Abstract
The indusium griseum (IG) is a cortical structure overlying the corpus callosum along its anterior–posterior extent. It has been classified either as a vestige of the hippocampus or as an extension of the dentate gyrus via the fasciola cinerea, but its attribution to a specific hippocampal subregion is still under debate. To specify the identity of IG neurons more precisely, we investigated the spatiotemporal expression of calbindin, secretagogin, Necab2, PCP4, and Prox1 in the postnatal mouse IG, fasciola cinerea, and hippocampus. We identified the calcium-binding protein Necab2 as a first reliable marker for the IG and fasciola cinerea throughout postnatal development into adulthood. In contrast, calbindin, secretagogin, and PCP4 were expressed each with a different individual time course during maturation, and at no time point, IG or fasciola cinerea principal neurons expressed Prox1, a transcription factor known to define dentate granule cell fate. Concordantly, in a transgenic mouse line expressing enhanced green fluorescent protein (eGFP) in dentate granule cells, neurons of IG and fasciola cinerea were eGFP-negative. Our findings preclude that IG neurons represent dentate granule cells, as earlier hypothesized, and strongly support the view that the IG is an own hippocampal subfield composed of a distinct neuronal population.
Highlights
The indusium griseum (IG) is a thin bilateral stripe-like allocortical brain area covering the entire anterior–posterior extent of the corpus callosum at the basis of the cingulate cortex
We detected a transition zone between IG and fasciola cinerea (FC) (Figure 1F), not observed between the IG and dentate gyrus (DG), which were always separated through the hippocampal fissure (Figure 1C)
The FC can be recognized by its pyramidal cell layer, which is oriented perpendicular to the hippocampal pyramidal cell layer (Figures 1I,O)
Summary
The indusium griseum (IG) is a thin bilateral stripe-like allocortical brain area covering the entire anterior–posterior extent of the corpus callosum at the basis of the cingulate cortex. IG neurons in the mouse brain expressing the transcription factor Rfx contribute to the formation of both the hippocampal fissure and the corpus callosum (Benadiba et al, 2012). Apart from this transient function as a source of guideposts, another role of the IG during development gets significant support by a recent study describing the IG as an important prenatal relay station in developing limbic and olfactory circuits (Fuzik et al, 2019). More specific functions of the mature IG remain elusive
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