The rat hippocampal formation is a C-shaped structure that is situated in the caudal part of the brain (see animation below). The cortex that forms the hippocampal formation has a three-layered appearance, typical of the so-called allocortex.

  • On top, the most superficial layer is situated - this layer is referred to as the molecular layer (stratum moleculare) in the DG and the Sub. In the CA-region, the molecular layer is subdivided into a number of sub-layers. In the CA3, three sub-layers are distinguished: The stratum lucidum, representing the input from the DG, stratum radiatum, i.e the apical dendrites of the neurons in stratum pyramidale and most superficially, the stratum lacunosum-moleculare comprising the apical tufts of the apical dendrites. The lamination in the CA2 and the CA1 is similar with the exception that the stratum lucidum is missing (see Figure 1 in our NRN Review).
  • Below the molecular layer and superficial to a deep, polymorph layer is the cell layer, which is composed of principal cells and interneurons.
  • A deep layer, comprising a mixture of afferent and efferent fibers and interneurons.

Which subdivisionsare are considered part of the hippocampal formation and which are part of the parahippocampal region is still a matter of debate. The 2 criteria that we used to include structures into the definition of the hippocampal formation are:

  • Laminar organization: the hippocampal cortical sheet has a three-layered appearance that is characteristic of the allocortex.
  • Connectivity: the connectivity between the structures in the hippocampal formation is largely unidirectional, which is extremely atypical for corticocortical connections.

An extensive overview of the criteria for subdividing the hippocampal formation and parahippocampal region is provided at http://www.rbwb.org (Kjonigsen et al, 2011).

 

Although the video above depicts the postion of the hippocampal formation in the mouse brain, it closely resembles the position of the hippocampal formation in the rat brain. In video lesson 1, three-dimensional position of the hippocampal formation is explained in greater detail. To better understand the position of the hippocampal formation in the rodent brain, take a look at this interactive model of the mouse brain.

The authors of these web pages express their gratitude to the many authors that helped provide order and overview in the field of hippocampal anatomy and connectivity. A special mention of thanks for the authors of the 1st, 2nd and 3rd edition of the hippocampus chapter in the book titles The Rat Nervous System  and The Parahippocampal Region. The authors of this website welcome feedback to improve the information that is displayed. Please use the contact form for such enquiries. These pages are currently (2013) under active development and will be frequently updated and expanded.

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