Vertebrate craniofacial development: the relation between ontogenetic process and morphological outcome

Many structures that are present, often transiently, in the head of extant vertebrate embryos appear to be segmentally organized. These include the brain, particularly the hindbrain (e.g., rhombomeres), and adjacent axial structures such as paraxial mesoderm (e.g., somites, somitomeres) and neural crest cells. Also present in the head are additional sets of serially arranged structures that develop in more ventral and lateral locations. Examples of these are epibranchial placodes, aortic arches, and pharyngeal pouches. All these embryonic structures are frequently used both individually and collectively as characters to assist in defining homologies. New cell labeling and identification methods are providing detailed accounts of cell movements and tissue lineages that reveal a range of disparate behaviors not previously appreciated. The well-known migrations of neural crest cells bring all but the neurogenic members of this mesenchymal population form dorsal, axial locations into ventral and rostral locations where they largely surround the pharynx, stomdeum, and prosencephalon. Equally dramatic movements of neural plate cells, myoblasts, angioblasts, and placode-derived cells have recently been documented. These movements may occur in concert with those of other nearby tissues (e.g., branchiomeric myoblasts, neural crest cells, and surface ectoderm) or may be independent (e.g., placodal neuroblasts). Migrating cells may be clustered and follow definable pathways towards their destination (e.g., neural crest cells), or they may be solitary and wander invasively without a prespecified destination (e.g., angioblasts). These extensive morphogenetic movements bring cells into contact with a greater variety of other tissues and matrix environments than has heretofore been recognized. Moreover, because of these rearrangements, the cells present in a particular location, such as a branchial arch, may trace their ancestry to many axial levels, which complicates the analyses of segmental relations. Comparative morphological studies of craniofacial development have recently been augmented by descriptions of the sites and times of expression of many matrix components, growth factors and their receptors, and regulatory genes. Particularly important has been the discovery of a network of genes called the homeobox family. These genes are similar in their sequence and their organization along a chromosome to genes that establish the spatial identity of prospective body parts in drosophila. The combination of cellular and molecular descriptive studies of vertebrate craniofacial development provide exciting opportunities to catalogue patterns of gene expression and morphogenesis during the gastrula, neurula, and early organogenesis stages. Moreover, such data form the basis for proposing and then testing hypotheses about the mechanisms controlling cell movements, tissue formation, and the assembly of functionally integrated sets of structures.(ABSTRACT TRUNCATED AT 400 WORDS)