Mossy cells in the mouse dentate gyrus: identification in the dorsal hilus and their distribution along the dorsoventral axis

Previously we showed that large multipolar cells immunoreactive for calretinin and subunits 2 and 3 of amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptors (GluR2/3) clustered in the ventral hilus of the mouse dentate gyrus and revealed that they were mossy cells. Although such large calretinin immunoreactive cells were not seen in the dorsal hilus, our Golgi study revealed the presence of mossy cells in the dorsal hilus. As we observed large intensely GluR2/3 immunoreactive cells in the dorsal hilus, we suggested that these calretinin negative but intensely GluR2/3 positive large cells in the dorsal hilus were also mossy cells. In the present study we confirmed this identification with several methods. The extracellular tracer labeling studies revealed that all of 47 mossy cells identified morphologically were intensely GluR2/3 positive but calretinin negative, whereas none of 22 non-mossy hilar neurons were intensely GluR2/3 positive. Electron microscopically most of intensely GluR2/3 positive somata and dendritic processes showed the characteristic ultrastructural features of mossy cells. Furthermore, the fimbria–fornix–hippocampal commissure transection procedures induced the calretinin expression in some of these dorsal GluR2/3 immunoreactive cells. On the basis of these observations, we concluded that the vast majority of intensely GluR2/3 immunoreactive large cells in the mouse dorsal hilus were mossy cells. Then we evaluated the presumed difference in the distribution of mossy cells along the dorsoventral axis by the disector. The numerical density of mossy cells was about 1.4 times larger at the ventral level than at the dorsal level, indicating that the dorsoventral difference in the distribution of mossy cells in the mouse hilus was far smaller than that previously speculated.