Cornu Ammonis of the dog: a rudimentary CA2 field is only present in a small part of the dorsal division, and is absent in the ventral division of the cornu Ammonis.

Abstract

The relationship between mossy fiber projections and cytoarchitecture was studied in the cornu Ammonis (CA) of three mongrel dogs and two Japanese white rabbits. Precise comparisons were accomplished by examining adjoining sections that were stained by two methods: 1) the Timm-Danscher method which stains for zinc, which is contained in high concentrations in the mossy fiber terminals, and 2) thionin staining of the Nissl substance, so that the mossy fiber projections and cytoarchitecture could be closely compared in each subdivision of the CA. There was a cytoarchitectonically transitional zone (TZ) between CA3 and CA1 in both dogs and rabbits. The TZ was composed of an intermingling of CA3-like large pyramidal cells and CA1-like small pyramidal cells. Across the dorso-ventral length of the rabbit CA, the mossy fiber projections emitted from the dentate granular cell layer extended as far as the distal border of CA3 toward the TZ but did not enter the TZ or CA1. The TZ of the rabbit was equivalent to CA2 of Lorente de Nó (1934), that was characterized by CA3-like large pyramidal cells which did not receive mossy fiber inputs. In the dorsal CA of the dog, the mossy fiber projections extended across the CA3/TZ border, as far as the most distal part of the TZ in one dog but only for a short distance in the other two dogs. That part of the TZ which did not receive mossy fiber projections corresponded with CA2. There were no mossy fiber projections to CA1. In the ventral CA of the dog, however, mossy fiber projections were more extensive than in the dorsal CA, extending across CA3/TZ as well as TZ/CA1 borders to enter the proximal part of CA1. These observations indicate that there is a distinct CA2 in the rabbit CA, and that a rudimentary CA2 is present only in a small part of the dorsal CA of the dog, whereas it is absent in the ventral CA.