Ephrin-dependent growth and pruning of hippocampal axons

Neuronal connections are arranged topographically such that the spatial org anization of neurons is preserved by their termini in the targets. During t he development of topographic projections, axons initially explore areas mu ch wider than the final targets, and mistargeted axons are pruned later. Th e molecules regulating these processes are not known, We report here that t he ligands of the Eph family tyrosine kinase receptors may regulate both th e initial outgrowth and the subsequent pruning of axons, In the presence of ephrins, the outgrowth and branching of the receptor-positive hippocampal aliens are enhanced. However, these axons are induced later to degenerate. These observations suggest that the ephrins and their receptors may regulat e topographic map formation by stimulating axonal arborization and by pruni ng mistargeted axons.