The indusium griseum in the mouse: architecture, Timm's histochemistry and some afferent connections.

The Indusium griseum (IG) is an enigmatic cortical field classically felt to be a part of the hippocampus (HC). In the mouse, IG lies just dorsal to the corpus callosum at the base of the anterior half of cingulate cortex. In coronal sections the field is small but constitutes a fairly long rostro-caudal strip. The connections of the IG are poorly understood. The Timm's staining pattern of the IG is reminiscent of a mini-dentate gyrus (DG) comprising a layer of granule cells with two bands of staining in the molecular layer. In the DG there are three bands which correspond to inputs from the lateral and medial entorhinal area (LEA and MEA) and the ipsi- and contralateral association systems. Using anterograde transport of HRP we have found that the LEA and MEA also terminate in the molecular layer of the IG. This suggests that the IG is a displaced portion of the DG. The olfactory system is known to have a strong indirect influence on the HC via primary and secondary bulbar projections to the LEA. Wheat germ agglutinin-HRP injections confined to the main olfactory bulb (MOB) show a direct projection from the MOB to IG. Both the olfactory bulb itself and retrobulbar structures such as the piriform cortex (PC) convey olfactory information to the LEA; the LEA supplies a major input to the DG. Our results suggest that there is a more direct pathway whereby olfactory information may influence a cortical region, IG, whose histochemistry and direct afferents from the entorhinal cortex suggest that it is part of or closely related to the DG. Thus, IG may represent a phylogenetically old olfacto-recipient outpost of the hippocampus.