ENTORHINAL AXONS PERFORATE HIPPOCAMPAL FIELD CA3 IN ORGANOTYPIC SLICECULTURE

In growing towards their hippocampal targets, incoming afferent axons from the entorhinal cortex arrive at the subicular pole of the hippoca mpus and normally turn pialwards from the alvear path, crossing (perfo rating) the subiculum and field CAI, but never the more distally situa ted field CA3. To address the question of whether a specific repulsive characteristic of field CA3 might explain this behaviour, artificial confrontations were set up in vitro. Embryonic entorhinal explants wer e placed in restricted contact with 8-day-old rat hippocampal slices, orientated so that outgrowing axons could only grow into either the de ntate gyrus, the subiculum/field CA1, or field CA3. Anterograde biotin -dextran labelling of projections after 2 weeks in culture showed that entorhinal axons perforated the stratum oriens, pyramidal cell layer, and stratum radiatum of CA3 just as readily as they did along their n ormal trajectory across CA1/subiculum. It is therefore concluded that spatiotemporal cues are more likely than specific chemorepulsive molec ules to be involved in setting up this part of the entorhinal pathway.