Fig. 1. The cephalic neural crest: fate map and Hox gene expression. (A) Presumptive diencephalic, mesencephalic and rhombencephalic territories of the neural fold in the avian embryo at 5 ss, as established by Grapin-Botton et al. (Grapin-Botton et al., 1995). (B) Migration map of cephalic neural crest cells in the avian embryo. The origin of neural crest cells found in the nasofrontal and periocular regions and in the branchial arches is color-coded as in A. Neural crest cells arising from the posterior diencephalon and mesencephalon populate the nasofrontal and periocular region. Posterior mesencephalon also participates in these structures, but in addition populates the anterodistal part of the first branchial arch. The complementary portion of the first branchial arch derives from r1/r2, together with a small contribution from r3. The major contribution to the second branchial arch comes from r4. Neural crest cells arising from r3 and r5 split into strains participating to two adjacent arches: r3 cells migrate to the first and second branchial arches; r5 cells migrate to the second and third branchial arches. r6 cells migrate to the third and fourth branchial arches, r7- and r8-derived cells migrate to the third and to the more caudal branchial arches. (C) Hox gene expression in the chick and quail embryo at E3 when the branchial arches are being colonized by neural crest cells originating from the posterior half of the mesencephalon and the rhombomeres (r1-r8). The arrows indicate the AP origin of the neural crest cells migrating to each branchial arch. Expression of Hox genes is also indicated in the superficial ectoderm, the endoderm and mesoderm. (D,E) Cartilages forming the upper face of a chick embryo at E8 and the contribution of the crest-derived cells according to the level from which they originate. (F,G) Lower jaw skeleton of E8.5 chick embryo: the participation of the crest derived cells is color coded as in A. Skeletal components of hyoid cartilages, which are formed by Hox-expressing crest cells (Hox +), are shown in black. (H) Bilateral surgical ablation of the Hox-negative domain of the skeletogenic neural fold, extending from the mid-diencephalon down to r2 included in 5 ss chick embryo. (I) Frontal view of E8 chick embryo subjected to the bilateral extirpation of the Hox-negative skeletogenic neural folds. In these embryos, which have virtually no face, structures anterior to the second branchial arch fail to develop: the nasal process, mandibular and maxillary buds are rudimentary. (J,K) In this context, the bilateral graft of the posterior diencephalic neural folds can regenerate the excised territory and form a normal face. BA, branchial arch; Ect., ectoderm; Endo., endoderm; Meso., mesoderm; NC, neural crest; NFB, nasofrontal bud; r, rhombomere. A, articular; Bb, basibranchial; Bh, basihyal; Cb, ceratobranchial; E, entoglossum; Eb, epibranchial; Mc, Meckel’s cartilage; Nc, nasal capsule; Q, quadrate; Sc, sclerotic.