Cell proliferation during morphogenetic change; analysis of frontonasal morphogenesis in the chick embryo employing DNA labeling indices.

The role of cell proliferation was analyzed in the chick embryo system employing DNA labeling indices during the invagination of the olfactory placode and the development of the lateral and medial nasal processes. Chick embryos were labeled for 1 h with [H]thymidine and processed histologically and autoradiographically. The percentage of labeled mesenchymal cells in delineated areas within and adjacent to the nasal processes was determined. From analysis of labeling indices of each area at successive developmental stages, it was concluded that cell proliferation of mesenchyme, as measured by DNA labeling indices, did not appear to increase during the formation of the nasal processes, and that cell proliferation actually declined during the later stages of nasal process formation. Differences were also found between the labeling indices of the mesenchyme of the nasal processes as compared to that of adjacent areas. These differences tended to become greater as development progressed. In all of the areas studied, cell proliferation declined during the Jater stages of development but the magnitude of the decline was greater in the areas adjacent to the nasal processes. Differential rates of decline, rather than acceleration of cell proliferation, therefore, appears to be operative as a morphogenetic mechanism during early primary palate formation.