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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.
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