Incorporation of tritiated thymidine in the hypophysis of the rhesus monkey (Macaca mulatta) embryo.

Proliferation and growth in the neurohypophysis and adenohypophysis, including the pars distalis, pars intermedia and pars tuberalis, were studied in a series of rhesus monkey embryos of stages 13 to 21 (from 26 to 41 days of gestation) and labelled for one hour with a single intrachorionic injection of 3H-thymidine. As early as stage 13 the anlage of the neurohypophysis is foreshadowed in the floor of the diencephalon by a relatively low level of proliferation, with a labelling index (LI) of 35.1% as compared with an LI of 66.7% in the lateral walls of the diencephalon and an LI of 59.5% in the hypophyseal pouch. By stage 16 the neurohypophyseal anlage consists of a well-defined infundibulum, most of whose cells are unlabelled, (LI = 2.8%), in contrast with the more heavily labelled hypophyseal pouch (lateral walls = 45.3%; dorsal wall = 40.3%; ventral wall = 41.0%). The wall of the anlage of the adenohypophysis is in close apposition with the wall of the neurohypophysis until stage 21 when mesenchymal cells can be seen penetrating between the two. Near the communication of the hypophyseal pouch with the stomodeum the ventral walls of the hypophyseal pouch show a drastic reduction in labelling (LI = 8.0%) at stage 16. Although the pouch loses its contact with the stomodeum at stage 18, well-defined remnants of its earlier connection can still be seen as an irregular strand of cells at stage 21.