Fig. 4. Forced expression of Hoxa3 or Hoxb4 and the early steps of neural crest cell migration. (A) To target Hoxa3 or/and Hoxb4 expression in posterior diencephalic neural crest, unilateral electroporations of anterior cephalic neural fold were alternatively carried out on each side in two 5 ss quail embryos. Then, transfected neural folds were bilaterally implanted in a stage-matched chick embryo, which had been subjected to the bilateral ablation of the Hox-negative domain of its skeletogenic neural folds (broken lines). Recipient chick embryos that are bilaterally engrafted with the untransfected contralateral quail neural folds are referred to as quail-grafted controls. In situ hybridization with Hoxb4 probe in control embryo (B) and embryo that has received Hoxb4 transfected neural folds (C): the endogenous expression of Hoxb4 at posterior rhombencephalic level was unperturbed (arrowheads). In experimental embryos engrafted with Hoxb4 neural folds, Hoxb4 transcript expression also features the exogenous neural crest cells that start to spread from the transplant (arrows). Twenty hours after the operation (HH14), quail cell detection show that the migratory behavior of implanted neural crest cells in control embryos (D) is equivalent to that observed in embryos carrying Hoxa3 (E) or Hoxb4 (F) transfected neural fold-derived cells. In all these embryos, quail cells move rostrally to populate the nasofrontal bud (arrows) as well as laterally to colonize the presumptive first branchial arch (arrowheads).