The alvear pathway of the rat hippocampus.

Neurons of the entorhinal cortex project to the hippocampus proper and dentate gyrus. This projection is called the "perforant pathway" because it perforates the subiculum; current usage applies this term to all entorhino-hippocampal fibers. However, entorhinal fibers also reach Ammon's horn via the alveus ("alvear pathway"), an alternative route first described by Cajal. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) was used in order to analyze the contribution of this pathway to the temporo-ammonic projection. In the temporal portion of the rat hippocampus, most of the entorhinal fibers reach Ammon's horn after perforating the subiculum (classical perforant pathway). At more septal levels, the number of entorhinal fibers that take the alvear pathway increases; in the septal portion of the hippocampal formation, most of the entorhinal fibers to hippocampal subfield CA1 reach this subfield via the alveus. These fibers make sharp right-angle turns in the alveus, perforate the pyramidal cell layer, and finally terminate in the stratum lacunosum-moleculare. The crossed temporo-ammonic fibers reach their termination area in the stratum lacunosum-moleculare of CA1 almost exclusively via the alveus. These data indicate that the alveus is a major route by which entorhinal fibers reach their targets in CA1.