The time of origin of the mesencephalic trigeminal neurons in Xenopus.

Time of origin of the mesencephalic trigeminal cells (MTC) in Xenopus 'has been studied with 3H-thymidine autoradiography. The first MTC's were formed at stage 20, and they continued to arise up to stage 58. The mitotic rate of MTC's was found to be ten per cent at any time from stages 20--50, with a slow decrease thereafter. MTC's in the tectum were distributed in a spatio-temporal manner. Cells generated earliest were located in the rostral pole of the tectum and were followed in caudomedial sequence by cells formed later. The generation times of MTC and the neurons of the sensory trigeminal ganglion (TGC) were strikingly dissimilar, indicating that they probably do not represent a common cell lineage, that is, the former may not arise from the mesencephalic neural crest. It is proposed that the majority of MTC's generated from stage 30 onward, derive through proliferative activity of precursor cells in the di-mesencephalic junction and in the tectal neuroepithelium.