Pyramidal neuron types in field CA2 of the guinea pig

The aim of the present study was to obtain information about the pyramidal neuron types in hippocampal field CA2 of the guinea pig. The apical and bas al dendritic trees and the somata of CA2 pyramidal neurons were analyzed an d quantified in Golgi-stained brains of adult guinea pigs of both sexes. Mo st of field CA2 pyramidal neurons (92%) had a single apical shaft (monoapic al neurons) and very few neurons (8%) had two apical shafts (biapical neuro ns). The monoapical neurons were subdivided into three classes on the basis of the pattern of their apical tree. Morphometric analysis showed differen ces among the different neuron classes in the number of apical and basal de ndritic branches, in the spread of the apical tree, in shaft dimensions and in soma diameters. Several morphometric sex differences were also found fo r two of the three monoapical neuron classes and for the biapical neurons. The results demonstrate that field CA2 of the guinea pig contains four diff erent types of pyramidal neurons and that also this small hippocampal field is characterized, like the other hippocampal fields, by the presence of a heterogeneous population of pyramidal neurons. (C) 1999 Elsevier Science In c.