The rat parahippcampal region (or retrohippocampal region) lies adjacent to the hippocampal formation (bordering with the subiculum). The subfields of the parahippocampal region can be distinguished from those of the hippocampal formation by:
- Laminar organization: An increase in the number of cell layers as compared with the hippocampal formation.
- Connectivity: the connectivity of the structures of the parahippocampal region typically show reciprical connections.
At the junction with the subiculum, superficially positioned cell layers become apparent and a cell-free zone called lamina dissecans exists in-between the two main neuronal sheets. The parahippocampal region is generally described as having six layers, although it consists of transition in cortex from peri-allocortex, through pro-isocortex to the isocortex. Based on cytoarchitectonics, the parahippocampal region is divided into five main sub-regions: the presubiculum (PrS), parasubiculum (PaS), entorhinal cortex (EC), perirhinal cortex (PER) and postrhinal cortex (POR). The coordinate systems that defines the position within the PHR is explained in Figure 1 of our NRN Review.
An extensive overview of the criteria for subdividing the rat hippocampal formation and parahippocampal region is provided by Kjonigsen et al, 2011. You can find an atlas representation of this information via this link: http://cmbn-navigator.uio.no/rat_hippocampus_atlas/structureindexA.html