ULTRASTRUCTURE OF MOUSE AND RAT PALATAL PROCESSES PRIOR TO AND DURING SECONDARY PALATE FORMATION

Formation of the secondary palate by the union of two palatine processes or shelves involves a series of rapid changes in the epithelial and mesenchymal shelf components. These alterations are described at the ultrastructural level just prior to, during, and after complete union. The palatal shelves exhibit an irregular outer epithelial plasmalemma which becomes more regular just prior to contact. Desmosomes form at the contacting shelf surfaces and appear to bind the two processes together until mesenchymal union is ultimately made. The newly established epithelial wall gradually breaks down, probably due to an autolytic process. Lysosome-like structures are evident within the epithelial cell cytoplasm as fragmentation of the wall proceeds. The basal lamina separating the epithelium from the mesenchyme remains intact and the underlying mesenchymal cells are unchanged until final epithelial cell disintegration occurs. Mergence rather than fusion is evident in some areas along the epithelial wall, particularly in the soft palate region where, soon after epithelial wall lysis, the basal lamina repairs across the shelves leaving a depression into the young secondary palate which is gradually reduced. Concomitantly hemi-desmosomes appear along the newly formed basal lamina. © 1968.