Cornu ammonis of the cat: lack of a separate field of CA2.

Close comparison of Timm or Nissl stained adjoining sections of the cat cornu Ammonis (CA) disclosed the lack of a separate CA2 field, which was defined as a field of CA characterized by giant pyramids similar to those of CA3 and by absence of mossy fiber inputs in mice, rats, monkeys and humans. In the cat CA, mossy fiber terminals stained by Timm's method filled the hilar region (CA4) of the dentate gyrus and extended through the stratum pyramidale and/or stratum lucidum of the giant pyramidal part of CA as far as the border with CA1, which was clearly delineated by sudden appearance of smaller pyramids. Thus, there was no separate giant pyramidal field devoid of mossy fiber inputs in the cat CA. Unlike other mammals, some parts of CA1 also received mossy fiber projections in the cat.