Entorhinal projections to the hippocampal CA1 region in the rat: an underestimated pathway.

The projections of the entorhinal cortex to CA1 in relation to the entorhinal-dentate projections were studied in the rat, using the anterograde transport of Phaseolus vulgaris leucoagglutinin. It was observed that the entorhinal cortex is heterogeneous with respect to the origin of these projections. Caudomedial portions of the entorhinal cortex mainly distribute fibers to the fascia dentata, whereas only a minor projection reaches CA1. Progressively more rostral and lateral parts of the entorhinal cortex project more strongly to CA1, at the expense of the number of fibers that terminate in the fascia dentata. The rostrolateral part of the entorhinal cortex, adjacent to the olfactory cortex and the amygdaloid complex, projects only to CA1.