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Cornu Ammonis

The Cornu Ammonis (CA) subfield forms the area between dentate gyrus and the subiculum and is also referred to as the 'hippocampus proper'. It is subdivided into subfields CA3, CA2 and CA1, according to the nomenclature of Lorente de Nó (1934). The polymorphic layer of the dentate gyrus is sometimes referred to as CA4, also defined by Lorente de Nó (1934). However, the existence of CA4 is disputed and we do not delineate this subdivision in this connectome.

  • CA3 borders with the dentate gyrus and CA2
  • CA2 borders with CA3 and CA1. The CA3/CA2 border is marked by a thickening of the stratum lucidum of CA3.
  • CA1 borders with CA2 and the subiculum

Stratification

In this description of the stratification/lamination of the Cornu Ammonis, the most superficial layer, close to the pial surface of the brain comes first, followed by the layers below. The stratification of the Cornu Ammonis is largely the same for all subfields. The main difference is that CA3 has a layer called the stratum lucidum, whereas this layer is absent in CA2 and CA1. Below, the layers of CA3, CA2 and CA1 are summarized.

CA3

  • Stratum lacunosum-moleculare; contains the terminations of, for example, the perforant pathway.
  • Stratum radiatum; contains CA3-CA3 associational connections and CA3-CA1 Schaffer collaterals.
  • Stratum lucidum; contains the mossy fiber axons from the dentate gyrus.
  • Pyramidal cell layer or stratum pyramidale; principal cell layer.
  • Stratum oriens.

CA2 / CA1

  • Stratum lacunosum-moleculare.
  • Stratum radiatum.
  • Pyramidal cell layer or stratum pyramidale; principal cell layer.
  • Stratum oriens.

Cells

Several good overviews exist, mostly tailored towards the physiology of interneurons (e.g. Freund and Buzsaki, 1996; Klausberger, 2009). An online repository containing all known cell types of the hippocampal formation and their characteristics exists at: wwww.hippocampome.org

The animation above displays a CA3 (left) and CA1 (right) pyramidal neuron. The cells were reconstructed by Ishizuka, 1995 and downloaded from neuromorpho.org (Ascoli, 2018).

Suggested reading

Boccara, C. N., Kjonigsen, L. J., Hammer, I. M., Bjaalie, J. G., Leergaard, T. B., & Witter, M. P. (2015). A three-plane architectonic atlas of the rat hippocampal region. Hippocampus, 25(7), 838–857. CA1 in Hippocampus Atlas | CA2 in Hippocampus Atlas | CA3 in Hippocampus Atlas

Cappaert, N. L. M., Van Strien, N. M., & Witter, M. P. (2015). Chapter 20 - Hippocampal Formation. In G. Paxinos (Ed.), The Rat Nervous System (Fourth Edition) (pp. 511–573). San Diego: Academic Press. € Buy from Elsevier

Dudek S.M., Alexander G.M. & Farris S. (2016) Rediscovering area CA2: unique properties and functions. Nat Rev Neurosci, 17(2), 89-102.

References

Akram M.A., Nanda S., Maraver P., Armañanzas R. & Ascoli G.A. (2018). An open repository for single-cell reconstructions of the brain forest. Sci Data Resolve DOI

Freund T.F. & Buzsáki G. (1996). Interneurons of the hippocampus. Hippocampus 6(4),347-470. Resolve DOI

Ishizuka N., Cowan W.M. & Amaral DG. (1995). A quantitative analysis of the dendritic organization of pyramidal cells in the rat hippocampus. J Comp Neurol 362(1),17-45. Resolve DOI

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