Fig. 3. Neural crest migration and ectomere alignment. (A,C,E) Schematics of a developing chick embryo illustrating neural crest migration during craniofacial development. (B,D,F) In situ hybridization showing Fgf8 expression (yellow) during chick craniofacial development. (A-D) As the closed neural tube begins to differentiate into the central nervous system, the neural crest begins to migrate anteriorly from specific rhombomeres (r1-r3) into discrete regions of the face. During this process, the neuroectoderm (ne) and surface ectoderm (se) components of the ectomeres continue to remain aligned (yellow arrows in C). Inset in A shows higher magnification of the boxed area in B (the direct contact between the anterior neuroectoderm and presumptive facial ectoderm, prior to neural crest cell migration between those two epithelial layers). (E,F) As neural crest migration nears completion, the neuroectoderm and facial ectoderm (fe; late-stage term for surface ectoderm) components of the ectomere are no longer aligned. is, isthmus; mn, mandible; PA, pharyngeal arch; pe, pharyngeal endoderm; rp, Rathke's pouch; tel ne, telencephalic neuroectoderm. (B,D,F) Unpublished data from J.A.H.'s laboratory.