked during early metamo rphic climax (stage 43). This pattern coincides well with the notion t hat thyroid hormones are central regulators of metamorphosis in B. mar inus as has been shown in other amphibians examined to date. Generally , both total content and concentration of the two hormones varied in p arallel. Overall, whereas individual mass and water changes may affect the magnitude of hormone changes, they appear to have little input in to their direction. a 1994 Academic Press, Inc.