PROLIFERATION OF NASAL EPITHELIAL AND MESENCHYMAL CELLS DURING PRIMARY PALATE FORMATION

Proliferation of nasal epithelial and mesenchymal cells in mouse embry os was analyzed during primary palate formation using immunohistochemi cal demonstration of the thymidine analogue, 5-bromodeoxyuridine. Puls e labeling was employed to determine cell proliferation rates, with ce ll density of the nasal mesenchyme also being measured. To represent t he 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, wh eras 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 additio n, the corresponding activity in the presumptive fusion area of the LN P epithelium was less than that in the MNP epithelium. The time at whi ch a definitive decrease in the labeling index of the presumptive fusi on area is believed to occur between tail somite (TS) stages TS5-7. A similar yet smaller decreasing tendency was observed in the labeling i ndices of the nasal mesenchyme. The cell density of the mesenchyme, ho wever, 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 rela tive decreases in the labeling indices of the nasal mesenchyme, as wel l 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, ther eby indicating that the LNP's mesenchymal cell growth may play a cruci al role at the initial contact between these prominences in the presum ptive fusion area associated with the epithelial differentiation.