Calretinin immunoreactive structures in the human hippocampal formation

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Abstract
The calcium‐binding protein calretinin is present in an intrinsic GABAergic and an extrinsic non‐GABAergic system in the rat and monkey hippocampal formation. Important species differences have been noted in hippocampal cell types immunostained for calretinin and the termination pattern of calretinin containing hypothalamic afferents in the hippocampus. In the present study, calretinin‐containing neurons were visualized using immunocytochemistry in the human hippocampal formation of individuals which showed no significant neuropathological alterations. Calretinin‐immunoreactivity was present exclusively in non‐granule cells of the dentate gyrus and in non‐pyramidal cells of Ammon's horn. Calretinin‐positive neurons were found most frequently in the hilus of the fascia dentata and in strate radiatum and lacunosum‐moleculare of CA1, whereas neurons in CA2 and CA3 were rarely immunostained. The majority of calretinin‐immunoreactive neurons were small, bipolar or fusiform neurons. The dendritic trees of the calretinin‐positive neurons were, for the most part, parallel to the dendrites of the principal cells. In the hilus, however, we observed cells with dendrites restricted to the hilar area. These dendrites were parallel to the granule cell layer. In the stratum lacunosum‐moleculare, neurons with dendrites oriented parallel to the hippocampal fissure were frequently detected. In general, dendrites were smooth or sparsely spiny, displaying small conventional spines. The axons usually emerged from the proximal dendrite and could be followed over long distances. Axons were thin, had small varicosities and displayed only few collaterals which branched relatively far away from the cell body. Distinct bands of darkly stained calretinin‐positive fibers occupied the innermost portion of the dentate molecular layer and the pyramidal cell layer of CA2. This distribution of calretinin‐immunoreactive structures in the human hippocampus is similar to that observed in other primates but differs from that described in lower mammals, i.e., the rat. Our findings suggest that primates may share a common hippocampal calrtinin‐containing system, presumably both the intrinsic GABAergic and the extrinsic hypothalamic non‐GABAergic components.