Redefining the head–trunk interface for the neural crest

Abstract

The head-trunk interface lies at the occipito-cervical boundary, which corresponds to the somite 5/6 level. Previous studies have demonstrated that neural crest cells also behave differently either side of this boundary and that this may be due to intrinsic differences between cranial and trunk crest. However, it is also possible that some of the observed differences between cranial and trunk crest are assigned by environmental cues. We have therefore scrutinised the behaviour of the neural crest cells generated either side of the occipito-cervical boundary in chick and, interestingly, find that both behave in a truncal fashion by traversing the anterior half of their adjacent somites. Furthermore, although not previously described, we find that transient DRGs form opposite somites 4 and 5. Crest cells produced anterior of the somite 3/4 boundary avoid the somites and behave in a non-truncal fashion; these cells populate the pharyngeal arches, and thus contribute to the developing head. We have further shown, via somite transplantations, that differential behaviour of the posterior versus anterior occipital crest is assigned by the somites. If somites 1 to 3 are replaced by trunk somites, then the anterior occipital crest will behave in a truncal fashion by invading the somites. Correspondingly, if these anterior occipital somites are transplanted in place of trunk somites, they perturb the migration of trunk crest. Thus, for the neural crest, the head-trunk interface does not lie at the occipito-cervical boundary, but rather lies at the somite 3/4 level and is defined by the somites. The fact that this boundary lies at the somite 3/4 level in chick is significant as it reflects the more ancient posterior occipital boundary; in fish, only the first three somites contribute to the occipital bone.

Recent advances in craniofacial morphogenesis

“…Studies using chick embryos suggest that cranial paraxial mesoderm is able to direct CNC cell movement independently of their epithelial or mesenchymal organization (Noden, 1986;Ferguson and Graham, 2004). Of interest, recent cell fate mapping analysis has suggested that myoblast precursors contain positional identity inherited from their somatic mesenchymal stem cell precursors and can help to determine skeletal homologies that are based on muscle attachments (Matsuoka et al, 2005).…”

The differentiation and morphogenesis of craniofacial muscles

“…Somites impose many spatial constraints upon the pathways available to migrating neural crest cells (Loring and Erickson, 1987;Kalcheim and Tiellet, 1989;Ferguson and Graham, 2004) and preclude the dissemination of large connective tissue-forming crest populations. This becomes especially problematic in unraveling the homologies in amniotes of muscles associated with ancestral caudal gill arches, which formed at the same axial levels as the rostral somites but (presumably) derived their connective tissues from the neural crest.…”

Neural crest cell-autonomous roles of fibronectin in cardiovascular development

“…Studies in chickens and mice show that NC cell fates are influenced by signals emanating from their host tissues (Chen et al, 2012;Donoghue et al, 2008;Ferguson and Graham, 2004;Itasaki et al, 1996;Trainor and Krumlauf, 2000;Trainor et al, 2002a,b). In the pharyngeal arches, Notch signaling from the PAA endothelium mediates differentiation of the adjacent NC cell layers into VSMCs, and the region of active Notch around the PAA endothelium corresponds to the area within which NC cells undergo VSMC differentiation (High et al, , 2007Manderfield et al, 2012).…”