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

Abstract

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.

Telencephalic binding sites for oxytocin and social organization: A comparative study of eusocial naked mole‐rats and solitary cape mole‐rats

“…The function of the indusium griseum remains unclear. Tract-tracing studies indicate that, in rats and mice (Wyss and Sripanidkulchai, 1983;Adamek et al, 1984), and to a lesser extent in Madagascan hedgehogs (Kunzle, 2004), the indusium griseum receives direct projections from the main olfactory bulb. In hedgehogs, efferents of the indusium griseum reach, among other sites, the minor islands of Calleja (Kunzle, 2004).…”

Contribution of Genoarchitecture to Understanding Hippocampal Evolution and Development

“…This previous proposal is also supported by the comparison with the avian medial pallium, showing the high similarity in terms of position and cytoarchitecture of reptilian MC and at least the avian ventral hippocampus. The fact that the latter expresses Prox1 is also a proof that a DG-like area was likely present in the medial pallium of stem amniotes [Abellán et al, 2014], instead of being a novel acquisition of mammals [Künzle, 2004;Striedter, 2016]. If confirmed that the MC of reptiles also expresses Prox1, this would also disagree with the proposal that this reptilian structure is comparable to the mammalian indusium griseum , as the latter does not express Prox1 [Abellán et al, 2014].…”

“…In all mammals, it consists of identical, similarly convoluted and interlocked 3-layered subdivisions, which from medial to lateral are the dentate gyrus (DG), the hippocampus proper or Ammon's horns ( cornu ammonis or CA fields 1-3), and the subiculum (Sub) [Witter and Amaral, 2004;Hevner, 2016]. It is rostrally continuous with a narrow strip of gray matter called indusium griseum , which is sometimes considered a hippocampal vestige [Künzle, 2004;Laplante et al, 2013]. For details on the structure, connections, and function of the HF, the readers are also referred to other articles of this special issue [for instance, see Fig.…”

Functional Connectivity in Rat Brain at 200 μm Resolution

“…In human brain, IG is found in the supracallosal gyrus and is made of a thin layer of gray matter. As a continuation of the hippocampus, it receives laminar-specific afferent inputs from the entorhinal cortex, the anterior and posterior piriform cortices, and the supramammillary region (Kunzle, 2004). This region was previously believed to be related to the olfactory system, but few studies have been reported.…”