Research on development of the secondary palate has, in the past, dealt primarily with morphological aspects of shelf elevation and fusion. The many factors thought to be involved in palatal elevation, such as fetal neuromuscular activity and growth of the cranial base and mandible, as well as production of extracellular matrix and contractile elements in the palate, are mostly based on gross, light microscopic, morphometric or histochemical observations. Recently, more biochemical procedures have been utilized to describe palatal shelf elevation. Although these studies strongly suggest that palatal extracellular matrix plays a major role in shelf movement, interpretation of these data remains difficult owing to the complexity of tissue interactions involved in craniofacial development. Shelf elevation does not appear to involve a single motive factor, but rather a coordinated interaction of all of the abovementioned developmental events. Further analysis of mechanisms of shelf elevation requires development of new, and refinement of existing, in vitro procedures. A system that enables one to examine shelf elevation in vitro would allow more meaningful analysis of the relative importance of the various components in shelf movement.
Much more is known about fusion of the palatal shelves, owing in large part to in vitro studies. Fusion of the apposing shelves, both in vivo and in vitro, is dependent upon adhesion and cell death of the midline epithelial cells. Adhesion between apposing epithelial surfaces appears to involve epithelial cell surface macromolecules. Further analysis of palatal epithelial adhesion should be directed towards characterization of those cell surface components responsible for this adhesive interaction. Midline epithelial cells cease DNA synthesis 24–36 h before shelf elevation and contact, become active in the synthesis of cell surface glycoproteins, and subsequently manifest morphological signs of necrosis. Death of the midline epithelial cells is thought to involve a programmed, lysosomal-mediated autolysis. Information regarding the appearance, distribution and quantitation of epithelial hydrolytic enzymes is needed.
The control mechanisms which regulate adhesiveness and cell death in the palatal epithelium are not fully understood. Although palatal epithelial-mesenchymal recombination experiments have demonstrated a close relationship between the underlying mesenchyme and the differentiating epithelium, the molecular mechanism of interaction remains unclear. Recently cyclic nucleotides have been implicated as possible mediators of palatal epithelial differentiation.
The developing secondary palate therefore offers a system whereby one can probe a variety of developmental phenomena. Cellular adhesion, programmed cell death and epithelial- mesenchymal interactions are all amenable to both morphological as well as bio- chemical analysis. Although research in the field of secondary palate development has been extensive, there still remain many provocative questions relating to normal development of this structure.
Immunocytochemical localization of glucocorticoid receptors in midgestation murine embryos and human embryonic cultured cells.