An in vitro quantitative analysis of the response of tadpole tissue to thyroxine

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Abstract
Dorsal tail fins of tadpoles, (Rana pipiens) at various stages of metamorphosis, were cut into pieces or discs and then cultured either in Hanks balanced salt solution or Hanks containing L‐thyroxine (T4). Six concentrations of T4 were tested: 3, 9, 27, 81, 250, and 750 parts per billion (ppb). In addition, early prometamorphic discs were cultured in medium containing 3 ppb of T4 for different periods of time: 1, 3, 5, and 7 days and then placed into control medium for the remainder of the experimental period. Controls for this experiment were maintained continuously in either 3 ppb of T4 or in control medium. To test the specificity of response to T4 early prometamorphic discs were cultured in medium containing KI, DL‐thyronine or DL‐diiodothyronine. Discs respond to treatment with thyroxine by shrinkage. The degree of shrinkage was determined quantitatively by measurement of area.The results show that a direct quantitative relationship exists between concentration of thyroxine and resorption. Resorption is dependent upon the continuous presence of T4. It was also demonstrated that tail discs change in their sensitivity to thyroxine at progressive stages of metamorphosis. It is proposed that the increased sensitivity of tail tissues to T4 during metamorphosis results from their previous exposure to a low but rising level of thyroid hormone that increases sharply in the donor tissues one to two days before the emergence of the forelegs. Finally, it was demonstrated that these results represent the specific action of thyroid hormone.