This is an EM of a smaller nerve bundle than the one shown HA26. This image shows several myelinated axons and two unmyelinated axon clusters. Surrounding these and the endoneurium is a cell layer called the perineurium. It is composed of four layers of flattened cells with external laminae on both their inner and outer surfaces. The cytoplasm of the perineurial cells is characterized by numerous white transcytotic vesicles. What is not shown in this image are the tight junctions between the perineurial cells that prevent molecules from using the paracellular pathway.