Segmental differentiation in the leech nervous system: specific phenotypic changes associated with ectopic targets.

The issue of whether interactions between central neurons and the targets they innervate are involved in establishing segment-specific phenotypic differences in the central nervous system was evaluated by studying the effect of naturally occurring ectopic targets on individual identified neurons of the leech Hirudo medicinalis. The specimens studied were found to have additional sex organs located in segments adjacent to those containing the normal structures. We report our observations on neuronal morphology, the presence or absence of specific neurons, and total ganglionic cell number and discuss some of the possible ways in which the observed changes might have arisen.