Rhythms of cell proliferation in the hindlimb epidermis of control and thyroxine-treated Rana pipiens tadpoles.

Labeling and mitotic index rhythms were studied in premetamorphic tadpoles under an LD 12:12 with the light phase beginning at 0800 hr. In a 72-hr experiment, control labeling and mitotic index curves showed a peak in the light and a peak in the dark with labeling index rhythms of 12.4, 17.7, and 23.6 hr and a 21.4-hr mitotic index rhythm. Thyroxine (T4) treatment resulted in a marked elevation of labeling index by 24 hr and of mitotic index by 48 hr, obscured the control bimodal pattern of peaks, and altered the rhythms. During the first 3 days of T4 treatment, a labeling index rhythm of 22 hr and a mitotic index rhythm of 37.5 hr occurred. However, additional work demonstrated that the dominant control rhythms of labeling and mitotic indices returned in the T4-treated during Days 4 and 5. The same pattern of change in labeling index occurred during Day 3 of T4 treatment when hormone administration began at different times in the diurnal phase of the light-dark cycle. The findings suggest that cell proliferation rhythms can be temporarily disturbed by an exogenous T4 stimulus without apparent reference to the phase of the circadian rhythm.