THE REGENERATION OF THE CEPHALIC NEURAL CREST, A PROBLEM REVISITED - THE REGENERATING CELLS ORIGINATE FROM THE CONTRALATERAL OR FROM THE ANTERIOR AND POSTERIOR NEURAL FOLD

The mesencephalic and rhombencephalic levels of origin of the hypobran chial skeleton (lower jaw and hyoid bone) within the neural fold have been determined at the 5-somite stage with a resolution corresponding to each single rhombomere, by means of the quail-chick chimera techniq ue. Expression of certain Hox genes (Hoxa-2, Hoxa-3 and Hoxb-4) was re corded in the branchial arches of chick and quail embryos at embryonic days 3 (E3) and E4. This was a prerequisite for studying the regenera tion capacities of the neural crest, after the dorsal neural tube was resected at the mesencephalic and rhombencephalic level. We found firs t that excisions at the 5-somite stage extending from the mid-mesencep halon down to r8 are followed by the regeneration of neural crest cell s able to compensate for the deficiencies so produced. This confirmed the results of previous authors who made similar excisions at comparab le (or older) developmental stages. When a bilateral excision was foll owed by the unilateral homotopic graft of the dorsal neural tube from a quail embryo, thus mimicking the situation created by a unilateral e xcision, we found that the migration of the grafted unilateral neural crest (quail-labelled) is bilateral and compensates massively for the missing crest derivatives. The capacity of the intermediate and ventra l neural tube to yield neural crest cells was tested by removing the c hick rhombencephalic neural tube and replacing it either uni- or bilat erally with a ventral tube coming from a stage-matched quail. No neura l crest cells exited from the ventral neural tube but no deficiency in neural crest derivatives was recorded, Crest cells were found to rege nerate from the ends of the operated region. This was demonstrated by grafting fragments of quail neural fold at the extremities of the exci sed territory. Quail neural crest cells were seen migrating longitudin ally from both the rostral and caudal ends of the operated region and filling the branchial arches located inbetween. Comparison of the beha viour of neural crest cells in this experimental situation with that s howed by their normal fate map revealed that crest cells increase thei r proliferation rate and change their migratory behaviour without modi fying their Hox code.