Palate Development: In Vitro Procedures

The mammalian secondary palate provides an excellent experimental paradigm with which to investigate regulation of the sequential expression and interaction of molecular signals during embryogenesis. The secondary palate arises as bilateral extensions of the oral aspect of the maxillary processes of the first arch. In mammalian embryos, these palatal extensions make contact, fuse with one another and give rise to the secondary palate (roof of the oral cavity) (1 ,2 ). The palatal processes, consisting of mesenchymal cells embedded in a loosely organized extracellular matrix and enclosed within a stratified epithelium, initially grow vertically on either side of the tongue. The processes subsequently undergo a series of morphogenetic movements that result in their reorientation above the tongue, thereby bringing the medial edge epithelium (MEE), on the apical surface of each process, into contact forming a midline epithelial seam. Cells of the MEE undergo a precise sequence of molecular changes that culminate in removal of the midline epithelial seam. These changes include cessation of DNA synthesis, increased synthesis of cell-surface glycoconjugates, increased synthesis of lysosomal enzymes, cell death of the peridermal (surface) cell layer, and transdifferentiation of the subjacent MEE from an epithelial to a mesenchymal phenotype. Fusion of the palatal processes, followed by peridermal cell death and transdifferentiation of the MEE and migration into the underlying mesenchyme results in formation of the definitive secondary palate (3 ,4 ).