Dendritic morphology of CA1 pyramidal neurones from the rat hippocampus: I. Branching patterns.

The aim of this study was to provide quantitative descriptions of the dendritic branching patterns of pyramidal neurones in the CA1 region of the rat hippocampus. Thirteen adult cells were filled with biocytin and reconstructed by using the light microscope. The number of basal trees arising from the soma of each cell ranged from two to eight. There was wide variation in the number of terminal segments per tree. Six cells had single apical trunks, and seven had trunks that bifurcated in stratum radiatum. The number of apical oblique trees ranged from nine to 30, with each tree usually showing a lower degree of branching than basal trees. Basal and oblique trees had similar branching patterns, with the majority of branch points occurring close to the origin of the tree. Both basal and oblique terminal segments were generally much longer than intermediate segments and constituted up to 90% of the combined dendritic length of the tree. The branching pattern of the apical tuft was different, with many relatively long intermediate segments; terminal segments contributed only some 66% of the combined dendritic length of these trees. The mean total combined dendritic length for six adult cells reconstructed and measured completely was 11,900 +/- 1,000 microns (standard deviation). The relative proportions of the different parts of the dendritic system, although not the total dendritic length, were correlated with the location of the soma relative to the cell body layer. Cells with somata close to the stratum pyramidale/stratum radiatum border had more dendrites terminating in stratum radiatum and fewer in stratum oriens than cells with somata further from it.