The significance of the latent period in thyroid hormone induced tissue regression during amphibian metamorphosis

Tail fin disks removed from tadpoles of Rana pipiens that were immersed in thyroxine or triiodothyronine for 3 days (referred to as donors) were fused to premetamorphic tail fin blocks that had never been exposed to this hormone (referred to as recipients) so that triplets were formed, consisting of one recipient tail block sandwiched between two donor tail fin blocks. Such recipient tail blocks responded with characteristic resorptive activity within 24 or 48 hr, instead of the minimum 72-hr latent period normally intervening in donor blocks, until shrinkage was initiated in response to triiodothyronine (T 3) or tetraiodothyronine (T 4). The presence of T 3 or T 4 hormone was not required continuously throughout the latent period. Hormone could be withdrawn after 30 hr contact in vivo and after 24 hr contact in vitro without interfering with the rate of tissue regression of tadpole tail fins, suggesting that the “latent period” probably does not coincide with the “critical period” during which subtle biochemical changes presumably occur that precede regression of the tadpole tail during metamorphosis. It is suggested that during the latent period active intermediates may be synthesized that are subsequently transferred from donor tail fins to recipients, thus reducing the latent period of the latter.