hippocampal anatomy

The hippocampus, or intralimbic gyrus, is formed by 2 cortical laminae embedded in each other: the cornu ammonis (CA), also called the hippocampus proper or Ammon horn, and the dentate gyrus. The CA can be divided into regions, or fields, depending on the appearance of pyramidal neurons.

The 4 fields (named by Lorente de No in 1934) are characterized as follows8 :
1-CA1, or the Sommer sector, is the most vulnerable region; it is the most sensitive to hypoxia.
2-CA2 is the most resistant and well-preserved sector.
3-CA3, which enters the concavity of the dentate gyrus, is slightly vulnerable.
4-CA4, sometimes called the endfolium, has intermediate vulnerability to insults.

The following 3 patterns of cell loss are described in the hippocampus:
1-Classic Ammon horn sclerosis - Primary neuronal loss involves CA1 and CA4; occurs less often in C3 and least often in CA2
2-Total Ammon horn sclerosis - Severe neuronal loss in all of the hippocampal zones, CA1 to CA4
3-Endfolium sclerosis - Cell loss restricted to CA4

http://emedicine.medscape.com/article/342150-overview