A morphometric analysis of cell densities in facial prominences of the rhesus monkey embryo during primary palate formation.

The mechanisms by which the craniofacial complex undergoes morphogenetic change during primary palate formation remain unknown. Although changes in cell dispersion and extracellular matrix content are known to be involved in growth of embryonic primordia, little information about these parameters is available for the mammalian face. The purpose of this study was to analyze cell densities in different facial regions of rhesus embryos during primary palate formation. Eight serially-sectioned embryos of stages 13 to 18 in the collection at the California Primary Research Center, most labeled with 3H-thymidine, were analyzed by making regional cell counts at x400 with an ocular micrometer. Three embryos with autoradiographic label preserved in every section were used to analyze patterns of labeling through the depth of the entire face. The results showed that different regions of an embryonic face have significantly different cell densities. At the early stages, the cell densities were high in the maxillary prominences, and the lateral and medial nasal prominences; whereas cell densities were low in midline tissues. At later stages, cell densities were lower in the lateral portion of the maxillary prominences than in the nasal prominences. Serial counts through the face showed regional variation in individual prominences, but central high-density cores did not appear to be present. Labeling with 3H-thymidine was present in all facial primordia. Labeling indices were consistently higher in facial prominences than in midline tissues ventral to the brain. The results suggest that regional changes in cell densities could be used to reflect net changes in cell dispersion associated with altered balances between cell proliferation and extracellular matrix content in embryonic facial primordia.