Region I of hippocampus proper (CA1) is a portion of the hippocampal formation. CA stands for the latincornus ammonis.
Region CA1, or Area CA1 is the first region in the hippocampal circuit that yields a significant output pathway, which goes to entorhinal cortex layer V. It also sends significant output forward to the subiculum. Like CA3, it receives input from superficial entorhinal cortex along the perforant pathway (note that some authors only consider the input to DG and CA3 to be the perforant pathway, referring to the input to CA1 as the temporoammonic pathway). Unlike CA3, however, it contains very few recurrent connections. In the rat, CA1 contains approximately 250,000 pyramidal cells.