Rhombomères, code Hox, crête neurale et malformations de la face

Abstract

The transient segmentation of the developing hindbrain into rhombomeres, already described at the turn of the century, has recently been confirmed and further documented at the cellular and molecular levels. Cell tracing experiments carried out in the chick embryo have demonstrated the polyclonal origin of the neurons within each rhombomere and the absence of cell mixing between adjacent rhombomeres after interrhombomeric boundaries are established. Each hindbrain segment is characterized during ontogeny by the transient expression of a definite combinatoire of Hox genes defining a Hox code. Experimental alteration of this code at the critical stages where Hox genes act as selectors of a particular developmental programme, results in malformations of a particular branchiorhombomeric compartment. Studies based on the use of the quail-chick marker system in the avian embryo have revealed that the facial and hypobranchial structures are derived from the neural crest. The cells originating from the neural crest express the same Hox code than those of the corresponding rhombomeres. Malformations of the facial and hypobranchial regions are relatively common in man. They ar interpreted here in the light of the recent discoveries concerning the metamerisation of the branchiorhombomeric area in the embryo and its control by selector genes of the Hox complex. We argue that each described syndrome results from developmental abnormalities of one or two consecutive hindbrain segments. As a consequence, facial abnormalities hav a predictive value concerning the development of the hindbrain itself. Facial structures being neural crest-derived, constitute a <<marker>> of neurogenesis in the transverse levels of the brain from which they arise.