DURING development of the nervous system, specific recognition molecules provide the cues necessary for the formation of neural connections. In some regions, guiding cues for axonal pathfinding and target selection are provided by specific cells that exist only transiently during development, such as the floorplate or the cortical subplate. In the hippocampus, distinct groups of fibres innervate different layers. We have tested the hypothesis that transient neurons in the hippocampus provide positional information for the targeting of these fibres. Here we report that ablation of Cajal-Retzius cells in organotypic slice cultures of hippocampus prevented the ingrowth of entorhinal but not of commissural afferents. Experiments inhibiting Reelin (an extracellular matrix protein expressed by Cajal-Retzius cells) and analysis of reeler mutant mice showed dramatic abnormalities in the development of entorhinal afferents. Thus Cajal-Retzius cells and reelin are essential for the formation of layer-specific hippocampal connections.
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