A ROLE FOR CAJAL-RETZIUS CELLS AND REELIN IN THE DEVELOPMENT OF HIPPOCAMPAL CONNECTIONS

DURING development ache nervous system, specific recognition molecules provide the cues necessary for the formation of neural connections, I n some regions, guiding cues for axonal pathfinding and target selecti on are provided by specific cells that exist only transiently during d evelopment, such as the floorplate or the cortical subplate(1-4). In t he hippocampus, distinct groups of fibres innervate different layers(5 ). We have tested the hypothesis that transient neurons in the hippoca mpus(6,7) provide positional information for the targeting of these fi bres, Here we report that ablation of Cajal-Retzius cells in organotyp ic slice cultures of hippocampus prevented the ingrowth of entorhinal but not of commissural afferents. Experiments Inhibiting ReeIin (an ex tracellular matrix protein expressed by Cajal-Retzius cells) and analy sis of reeler mutant mice showed dramatic abnormalities in the develop ment of entorhinal afferents, Thus Cajal-Retzius cells and reelin are essential for the formation of layer-specific hippocampal connections.