The objective of this research project is to understand the morphological, cellular, and molecular aspects of normal and abnormal rodent and human embryonic craniofacial development. Retinoic acid (RA) is known to be essential for normal development, but non-physiological levels are teratogenic in both the rodent and the human. Our studies have revealed that there are several cell types in the developing craniofacial region which are sensitive to retinoid-induced alterations: 1) RA inhibits the migration of cranial neural crest cells in whole embryo culture; 2) RA decreases the proliferation of primary and secondary palatal mesenchymal cells in vivo and in vitro; 3) RA decreases proliferation and also increases the synthesis of a 97 Kd protein in an established cell line derived from the human embryonic palatal mesenchyme; and 4) RA alters the growth and differentiation of rodent and human embryonic palatal medial epithelial cells in vitro. Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) appear to play an important role in development. Our cDNA hybridization studies have demonstrated that the implanting rodent embryo initiates the de nova expression of the TGF-alpha gene in the maternal endometrium. TGF-alpha may serve as an autocrine factor for the maternal decidual cells as well as a paracrine factor for the embryo. EGF influences the proliferation and differentiation of rodent and human palatal medial epithelial cells in cell and organ culture. In normal development, EGF receptors are present in medial, nasal, and oral epithelial cells in the early mouse palatal shelf (day 12); a dramatic reduction in the number of these receptors takes place in the medial epithelium on days 13 and 14. RA and EGF prevent this decline in medial EGF receptors and also inhibit medial epithelial programmed cell death. Additional studies have indicated that palatal mesenchymal cells can express the chondrogenic phenotype and the extracellular matrix (ECM) protein laminin under certain conditions in culture. Our results indicate that retinoids, ECM, and EGF-related growth factors play a key role in mammalian craniofacial development.
