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 f rom embryonic day (E) 5 to E7, but weakly on the motoneurons of the no n-limbinnervating segments, We determined by means of heterotopic neur al tube transplantation, that the expression of these molecules was al ready intrinsically determined at E2, We used these spatiotemporal spe cific molecules as markers of motoneuron subpopulations. To analyze ho w motoneurons acquire regional specification along the anterior;poster ior (A-P) axis and in the transverse plane, we observed the expression of these molecules on ectopic motoneurons induced by implanting a sup ernumerary notochord or floor plate at E2, The ectopic motoneurons ind uced 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 motoneur ons, at least of Cek8 and LNGFR expression, is independent of the noto chord 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.