Current concepts in embryonic craniofacial development.

Abstract

Embryology mirrors phylogeny. The phenotypic expression of the genome is the result of differential gene transcription, the critically timed turning on and off of specific genes by transcription factors to produce cyto-, histo-, and morpho-differentiation that fleetingly reflects evolutionary stages of development during ontogeny. Hox genes regulate transcription of other structural genes and are responsible for patterning of the facial primordia. Cephalic development involves extremely complex morphogenetic mechanisms built on conserved elements that have undergone enormous evolutionary changes. Transient expression of phylogenetic origins characterize ontogeny and are reflected in defective development that may be due to inappropriate expression of Hox genes or distorted or disrupted epignetic processes. The mechanisms by which genetic information is transformed into morphological patterning by the actions of growth factors, morphogenes, and receptors are currently being identified. Biochemical, immunological, and allometric analyses of embryos and fetuses in experimental and descriptive studies are elucidating details of units of craniofacial morphogenesis--faciogenesis, palatogenesis, gnathogenesis, odontogenesis. Three-dimensional model computer-assisted reconstruction of sectioned embryos and fetuses provides a further technique for understanding the complex configurations of tissue migratory patterns and growth sites that account for normal and abnormal craniofaciogenesis.