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Alterations in the epithelial surface of human palatal shelves prior to and during fusion: A scanning electron microscopic study

The surface topography of palatal shelves was examined by scanning electron microscopy prior to and during secondary palate formation in 72 therapeutically aborted human embryos and fetuses ranging in age from 37 days to 12 weeks (post‐fertilization). Alterations in epithelial surface appearance were observed in the prospective region of fusion prior to contact between the shelves. First seen as an elliptical area along the anterior portion of the presumptive medial edge, a zone of alteration spread progressively along the anterior four‐fifths of the medial edge prior to shelf contact, and continued along the posterior one‐fifth of the shelf as fusion proceeded. The altered zone eventually extended onto the oral surface in the middle of the shelf but was restricted to a narrow zone along the medial edge posteriorly. Initially characterized by elongation and “intertwining” of superficial cells, alterations later included cell death, desquamation and possible active migration of surface cells. The shelf epithelium subjacent to the zone of alteration exhibited increased irregularity in thickness and numerous “invaginations” into the underlying mesenchyme. These alterations are thought to reflect intrinsic changes within the shelf and may play a role in the fusion process. Evidence for surface remodeling following fusion was also observed.