Reduction in the latent period of the response to thyroxin by tadpole tail discs fused to discs pretreated with thyroxin.

It has been previously shown that tissue blocks prepared from the tail fins of tadpoles (Rana pipiens) survive in vitro for several weeks and respond to thyroxin by shrinkage after a latent period of four or more days (20°C). It has been postulated that this shrinkage corresponds to that of normal tail tissue during metamorphic climax. The long latent period of thyroid action is presumed to depend upon the time required to form the necessary cellular or biochemical intermediates. If such intermediates are activated by thyroxin in the amphibian tail tissue, it may be possible to shorten the latent period of the thyroxin response of fin tissue by fusing untreated blocks to blocks previously treated with thyroxin. Experiments are reported here in which tail fin blocks obtained from tadpoles previously immersed in thyroxin for 3 days were docked to recipient blocks not exposed to this hormone. Such recipient tissues responded with characteristic resorptive activity within 24 hr, instead of the minimum of 4 days required by control tissues exposed directly to thyroxin. It is inferred that some component other than thyroxin which is active in inducing resorption is transmitted from the treated to the untreated block. Presumably, this is a product normally present late in the latent period of thyroxin action.