DNA synthesis is unaffected but subsequent cell division is delayed in tadpole hindlimb epidermis when thyroxine is...

In Rana pipiens tadpoles, thyroxine (T4) injection in the early light promotes faster hindlimb growth and development than injection early in the dark. T4 also stimulates a significant increase in cell proliferation in the basal epidermal cells of the limb. Using autoradiography, we studied the timing of the T4-induced rise in the labeling and mitotic indices on 12L:12D cycles with 0.2 micrograms T4 injection early in the light or dark and on a 12L:3D:1L:8D cycle with a light pulse early in the dark shortly after a T4 injection. In all instances, the labeling index peaked at the same time after the start of T4, so diurnal differences in the binding or initial actions of T4 leading up to the entrance of epidermal cells into the DNA synthetic (S) phase of the cell cycle are not indicated. However, with T4 injection in the dark the subsequent mitotic index peak was delayed, to a greater extent on 12L:12D than on 12L:3D:1L:8D. Cells induced to proliferate following T4 injection in the dark evidently had longer cell cycles, probably at the expense of the S or G2 phase, than when T4 was given in the light.