The central aim of this project is to explore in vivo the cellular mechanisms that are responsible for the retraction of certain axonal projections by identified neurons in the leech central nervous system. Based on recent experiments, Dr. Macagno believes that the loss of these projections is the result of interactions between segmentally homologous neurons. He suggests that these interactions are mediated by a signal that the neurons exchange at or near their growth cones, probably through direct cell-to-cell contact. The experiments are designed to test this hypothesis and to provide the bases for the future examination of the mechanisms involved in the signalling and response of the cells. The principal approaches will be (1) to examine ultrastructurally the interaction between processes by dye- marking and computer-aided serial section electron microscopy and (2) to photoablate, wholly or partially, the interganglionic process of one cell and examine the response of its homologues. These observations will provide further insight into overproduction and selective retraction of axonal projections, a common and important feature of the development of neuronal circuitry.
