Mesencephalic fifth nucleus cell responses to thyroid hormone: one population or two?

Hypophysectomized Rana pipiens tadpoles 3-6 months old were placed in dl-thyroxine (T4) solutions of 4 to 200 micrograms/l for 1-18 days and fixed 1 day after removal from the hormone solution. Exposure times varied inversely with T4 concentration. Mesencephalic fifth nucleus (M-V) cells were counted on both sides, and cell and nuclear sizes were drawn and measured for each tadpole. Changes in M-V cell characteristics correlated well with T4 exposure times and concentrations, as did changes in external tadpole morphology. All T4 concentrations were effective. Cells and nuclei were distinctly larger in all T4-treated groups. The changes were greatest for cell sizes, less for nuclear dimensions, and still less for nucleo-cytoplasmic ratios. Significant changes were seen for minimum and maximum sizes as well as for the mean values. The greatest mean changes were seen at dosages of 50 micrograms/l for 7-9 days. Mean M-V cell numbers are significantly smaller in hypophysectomized tadpoles than in controls (about 420 vs. 650). Thyroid hormone treatment of the hypophysectomized animals abolishes much of the deficit, though M-V cell deaths at the higher concentrations and longer treatment times reduce the apparent increase in numbers. Are the additional cells obtained through cell division, or do they represent a preexisting subpopulation of prospective M-V cells that require stimulation by thyroid hormone for their full differentiation?