The hippocampal continuation (indusium griseum): its connectivity in the hedgehog tenrec and its status within the hippocampal formation of higher vertebrates.

The indusium griseum and its precallosal extension are usually considered poorly differentiated portions of the hippocampus. The connections of this so-called 'hippocampal continuation' (HCt) have only been analyzed so far in rodents, which show one of the least-developed HCt among mammals. In this study we have investigated the relatively well differentiated HCt of the small Madagascan hedgehog tenrec (Afrotheria) using histochemical and axonal transport techniques. The tenrec's HCt shows associative and commissural connections. It receives laminar specific afferents from the entorhinal cortex (collaterals from neurons projecting to the dentate area), the anterior and posterior piriform cortices as well as the supramammillary region. A few fibers also originate in the olfactory bulb and the dentate hilus. Among these input areas only the dentate hilus receives a significant reciprocal projection from the HCt. Additional HCt efferents are directed to the subcallosal septum (presumed septohippocampal nucleus), the olfactory tubercle and the islands of Calleja. With the exception of the supramammillary afferents and possible efferents to the supraoptic nucleus we failed, however, to demonstrate distinct thalamic and hypothalamic connections. A comparison of the connections of the HCt with those of the hippocampal subdivisions reveal some similarity between the HCt and the dentate area, but the overall pattern of connectivity does not permit a correlation of the HCt with the dentate area, let alone the cornu ammonis and the subiculum. This view is supported by histochemical findings in the tenrec (immunoreactivity to calcium binding proteins) as well as the rat (data taken from the literature). The HCt is therefore considered a region in its own right within the hippocampal formation. It may be tentatively correlated with the medial cortex of reptiles, while the dentate area and the cornu ammonis may have evolved de novo in mammals.