Distribution of calretinin immunoreactivity in the mouse dentate gyrus. I. General description.

Calretinin-containing elements were visualized with immunocytochemistry in the adult mouse dentate gyrus (DG). In the ventral DG calretinin immunoreactive (CR-IR) large multipolar cells were clustered; they extended between two and four thick cylindrical dendrites which further branched into several thinner processes. Characteristic grape-like spiny appendages were occasionally observed on these thick and thinner dendritic processes. On the basis of these structural features these large CR-IR cells were identified as hilar mossy cells. At the supragranular zone a dense CR-IR band was seen, where numerous CR-IR punctae and fibers were packed tightly among putative granule cell dendrites. In the granule cell layer, especially at the dorsal DG, numerous faintly CR-IR cells were located at the interface with the hilus. They were triangular in shape and neither calbindin D28k nor GABA positive, but were immunoreactive for highly polysialylated neural cell adhesion molecule (NCAM-H) and thus considered as newly generated neurons. In the molecular layer CR-IR cells were also scattered; they were mainly located near the pial surface and the hippocampal fissure, small in size, ovoid in shape and usually gave rise to one very thin axon-like and one thin cylindrical dendritic process. These cells were assumed to be Cajal-Retzius cells. Throughout the layers, that is, the molecular layer, the granule cell layer and the hilus, CR-IR multipolar and/or fusiform cells were encountered. They resembled those reported in the rat DG in their structural features and usually extended smooth or varicose or sparsely spiny dendritic processes; some of them were confirmed to be GABA-like immunoreactive and/or glutamic acid decarboxylase immunoreactive. The present study showed that CR immunoreactivity in the mouse DG differed significantly from that in the rat and monkey dentate gyri reported previously.