Entorhinal Cortex (EC)

Boundaries associated with this structure:
Olfactory and periamygdaloid cortex
Postrhinal (POR)
Parasubiculum (PaS)
Perirhinal cortex, area 35 (PER35)

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Entorhinal Cortex (EC)

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The entorhinal cortex (EC) occupies the ventro-caudal part of the cerebral hemisphere where it forms a cap-like structure. Its surface can be viewed as an ellipsoid with the white matter of the angular bundle as its center. The entorhinal cortex is hodologically defined by axonal projections from layer II neurons to the dentate gyrus of the hippocampal formation. Anteriorly, the entorhinal cortex is flanked by the piriform cortex laterally, and by the periamygdaloid cortex and the posterior cortical nucleus of the amygdala, medially (non-hippocampal regions not further described in this atlas). The transition between the entorhinal cortex and its anterior neighbors is approximately at the midst of the amygdaloid fissure, where the entorhinal cortex progressively decreases in width, such that it eventually extends anteriorly for approximately 2 mm as a narrow strip.

This anterior extension is delimited dorsolaterally by the perirhinal cortex and ventromedially by the piriform cortex. At its laterocaudal site, the entorhinal cortex is surrounded by the perirhinal and postrhinal cortices. Medially, the entorhinal cortex is bordered over most of its rostrocaudal extent by the parasubiculum. For further cytoarchitectonic descriptions, see its 5 subdivisions; dorsal-lateral entorhinal area (DLE), dorsal-intermediate entorhinal area (DIE), ventral-intermediate entorhinal area (VIE), medial entorhinal area (ME) and caudal entorhinal area (CE).

The lateral and caudal borders between EC and the perirhinal and postrhinal cortices are characterized by:

1. the disappearance of the lamina dissecans
2. the part of EC at the border stains densely for parvalbumin whereas the adjacent perirhinal and postrhinal cortex are almost devoid of positive staining
3. the latter areas stain for calbindin which is much less conspicuously present in the adjacent parts of EC
4. layer II of EC is characterized by a population of large to medium sized neurons that stain very densely for neuronal markers such as Nissl or NeuN. The adjacent parts of perirhinal and postrhinal cortices are characterized by blended layers II and III consisting of small, lightly stained neurons

The anterior border of EC is indicated by:

1. a reduction in the number of cell layers from six down to three
2. weak overall staining for parvalbumin in both anterior parts of EC as well as the neighboring areas, so this does not provide a clear indication of the border
3. staining for calbindin that shows less dense staining in the anterior portions of EC compared to the adjacent regions

The medial border with the parasubiculum features:

1. a striking merge between layers II and III in the parasubiculum
2. decreased staining intensity and homogeneity for calbindin in the neuropil of all layers
3. dense staining for acetylcholinesterase in layers I-III in parasubiculum

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