Fig. 8. Schematic representation of palate shelf fusion. (A) Initially, shelves approach each other at the time the periderm cells (yellow cells) overlying the basal MEE cells (white cells) emit filopodia. (B) First contact and adhesion occurs between periderm cells; proteoglycans appear to be important at this stage. Adhesion becomes stronger as periderm cells move up and down (arrows) the MES (bracket) forming the epithelial triangles (et). (C) Basal MEE cells of each shelf intercalate (convergent extension) resulting in a single epithelial layer. (D) MES breaks up and epithelial pearls (ep) form; periderm and MEE cells start to die within epithelial triangles and epithelial pearls, respectively (red cells). (E) MES, which is composed of periderm and basal MEE cells, essentially degenerates by cell death; dying cells activate basal lamina degradation (cataptosis; broken orange line). (F) Fusion is complete without a major mesenchymal cell movement across the midline; some oral and nasal epithelial cells do move across the middle line (doubleheaded arrows). Pink cells represent mesenchymal cells. Orange lines represent basal lamina.