Light-microscopic investigations on the problem of a dendritic bundling in the rat hippocampus

Light-microscopic investigations were devoted to the arrangement of the apical dendrites of the pyramidal cells of the CA 1 and CA 3 fields and the subiculum in the hippocampus of adult rats. By means of sectional reconstructions, a dendritic bundling was found to occur in the above mentioned regions. The bundles of the CA 1 region, with a length of 50-80 microns, are detectable in the stratum pyramidale and in the transitional region to the stratum radiatum. They include 2-5, and in single cases up to 9, dendrites. The groupings of the apical dendrites of the CA 3 pyramidal cells, with a length of appr. 50 microns, extend from the stratum pyramidale into the stratum lucidum. The number of dendrites grouped together is larger than in the CA 1 region; a more pronounced row configuration of the dendrites in the bundle prevails. The formation of dendrite bundles is most pronounced in the subiculum: appr. 10 dendrites form 80 micron long units. Their appearance resembles that of the bundles known from the neocortex very closely. Referring to dendrodendritic membrane contacts (gap junctions or casual appositions), which allow an electrotonic coupling, and hence the development of a synchronous activity within the groups of neurons defined by the dendrite bundle, a possible functional significance of the bundles in the hippocampus is pointed out.