The histological fine structure of perineurium

Abstract Extraocular muscles of the rat possess numerous nerves suitable for the study of fine structure. In these muscles, small nerves made up of one to ten myelinated and unmyelinated nerve fibers are surrounded by two or three layers of perineurium. The perineurium is arranged in concentric sleeves, each one cell thick. Continuous boundary membranes separate the perineural sleeves from the epineural and endoneural tissue space, but the boundary membranes may be spotty or absent between individual sleeves. The presence of boundary membranes around perineural cells distinguishes them from nearby fibroblasts which lack similar membranous investment. Tight intercellular junctions join the cells comprising each sleeve so that the nerves are completely ensheathed in perineurium. The number of sleeves decreases as the nerve becomes smaller, either by the termination of the innermost sleeve or by the loss of a sleeve as the nerve branches. The last sleeve ends shortly before the termination of the nerve. The perineurium is thus open‐ended peripherally and, at these places, the epineurium and endoneurium are continuous. Continuities between the epineurium and endoneurium also exist at the entrance and exit of blood vessels supplying the nerve and at points where reticular fibers pierce the perineurium. These structural features correlate well with the action of the perineurium as a diffusion barrier and as a pathway in the transmission of infections.