Electron microscopy of cell islands in layer II of the primate entorhinal cortex

An electron microscopic analysis of cell islands in layer II of the entorhinal cortex from rhesus monkeys was made to determine the ultrastructural features of these unique neuronal clusters. The rostral, intermediate, and caudal divisions of the entorhinal cortex were selected for electron microscopic examination. In the rostral division, neurons were grouped together in prominent clusters, often with 10 or more contiguous somata. Somatic and dendrosomatic appositions were frequent, without intervening cellular processes or specialized junctions. Somata were relatively small, typically 10-15 microns in diameter, with oval or circular nuclei that were euchromatic and contained nucleoli. Small nuclear infoldings were commonly seen. A thin shell of perikaryal cytoplasm contained numerous organelles. Axosomatic synapses were infrequent, with a mean of only 1.0 synapse per neuron per thin section. The neuropil contained numerous synapses, and myelinated axons were seen infrequently. In the intermediate division, somatic appositions were rarely observed. Somata were relatively large, typically 15-20 microns in diameter, and displayed a moderate amount of cytoplasm. Axosomatic synapses were relatively common, with a mean of 3.3 synapses per neuron per thin section. In the caudal division, neurons were typically grouped in clusters of two to three contiguous somata. Neurons were about 15 microns in diameter and displayed a moderate amount of cytoplasm. Axosomatic synapses were of moderate frequency, with a mean of 2.5 synapses per neuron per thin section. The neuropil in the caudal division displayed a relatively high frequency of myelinated axons. Our analysis of three regions of the entorhinal cortex revealed significant differences in the frequency of somatic appositions and axosomatic synapses, and in certain ultrastructural features of the somata and neuropil. These results showed that cell islands in layer II of the entorhinal cortex display regional morphologic differences. The paucity of symmetric axosomatic synapses in the rostral division may correlate with this region's vulnerability in certain diseases.