Fig. 1. Morphology of stratum oriens interneurons. Photomicrographs of the four subtypes of biocytin-filled stratum oriens interneurons analyzed in this study. A, An interneuron with its axon almost exclusively innervating stratum pyramidale, in which the CA1 pyramidal cell bodies are located.B, An OLM or horizontal cell. Note the dense axon termination in stratum-lacunosum moleculare and the horizontally oriented dendrites in stratum oriens. These are the defining anatomical characteristics of this cell type. C, An interneuron with its primary axonal ramification in stratum radiatum.D, An interneuron with its primary axonal projection confined to stratum oriens. The arrowheads denote stratum pyramidale in A, C, andD. Scale bar, 100 μm. a, Alveus;SO, stratum oriens; SP, stratum pyramidale; SR, stratum radiatum;SL-M, stratum lacunosum-moleculare.