The term CA4 field refers to a cell group in the hilus of the dentate gyrus of the human ( Duvernoy-2005 ) and the macaque ( Paxinos-2009a ). It is not found in the rat and mouse where the dentate gyrus has no hilus ( Insausti-2012 ). On one side CA4 borders the polymorphic layer of the dentate gyrus; on the other it borders the CA3 field ( 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. Updated 31 May 2024.