Columnar organization in the subiculum formed by axon branches originating from single CA1 pyramidal neurons in the rat hippocampus.

An intracellular horseradish peroxidase study combined with immunoperoxidase techniques was carried out on hippocampal CA1 pyramidal neurons in the rat. Most axon branches originating from a single CA1 pyramidal neuron ran caudally and terminated in the subiculum. The individual axon branches of the single pyramidal neurons bifurcated repeatedly in the subiculum and finally formed a slab-like or columnar terminal arborization (250-300 microns wide, 500-550 microns high and 1.8-2.2 mm long). The present results suggest, in association with other data, that the CA1 pyramidal neurons receive afferent information through lamellar organized connections and they send efferent information to the subiculum through columnar organized connections.