The dendritic morphology of pyramidal neurons in the rat hippocampal CA3 area. I. Cell types.

Pyramidal neurons from the hippocampal CA3 area of hooded rats were qualitatively and quantitatively described from Golgi-stained tissue. The most numerous of the pyramidal neurons, those with a single apical shaft, fell into two major categories. One category, termed short-shaft pyramidal neurons, is characterized by short apical shafts, a large number of thorny excrescences, and densely branched apical and basilar trees. The second category, long-shaft pyramidal neurons, is characterized by a long apical shaft, a small number of thorny excrescences and relatively less dendritic branching in both the apical and basilar trees. The somata of short-shaft pyramidal neurons tend to be located higher in stratum pyramidale than the somata of long-shaft neurons. Quantitative measurements, which included both analysis of dendritic branching and the distribution of dendritic material sampled at 10% intervals from the cell body, confirmed the qualitative observation that short-shaft neurons had more total dendritic length than long-shaft neurons. The difference in the total dendritic length observed between long- and short-shaft pyramidal neurons could be an indication that each type receives a different number of synapses per neuron. The less commonly observed variants of pyramidal neurons were briefly described but not quantified. This study demonstrates that CA3 pyramidal neurons are not a homogeneous group but that their heterogeneous characteristics fall into two major categories.