Retrograde labeling of migrating spinal motoneurons in bullfrog larvae

Horseradish peroxidase (HRP) was applied to the ventral root or developing limb bud of bullfrog tadpoles. In animals younger than Stage XII/XIII [9] ventral root application consistently labeled neurons located between the lateral motor column (LMC) and the neuroepithelium. Only occasional motoneurons outside the LMC were labeled in animals older than Stage XIV/XV. Limb bud application of HRP in any-stage tadpole did not label ectopic LMC motoneurons. These results indicate that LMC motoneurons have sent their axons out the ventral root before completion of migration but that these axons may not as yet have reached the limb bud.