A quantitative analysis of the dendritic organization of pyramidal cells in the rat hippocampus

Abstract

The three dimensional organization of the dendritic trees of pyramidal cells in the rat hippocampus was investigated using intracellular injection of horseradish peroxidase in the in vitro hippocampal slice preparation and computer‐aided reconstruction. The total dendritic length, dendritic length in each of the hippocampal laminae, and the number of dendritic branches were measured in 20 CAl pyramidal cells, 7 neurons in CA2 and 20 CA3 pyramidal cells. The total dendritic length of CA3 pyramidal cells varied in a consistent fashion depending on their position within the field. Cells located close to the dentate gyrus had the smallest dendritic trees which averaged 9,300 μm in total length. Cells in the distal part of CA3 (near CA2) had the largest dendritic trees, averaging 15,800 μm. The CA2 field contained cells which resembled CA3 pyramidal cells in most respects except for the absence of thorny excrescences on their proximal dendrites. There were also smaller pyramidal cells that resembled CAl neurons. CAl pyramidal cells tended to be more homogeneous. Pyramidal neurons throughout the transverse extent of CAl had a total dendritic length on the order of 13,500 μm. The quantitative analysis of the laminar distribution of dendrites demonstrated that the stratum oriens and stratum radiatum contained significant portions of the pyramidal cell dendritic trees. In CA3, for example, 42–51% of the total dendritic length was located in stratum oriens; about 34% of the dendritic tree was located in stratum radiatum. The amount of dendritic length in stratum lacunosum‐moleculare of CA3 varied depending on the location of the cell. Many CA3 cells located within the limbs of the dentate gyrus, for example, had no dendrites extending into stratum lacunosum‐moleculare whereas those located distally in CA3 had about the same percentage of their dendritic tree in stratum lacunosum‐moleculare as in stratum radiatum. In CAl, nearly half of the dendritic length was located in stratum radiatum, 34% was in stratum oriens and 18% was in stratum lacunosum‐moleculare. These studies identified distinctive dendritic branching patterns, in the stratum radiatum and stratum lacunosum moleculare, which clearly distinguished CA3 from CAl neurons. © Wiley‐Liss, Inc.