Presence of a blood-nerve barrier within blood vessels of frog sciatic nerve

The morphological correlates of protein permeability in endoneurial blood vessels of the frog sciatic nerve were investigated. The endothelial cells of these vessels possess numerous free and anastomosing vesicles at both their lumenal and ablumenal surfaces. The cells are joined by junctions characterized by one or two areas of membrane apposition. However, vesicular transport, transcellular channels or open junctions are not found. The protein tracers horseradish peroxidase and microperoxidase remained confined in the vessels after intravascular injection. Likewise, if injected into the endoneurium, horseradish peroxidase did not enter the vascular lumen. The endothelium of frog endoneurial blood vessels forms an effective barrier to protein tracers.