The perineurium is an important sheath that protects and maintains a microenvironment for nerve fascicles, which are bundles of axons. It is a component of the blood-nerve barrier and has glial cell properties. The perineurium also forms tight junctions to act as a diffusion barrier and is made up of layers of flattened cells and collagen. It surrounds nerve bundles and is found in various parts of the central nervous system, including sensory corpuscles and brainstem centers.