[The blood-nerve barrier in peripheral nerves]

Abstract

Aim: To study ultrastructural details of perineurium and endothelium samples from the endoneural vessels that form part of the blood-nerve barrier of peripheral nerves, with the intention of furthering our understanding of how these natural structures protect axons against foreign substances.

Methods: We obtained samples from the sciatic nerve at the superior angle of the popliteal fossa. The samples were first fixed in glutaraldehyde and then in osmium tetroxide; later they were dehydrated with acetone and soaked in resin epoxy (Epon 812). Ultra-thin sections were treated with uranyl acetate and lead citrate in solution. The slides were observed under a transmission electron microscope.

Results: The perineurium has a thickness of 10 to 25 microns and is composed of 8 to 15 continuous cell layers lying concentrically around each nerve fascicle. Each perineurial cell layer consists of a single layer of flat cells joined together by specialized junctions to provide a barrier against diffusion. Most of the endoneural vessels found near the axons were capillaries measuring 6 to 10 microns in diameter and composed of 6 to 8 endothelial cells with specialized junctions without fenestrations.

Conclusions: The blood-nerve barrier is a cylindrical structure formed partly by membranes composed of tightly joined perineurial cell layers whose union is reinforced by specialized junctions that tend to isolate each fascicle. In addition, there is a cylindrical structure made up of endoneural endothelial cells also united by specialized junctures. These tend to keep blood away from axons and to impede the passage of circulating substances into the endoneural environment. Systemic diseases that alter and diminish the efficacy of the barrier in peripheral nerves may have implications for the creation of peripheral nerve blocks.