Cell formation in the motor nucleus and mesencephalic nucleus of the trigeminal nerve of rats made hypothyroid by propylthiouracil.

Neurogenesis in the motor and mesencephalic nuclei of the trigeminal nerve was examined using autoradiographic techniques. Two groups of pregnant rats (control and experimental) were injected with two successive daily doses of 3H thymidine in an overlapping series starting from day nine of gestation in order to label in their progeny, the dividing precursor of neurons of the motor nucleus and mesencephalic nucleus of the trigeminal nerve. Control group of rats was raised on a standard diet, while the experimental group was made hypothyroid by propylthiouracil (PTU). At postnatal ages ranging between 20-30 days in the pups of both the control group and experimental group, the percentage of cells labelled and the proportion of cells added during each embryonic day were determined quantitatively throughout the rostro-caudal extent for both motor and mesencephalic nuclei. The neurons of the mesencephalic nucleus undergo their final cell divisions between gestational days 9 and 10 (E9 and E10). More than 80% of the population is generated by E10. The neurons of the motor nucleus undergo their final cell divisions between E9 and E11, and nearly 88% of the cells is generated by E11. In the thyroid deficient rats, in both nuclear centers, only 61% of the cells is generated by E12, and labelled cells are observed even as late as E18 and E19. In the hypothyroid state, there is a significant lengthening of the proliferative period. On the basis of absolute datings and duration of neuron production, it is postulated that in normal development, thyroid hormone determines the duration of the proliferative period, and push cells into the differentiative phase by taking them out of the proliferative phase.