Ultrastructural characterizations of olfactory pathway neurons in layer II of the entorhinal cortex in monkey

The somatic size, shape, dendritic and axonal morphology, and synaptology of representative neurons in layer II of the primate entorhinal cortex (EC) were analyzed. Layer II "islands" contained large spinous multipolar cells with triangular somata and local circuit axons in addition to multipolar neurons with large, radially arrayed, aspinous, primary processes and thick tapering axons. Small pyramidal neurons with a single, spinous, apical primary segment that bifurcated a short distance from the somata were also found in layer II. Subsequent spinous segments of these neurons with long terminal segments exhibited a paucity of branching in addition to having thick axons tapering into subjacent layers. The importance of providing these additional axonal, dendritic, and synaptic characterizations lies in the contextual role these neurons play in the connectional patterns of the EC with regard to olfaction, olfactory memory, and pathological variations.