Inhibition of thyroxine induced tail resorption in vitro by pituitary grafts from different stages of metamorphosis

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AbstractIt has been previously demonstrated that isolated pieces (discs) from the tail fins of tadpoles (R. pipiens), survive in vitro for several weeks and respond to thyroxine by undergoing resorption. In addition, discs into which tadpole pituitary glands have been implanted or discs that have been injected with either prolactin or growth hormone undergo a delayed pattern of resorption on expsoure to thyroxine when compared to discs treated with thyroxine alone.Pituitaries were taken from early prometamorphic, climax and froglet stages and were implanted into tail discs cut from tadpoles in early prometamorphosis. These discs were maintained in thyroxine solutions. As controls, unimplanted discs were treated with thyroxine or maintained in Hanks solution. In a second series of experiments, tail discs were cut from tadpoles at progressive stages of prometamorphosis and treated with thyroxine solutions.The results demonstrate that pituitaries taken from different stages in the life cycle of this animal differ in their capacity to inhibit thyroxine induced tail resorption. The most effective inhibition is demonstrated by early prometamorphic pituitaries, followed by climax pituitaries. Froglet glands exhibit little activity. The results also demonstrate that the response of tail tissues to thyroxine increases during early prometamorphosis and with increasing rapidity during the last third of this period. These results are in harmony with the concept that amphibian metamorphosis may be dependent upon a bihormonal system in which thyroid hormone promotes while a prolactin‐like hormone counteracts this process.