Topographic organization of the projections from the entorhinal area to the hippocampal formation of the rat

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Abstract
The present study re‐examines, with autoradiographic methods, the pattern of termination of fibers originating from various medio‐lateral divisions of the entorhinal cortex on dentate granule cells and on hippocampa pyramidal cells of the rat. Entorhinal fibers were found to distribute in a proximodistal gradient along the dendrites of dentate granule cells, with afferents from the medial entorhinal area terminating in the innermost portion of the entorhinal synaptic field, afferents from the lateral entorhinal area terminating in the most superficial portions of the entorhinal synaptic field, and intermediate medio‐lateral locations in the entorhinal area terminating in intermediate loca tions in the entorhinal synaptic zone. A similar graded pattern of termination of medial and lateral entorhinal fibers was apparent in the very slight crossed projection of the entorhinal area to the contralateral dentate gyrus. In addition, a comparable gradient in the pattern of termination of entorhinal fibers was evident in the entorhinal projection field in the distal dendritic regions of the pyramidal cells of regio inferior of the hippocampus proper. Entorhinal projections to regio superior were, however, organized in quite a different fashion. In this zone, there was no evidence of a proximo‐distal gradient in the patterns of termination of medial and lateral entorhinal areas along the dendrites of regio superior pyramidal cells. Rather, the medio‐lateral organization was in a longitudinal dimension, with medial entorhinal afferents terminating in the portions of regio superior near the CA1‐CA2 transition, and lateral entorhinal afferents terminating furthest from the CA1‐CA2 transition, immediately adjacent to the CA1‐subicular transition, and in the molecular layer of the subiculum proper. A comparable longitudinal organization of entorhinal projections to regio superior was also evident in the zones of termination of the crossed temporo‐ammonic tract, contralateral to the injection. These results demonstrate a heretofore unrecognized complexity in the patterns of projection of the entorhinal area to the hippocampal formation, and illustrate that the entorhinal cortex cannot be divided into only two discrete divisions on the basis of the pattern of projection.