Programmed cell death and cell transformation in craniofacial development.

Fusion of branchial arch derivatives is an essential component in the development of craniofacial structures. Bilaterally symmetric branchial arch processes fuse in the midline to form the mandible, lips, and palate. The mechanism for fusion requires several different morphologic and molecular events prior to the completion of the mesenchymal continuity between opposing tissue processes. The ectodermal covering of the branchial arches is one of the cell types that has an important role during craniofacial development. The surface epithelia provide the initial adherence between the processes; however, this population of cells is ultimately absent from the fusion zone. The medial edge epithelium of the secondary palatal shelves is one example of such an epithelium that must disappear from the fusion zone of the secondary palate during development in order to complete palatal fusion. The mechanisms for removal of the epithelial cells from the fusion zone could include either programmed cell death, epithelial-mesenchymal transformation, or migration to adjacent epithelia. All three of these fates have been hypothesized as a mechanism for the removal of the palatal medial edge epithelia. The processes of programmed cell death, epithelial-mesenchymal transformation, and epithelial migration are reviewed with respect to both palatal fusion and results reported in other model systems.