Neural crest origins of the neck and shoulder

Abstract

The neck and shoulder region of vertebrates has undergone a complex evolutionary history. To identify its underlying mechanisms we map the destinations of embryonic neural crest and mesodermal stem cells using Cre-recombinase-mediated transgenesis. The single-cell resolution of this genetic labelling reveals cryptic cell boundaries traversing the seemingly homogeneous skeleton of the neck and shoulders. Within this assembly of bones and muscles we discern a precise code of connectivity that mesenchymal stem cells of both neural crest and mesodermal origin obey as they form muscle scaffolds. The neural crest anchors the head onto the anterior lining of the shoulder girdle, while a Hox-gene-controlled mesoderm links trunk muscles to the posterior neck and shoulder skeleton. The skeleton that we identify as neural crest-derived is specifically affected in human Klippel-Feil syndrome, Sprengel's deformity and Arnold-Chiari I/II malformation, providing insights into their likely aetiology. We identify genes involved in the cellular modularity of the neck and shoulder skeleton and propose a new method for determining skeletal homologies that is based on muscle attachments. This has allowed us to trace the whereabouts of the cleithrum, the major shoulder bone of extinct land vertebrate ancestors, which seems to survive as the scapular spine in living mammals.

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells

“…Descriptive accounts in avian (Köntges and Lumsden, 1996;Noden and Francis-West, 2006) and mammalian (Matsuoka et al, 2005) embryos indicate that crestmuscle associations are often maintained even when muscles move out of the immediate branchial region. Whether these crest cells are determinative with respect to selecting sites of muscle attachment or are only necessary to provide connective tissues at these sites has not been resolved.…”

“…Detailed mapping studies using quail-chick transplantations have defined neural crest cells as the exclusive source of connective tissue components in the entire midfacial, oral, and branchial regions of the head (LeLiévre and Le Douarin, 1975;Le Liévre, 1978;Noden, 1978Noden, , 1983aNoden, , 1988Couly et al, 1992Couly et al, , 1993Le Douarin et al, 1993;Köntges and Lumsden, 1996). Indeed, this process appears to be a common developmental theme in all vertebrates so far examined, including mice (Jiang et al, 2002;Matsuoka et al, 2005;O'Gorman, 2005;Gage et al, 2005). These properties have elevated the neural crest to the status of vertebrate synaptomorph (Gans and Northcutt, 1983;Kuratani, 2003;Santagati and Rijli, 2003;Northcutt, 2005).…”

“…This mesoderm is the sole source of skeletal muscles in the head and, together with lateral mesoderm, generates all craniofacial plus some cardiac endothelial precursors (Noden, 1983b(Noden, , 1986(Noden, , 1991aCouly et al, 1992Couly et al, , 1995. Extraocular and branchial muscles initiate their development within paraxial mesoderm, but complete their differentiation and morphogenesis at peripheral locations populated by neural crest cells (Noden, 1983b(Noden, , 1986Couly et al, 1992;Köntges and Lumsden, 1996;Matsuoka et al, 2005;Gage et al, 2005).…”

“…Within the arches, myogenic cells remain segregated until later stages, when crest cells penetrate differentiating muscles to establish endo-, peri-, and epimysial layers (Noden, 1983a;McClearn and Noden, 1988;Trainor and Tam, 1995;Noden and Francis-West, 2006). Köntges and colleagues (Könt-ges and Lumsden, 1996;Matsuoka et al, 2005) have shown that, at later stages, the crest-derived progenitors of connective tissues associated with some branchial muscles will distort the original crest:mesoderm interface to reach distant attachment sites.…”

“…Precursors of individual muscles and their connective tissues arise at the same axial level and maintain these relations throughout their development (reviewed by Noden, 1991b;Noden and Trainor, 2005). When myoblasts leave the branchial arch environment, for example the progenitors of the trapezius, both muscle and associated connective tissue primordia remain as a constant, closed compartment (Matsuoka et al, 2005; but see O'Gorman, 2005, for exceptions). 3.…”