Induction of ectopic engrailed expression and fate change in avian rhombomeres: intersegmental boundaries as barriers

We tested the possibility of inducing ectopic expression of the gene Engrailed-2 (En-2) in the rhombomeres of the 2-days-old chick embryo. The experimental procedure consisted of grafting En-2-expressing neuroepithelium (prospective isthmocerebellum) from a quail or a mouse embryo into different rostrocaudal levels in the hindbrain of a host chick embryo. The graft replaced a given excised host rhombomere, the rostral and caudal limits of which were either also excised or left intact in different experiments. Induction of En-2 occurred in the host hindbrain, but only when the graft did not contact with host interrhombomeric boundaries and only in the alar plate of the rhombomeres immediately contacting the graft. Long survival experimental embryos showed that induced rhombomeres give rise to a cerebellar phenotype in their transformed alar plates. We thus demonstrate here a pluripotential state of the early rhombomeres as well as a possible role of the interrhombomeric limits as barriers to morphogenetic influences.