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 (RTKs) and mRNA differential display with embryonic day 5 (E5) chick motoneurons enriched by panning. In situ hybridization revealed that Cek8 RTK among the cloned molecules was specifically expressed on motoneurons at the brachial and lumbar segments of the spinal cord which innervate limb muscles. We found that the expression of Cek8 in limb motoneurons was already intrinsically determined at E2. To analyze how motoneurons 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 induced by implanting a supernumerary notochord or floor plate. The ectopic motoneurons induced by the graft obtained from either the thoracic or lumbar segments had the same expression profile as the in situ motoneurons at each A-P level. These findings suggest that regional specification of motoneurons is independent of the notochord and the floor plate and that the whole neural tube is committed to differentiate into the motoneuron subtypes along the A-P axis at the operative stages.