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Fig. 3. Hox gene expression restricts skeletogenic properties of the cephalic
neural crest. (A) In a 5-somite stage (ss) chick embryo, the cephalic neural
crest (NC) is divided into an anterior Hox-negative (Hox–)
domain (red) and a posterior Hox-positive (Hox+) domain
(blue). The transition between these two domains corresponds to rhombomere (r)
3 (orange). The neural fold rostral to the mid-diencephalon does not produce
NC cells (NCCs). (B) Postmigratory Hox– NCCs (red) yield
cartilages, as well as endochondral and membrane bones of the entire upper
face and jaws. By contrast, skeletogenic functions of Hox+ NCCs
(blue) are limited to chondrogenesis and endochondral ossification in the
hyoid structure. (C-H) Facial development at embryonic day (E) 7 after
resection and/or exchange of cephalic NC domains in 5 ss chick embryo. The
removal of Hox– FSNC (facial skeletogenic neural crest;
broken lines) (C) abolishes head development (D). Replacement of FSNC by
Hox+ neural fold (E) severely hampers head morphogenesis (F).
Following removal of whole FSNC (as in E) (G), implantation of only a fragment
of the FSNC (from either di-, mes- or metencephalic level) restores normal
development of complete face and forebrain (H). Reproduced, with permission,
from Couly et al. (Couly et al.,
2002).
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