Plasticity of transposed rhombomeres: Hox gene induction is correlated with phenotypic modifications

Regional specification of motoneurons along the anterior-posterior axis is independent of the notochord

Cek8 and low affinity NGF receptor (LNGFR) are expressed at high levels on the chick spinal motoneurons of the brachial and lumbar segments from embryonic day (E) 5 to E7, but weakly on the motoneurons of the non-limb-innervating segments. We determined by means of heterotopic neural tube transplantation, that the expression of these molecules was already intrinsically determined at E2. We used these spatiotemporal specific molecules as markers of motoneuron subpopulations. To analyze how motoneurons acquire regional specification along the anterior-posterior (A-P) axis and in the transverse plane, we observed the expression of these molecules on ectopic motoneurons induced by implanting a supernumerary notochord or floor plate at E2. The ectopic motoneurons induced by the graft obtained from either the thoracic or lumbar segments had the same expression profile as the normal motoneurons at each A-P level. These findings suggest that regional specification of motoneurons, at least of Cek8 and LNGFR expression, is independent of the notochord and the floor plate and that the whole neural tube appears to be committed to differentiate into the motoneuron subtypes along the A-P axis at the operative stages.

Patterning the hindbrain

Of paramount importance for hindbrain patterning is positional information that is laid down along the rostrocaudal axis. Recent findings suggest that retinoic acid may establish rostrocaudal domains of gene expression that confer on rhombomeres their specific identities; these domains display different responses to dorsoventral signals that further refine the repertoire of cellular fates therein. After rhombomere boundaries form, a high degree of segmental autonomy is balanced by a continuing capacity for interaction along the rostrocaudal axis, exemplified by the generation of the neural crest.