Proliferation of nasal epithelial and mesenchymal cells during primary palate formation

Proliferation of nasal epithelial and mesenchymal cells in mouse embryos was analyzed during primary palate formation using immunohistochemical demonstration of the thymidine analogue, 5-bromodeoxyuridine. Pulse labeling was employed to determine cell proliferation rates, with cell density of the nasal mesenchyme also being measured. To represent the entire nasal groove and prominences, four levels along the superior-inferior direction of three regions were utilized, i.e., the lateral and medial nasal prominences (LNP and MNP) and the bottom of the nasal groove. During the formation period, the labeling indices of the LNP and MNP epithelium decreased with respect to the development stage, wheras those of the bottom epithelium only slightly did. The epithelial cells in the prospective fusion area particularly showed decreased DNA synthesis in comparison with those in the nonfusing areas. In addition, the corresponding activity in the presumptive fusion area of the LNP epithelium was less than that in the MNP epithelium. The time at which a definitive decrease in the labeling index of the presumptive fusion area is believed to occur between tail somite (TS) stages TS5-7. A similar yet smaller decreasing tendency was observed in the labeling indices of the nasal mesenchyme. The cell density of the mesenchyme, however, slightly increased in all examined regions. Our results suggest that epithelial cell proliferation converts to a differentiation-type pattern, especially in the presumptive fusion area. By comparing relative decreases in the labeling indices of the nasal mesenchyme, as well as the resultant increase in nasal mesenchyme cell density, the bulk of the LNP is assumed to grow more rapidly than that in the MNP, thereby indicating that the LNP's mesenchymal cell growth may play a crucial role at the initial contact between these prominences in the presumptive fusion area associated with the epithelial differentiation.