Freeze-fracture faces of the perineurial sheath of the rabbit sciatic nerve.

Replicas of the perineurium of rabbit sciatic nerve fascicles show extensive zonulae occludentes between the flattened cells of each perineuriallamella. The junctions formed by the zonulae occludentes are of the morphologically 'tight' type. Therefore, theyare likely to represent the anatomical site of the diffusion barrier previously demonstrated by other morphological and physiological methods. Neighbouring perineurial lamellae are connected to one another by numerous maculae occludentes, some of which lie on thelateral processes of the perineurial cells. Within the meshes of the maculae occludentes and close tothem, small gap junctions can be observed.