COMMITMENT OF MOTONEURON PROGENITORS OF CHICK-EMBRYO ALONG THE ANTERIOR-POSTERIOR AXIS

To identify molecules that might be involved in the early development of motoneurons, we performed RT-PCR for receptor tyrosine kinases (RTK s) and mRNA differential display with embryonic day 5 (E5) chick moton eurons enriched by panning. In situ hybridization revealed that Cek8 R TK among the cloned molecules was specifically expressed on motoneuron s at the brachial and lumbar segments of the spinal cord which innerva te limb muscles. We found that the expression of Cek8 in limb motoneur ons was already intrinsically determined at E2. To analyze how motoneu rons acquire regional specification along the anterior-posterior (A-P) axis and in the transverse plane, we observed the expression of Cek8, Islet-1, SC1 and low-affinity NGF receptor on ectopic motoneurons ind uced by implanting a supernumerary notochord or floor plate. The ectop ic motoneurons induced by the graft obtained from either the thoracic or lumbar segments had the same expression profile as the in situ moto neurons at each A-P level. These findings suggest that regional specif ication of motoneurons is independent of the notochord and the floor p late and that the whole neural tube is committed to differentiate into the motoneuron subtypes along the A-P axis at the operative stages.