The CA2 hippocampal subfield in humans: A review

CA2 is probably the most enigmatic of the hippocampal fields. It is small in size (in humans about 500 μm across the mediolateral axis), and yet, it is involved in important functions, such as in social memory and anxiety. This study offers a glimpse of several significant aspects of the anatomical organization of CA2. We present an overview of the anatomical structure of CA2, imbued in the general organization of the human hippocampal formation. The location and distinctiveness of CA2 is presented in relation with CA3 and CA1, based in a total of 23 human control cases serially sectioned throughout the whole longitudinal axis of the hippocampus, examined every 500 μm in Nissl-stained sections. The longitudinal extent of CA2 is close to 30 mm, starting in the hippocampal head, 2.5 mm caudal to the DG and 3.5 mm caudal to the start of CA3, approximately 10 mm from the hippocampus rostral end. The connectional information of human CA2 is very scarce, thereby we relied on nonhuman primate tract tracing studies of the hippocampal formation, given its resemblance to the human brain. Human CA2 is subject of neuropathological studies, and we chose to present Alzheimer's disease, schizophrenia, and Mesial Temporal Lobe Epilepsy with hippocampal sclerosis in those aspects that impinge directly into CA2.