Structure and barrier functions of the perineurium and its relationship with associated sensory corpuscles: A review

Abstract

Peripheral nerves are provided with a blood-nerve barrier which prevents the invasion of harmful substances and pathogens, and also regulates metabolic and ionic homeostasis within nerve fasci-cles. The barrier functions are attributed to both the concentric layer of flattened cells in the peri-neurium and blood vessels running in the endoneurium. The perineurial cells develop continuous tight junctions as a diffusion barrier. In order to take up a predominant nutrient, glucose, the peri-neurium as well as endoneurial capillaries expresses GLUT1, a glucose transporter. An axon-Schwann cell complex within peripheral nerves utilizes glucose as a major energy source via the GLUT1, as does the brain. Under conditions of a reduced utilization of glucose, only the perineur-ial cells can transfer other nutrients, namely monocarboxylates such as ketone bodies and lactate via MCT1. Thus, MCT1 colocalizes with GLUT1 in the perineurium but not in endoneurial capil-laries. To identify the cellular origins of the nerve sheath, marker proteins such as glial specific S100 protein, GLUT1, endoneurial CD34, and EMA (epithelial membrane antigen) are useful. Im-munohistochemical findings for these markers are reviewed in this paper, focusing on the perineu-rium and endoneurium and their derivatives, Pacinian and Meissner corpuscles. Growing evidence throws light on the critical involvement of the nerve sheaths in the development, maintenance, and diseases of peripheral nerves.