Abstract
The Gyrus ambiens is a gross anatomical prominence in the medial temporal lobe (MTL), associated closely with Brodmann area 34 (BA34). It is formed largely by the medial intermediate subfield of the entorhinal cortex (EC) [Brodmann area 28 (BA28)]. Although the MTL has been widely studied due to its well-known role on memory and spatial information, the anatomical relationship between G. ambiens, BA34, and medial intermediate EC subfield has not been completely defined, in particular whether BA34 is part of the EC or a different type of cortex. In order to clarify this issue, we carried out a detailed analysis of 37 human MTLs, determining the exact location of medial intermediate EC subfield and its extent within the G. ambiens, its cortical thickness, and the histological–MRI correspondence of the G. ambiens with the medial intermediate EC subfield in 10 ex vivo MRI. Our results show that the G. ambiens is limited between two small sulci in the medial aspect of the MTL, which correspond almost perfectly to the extent of the medial intermediate EC subfield, although the rostral and caudal extensions of the G. ambiens may extend to the olfactory (rostrally) and intermediate (caudally) entorhinal subfields. Moreover, the cortical thickness averaged 2.5 mm (1.3 mm for layers I–III and 1 mm for layers V–VI). Moreover, distance among different landmarks visible in the MRI scans which are relevant to the identification of the G. ambiens in MRI are provided. These results suggest that BA34 is a part of the EC that fits best with the medial intermediate subfield. The histological data, together with the ex vivo MRI identification and thickness of these structures may be of use when assessing changes in MRI scans in clinical settings, such as Alzheimer disease.
Highlights
The entorhinal cortex (EC) or Cortex Entorhinalis1 is a component of the hippocampal formation (HF), which is formed by different archicortical and periarchicortical areas
In the present report we show that the ventral limit of EC subfield Entorhinal Medial Intermediate subfield (EMI) corresponds topographically to the intrarhinal sulcus
Our study shows that the Gyrus ambiens (GA) is an EC subfield (EMI) which, very likely, coincides with Brodmann area 34 (BA34) (Brodmann, 1909)
Summary
The entorhinal cortex (EC) or Cortex Entorhinalis is a component of the hippocampal formation (HF), which is formed by different archicortical (dentate gyrus, CA fields, subiculum) and periarchicortical areas (presubiculum, parasubiculum, EC). The EC laminar structure is known since Hammarberg (1895) and can be found in classical neuroanatomical work (Cajal, 1901; Lorente de Nó, 1933), it was not topographically described until Brodmann (1909) He divided the MTL into area 28 (EC), located in the anterior portion of the PHG, and area 34, which lies medial to area 28. The GA forms in the human brain a visible protuberance situated in the medialmost portion of the anterior temporal lobe This parcellation of the cerebral cortex is not completely accepted by contemporary authors who challenge the correspondence of Brodmann’s cortical areas as defined by functional studies (Nieuwenhuys et al, 2015). We wanted to determine the cytoarchitectonic fields that conform the GA, both at the mediolateral and rostrocaudal extents, as our previous cytoarchitectonic parcellation of the human EC suggest that the GA may contain more than one EC subfield (Insausti et al, 1995)
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