Cell proliferation during formation of the embryonic facial primordia

Cell proliferation of mesenchyme in the developing primary palate of the chick embryo was analyzed by tritiated thymidine autoradiography. Pulse labeling, repeated labeling, and label dilution techniques were employed to determine generation times, transit times, growth fractions, and other parameters of the cell cycle. In vivo and in vitro studies were performed to evaluate the role of tissue interactions during outgrowth of the facial primordia. These studies indicated that initially, during early stages of primary palate formation, virtually all mesenchymal cells are in the division cycle with relatively short generation times. As development proceeds, mesenchymal cell populations in the facial primordia, such as the maxillary process, retain cycle characteristics comparable to those of the progenitor cell populations. In regions adjacent to the facial primordia, such as the roof of the stomodeum, cell cycle times become more heterogeneous and result in removal of cells from rapidly cycling cell populations into subpopulations that are cycling more slowly and that, in some instances, become quiescent. Regional analysis of cell proliferation in the maxillary process indicated that growth rates of mesenchyme differ based on proximity to the overlying epithelium. Correlative in vitro studies of epithelial-mesenchymal separation and recombination experiments in organ culture revealed that the viability of mesenchyme was dependent on the presence of epithelium and that this effect was strongly stage-dependent. These and other results lead us to the conclusion that epithelial-mesenchymal interaction is significant to the maintenance of growth rates in the facial primordia and that the effects observed are mediated, at least in part, by developmental signals at the epithelial-mesenchymal interface.