A comparative analysis of [3H]thymidine labeling in the embryonic tectum of the rhesus monkey (Macaca mulatta) and C57BL mouse.

The pattern of [3H]thymidine incorporation in the dorsolateral wall of the embryonic tectum was studied and compared one hour after injection of the label in the rhesus monkey (Macaca mulatta) at stages 11-20 (25-37 days of gestation) and in the C57BL mouse at stages 14-22 (9-14 days of gestation). During the early stages of development, the labeled nuclei were located peripherally in the ventricular zone in both the rhesus monkey and mouse embryo, although a number of labeled nuclei tended to occur closer to the ventricular border in the mouse, whereas there was little or no encroachment at the ventricular border in the rhesus monkey. The ventricular zone of the rhesus monkey and mouse embryos initially showed a high labeling index (LI) of about 59% which subsequently declined with increasing age. However, the decline occurred earlier and more precipitously in the rhesus monkey. At stage 17 of the rhesus monkey the LI had dropped to about 42%, whereas it still remained at 59% in the 12-day mouse, and by stage 20 of the monkey the LI was approximately 26%, in contrast to 41% in the stage 22 (14-day) mouse. At stage 20 of the mouse (12 days of gestation) the intermediate zone became much thicker than in the comparable stage (17) of the rhesus monkey, and this discrepancy continued at each successive stage observed in the current study. Also, whereas lamination became apparent in the intermediate zone of the mouse at stage 22, the monkey tectum at a comparable stage (20) was poorly differentiated.