Early events in mammalian craniofacial morphogenesis.

Head-trunk differences are well established in the most primitive vertebrates, and are clear from early developmental stages of all modern forms. The boundary between the two regions is not constant in all vertebrate classes in terms of the number of occipital somites. The occipital region is in some respects a transitional zone, giving rise to trunk-like somitic derivatives in the head. It is also highly specialised, providing a unique population of neural crest cells that are essential for formation of the aorticopulmonary septum (which divides the outflow tract of the heart) in mammals and birds. In the preoccipital hindbrain, rhombomeres represent a segmental structural pattern that is quite distinct from that of the somites, with a segment-specific pattern of gene expression. Expression of some of these genes in mesenchyme close to the primitive streak at earlier stages suggests that this pattern may be established at the time of neural induction. Mammalian embryos have taken cranial specialization further than other classes of vertebrate, particularly in relation to the pattern of development and eventual structural complexity of the forebrain. Mammalian specialisations of craniofacial development are described through references to studies on cranial neurulation, on cranial neural crest cell migration, and on the possible morphogenetic roles of extracellular matrix components.