Cell release from the palatal shelves and the fusion line.

Cell release from the palatal shelves and the fusion line prior to and during closure of the secondary palate was studied in Sprague-Dawley rat fetuses of a precise, individually determined age. Fetuses with normal and with experimentally retarded or failing palatal closure were examined by light- and electronmicroscopy. Three different phenomena were observed involving either locally restricted areas or the entire medial shelf edge: (1) desquamation of superficial cells, (2) extrusion of cell blebs, and (3) surface accumulation of basal cells. It is suggested, that desquamation is a process facilitating primary contact and fusion between undifferentiated cells, while extrusion of cell blebs probably represents degradation of the epithelial seam by cellular escape. Basal cells were seen at the surface of unfused shelf edges in fetuses with partial or total palatal clefting, as well as in fetuses with a normal but timewise delayed palatogenesis. It was assumed that, under normal conditions, epithelial surface desquamation is followed or accompanied by basal cell migration and that this may provide an additional mechanism assuring shelf contact between deep-seated cells.