Projections from the hippocampal and parahippocampal regions to the entorhinal cortex. An anterograde and retrograde tract-tracing study in the cat

Projections from the hippocampal and parahippocampal regions to the entorhinal cortex (EC) were examined in the cat by anterograde and retrograde tract-tracing with Phaseolus vulgaris leucoagglutinin and cholera toxin B subunit. CA1 fibers to EC were distributed more densely in the medial EC than in the lateral EC; these were seen in all EC layers, but most densely in layers II and III. The septotemporal axis of the area of origin of CA1-EC fibers corresponded to a caudal-to-rostral axis of the area of their termination in the EC. CA2 and CA4 also sent a small number of fibers to the EC. The subiculum sent fibers mainly to the lateral EC; more densely to layers IV-VI than to layers I-III. The septotemporal axis of the area of origin of subiculum-EC fibers corresponded to a caudolateral-to-rostromedial axis of their termination in the EC. Distribution pattern of fibers from the prosubiculum regions close to CA1 or from prosubiculum regions close to the subiculum was similar to that of CA1 fibers or subiculum fibers, respectively. The presubiculum sent fibers mainly to the medial EC; most densely to layers I and III. The parasubiculum sent fibers mainly to the medial EC; most densely to layer II. Fibers to the contralateral EC were detected only from the presubiculum; they originated from the superficial layers and terminated in layer III of the medial entorhinal area.