Ectopic CNS projections guide peripheral neuron axons along novel pathwaysin leech embryos

Previous studies have indicated that the formation of stereotyped segmental nerves in leech embryos depends on the interactions between CNS projection s and ingrowing afferents from peripheral neurons. Especially, CNS-ablation experiments have suggested that CNS-derived guidance cues are required for the correct navigation of several groups of peripheral sensory neurons. In order to directly test this hypothesis we have performed transplantations of CNS ganglia into ectopic sites in segments from which the resident gangl ia have been removed. We find that the transplanted ganglia extend numerous axons distributed roughly equally in all directions. When these CNS projec tions reach and make contact with peripheral sensory axons they are used as guides for peripheral neurons to grow toward and into the ectopic ganglia even when this means following novel pathways that cross the midline and/or segmental boundaries. The peripheral sensory axons turn and grow toward th e ectopic ganglia only when in physical contact with CNS axons, suggesting that diffusible chemoattractants are not a factor. These results demonstrat e that the guidance cues provided by ectopic CNS projections are both neces sary and sufficient to steer peripheral sensory neuron axons into the CNS. (C) 2000 Academic Press.