Entorhinal axons perforate hippocampal field CA3 in organotypic slice culture

In growing towards their hippocampal targets, incoming afferent axons from the entorhinal cortex arrive at the subicular pole of the hippocampus and normally turn pialwards from the alvear path, crossing (perforating) the subiculum and field CA1, but never the more distally situated field CA3. To address the question of whether a specific repulsive characteristic of field CA3 might explain this behaviour, artificial confrontation were set up in vitro. Embryonic entorhinal explants were placed in restricted contact with 8-day-old rat hippocampal slices, orientated so that outgrowing axons could only grow into either the dentate 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 normal trajectory across CA1/subiculum. It is therefore concluded that spatiotemporal cues are more likely than specific chemorepulsive molecules to be involved in setting up this part of the entorhinal pathway.