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refers to a cortical area defined on the basis of internal structure and located in the hilus of the dentate gyrus in the human ( Duvernoy-2005 ) and the macaque ( Rosene-1987 ). It is not clearly distinguishable in the rat and is often designated as part of the polymorphic layer of the dentate gyrus ( Amaral-1978 ).
CA4 borders the CA3 field where the pyramidal cell layer of the CA fields enters the hilus of the dentate gyrus and spreads out to occupy most of the hilus ( de No-1934 ). The polymorphic layer of the dentate gyrus is clearly distinct from CA4 and serves as its border with the dentate gyrus ( Rosene-1987 ). The internal structure of CA4 is not uniform, the neuronal cell bodies are ovoid, large, few in number and scattered among intertwined myelinated fibers.
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