Figure 2. Morphological characterization of O-LM and trilaminar interneurones in stratum oriens.
Reconstructions of representative biocytin-filled O-LM (A) and three trilaminar (B–D) cells. The soma and dendrites are drawn in red, whereas the axons are in black. The horizontal dendritic branches from both interneurone types are restricted to the stratum oriens. The O-LM cell had an axon that crossed the pyramidal cell layer and extensively arborized in the stratum lacunosum-moleculare of the area CA3, while the axons from the trilaminar cell arborized in three layers (str. oriens, pyramidale and radiatum). Axon collaterals of the trilaminar cell in B extend horizontally (∼1.3 mm) throughout the CA1 area of the hippocampus, whereas a ∼1.5 mm long axonal branch of another trilaminar cell in D traverses the entire extent of the CA1 are a and reaches the subiculum. Note the axon initial segment (arrow) in the trilaminar interneurone (D, inset). Hippocampal layers are depicted schematically. CA3, CA3 area; CA1, CA1 area; Sub, subiculum; str. or., stratum oriens; str. pyr., stratum pyramidale; str. rad., stratum radiatum; str. l.-m., stratum lacunosum-moleculare.