Ultrastructural characterization of the mesenchyme of the facial processes during development of the rat intermaxillary segment.

This study quantifies ultrastructural changes within the mesenchyme of the maxillary and medial/lateral nasal processes before and after formation of the intermaxillary segment. At both day 11 and day 13 of development, 4 fetal rat heads were fixed with 3% glutaraldehyde and processed for electron microscopy. Differences between cells within the facial processes were discerned at day 11 of gestation. Significantly more rough endoplasmic reticulum was present in the cells of the maxillary processes than in the lateral nasal processes (p < .05), and a greater number of cell projections was found in the maxillary processes than in the medial nasal processes (p < .05). At day 13, no significant differences were found in the facial processes. Of particular note, a consistent increase in the number of cell projections within all the facial processes was found on day 13 when compared with day 11 (p < .05). This change may be related to intercellular signaling between the mesenchymal cells at the time of onset of major histogenic changes.