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 branchia l arch processes fuse in the midline to form the mandible, lips, and p alate. The mechanism for fusion requires several different morphologic and molecular events prior to the completion of the mesenchymal conti nuity between opposing tissue processes. The ectodermal covering of th e branchial arches is one of the cell types that has an important role during craniofacial development. The surface epithelia provide the in itial adherence between the processes; however, this population of cel ls is ultimately absent from the fusion zone. The medial edge epitheli um of the secondary palatal shelves is one example of such an epitheli um that must disappear from the fusion zone of the secondary palate du ring development in order to complete palatal fusion. The mechanisms f or removal of the epithelial cells from the fusion zone could include either programmed cell death, epithelial-mesenchymal transformation, o r 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-mesench ymal transformation, and epithelial migration are reviewed with respec t to both palatal fusion and results reported in other model systems.