Na-fluorescein and fluorescein-labelled dextrans of various molecular weights were injected i.v. to localize the blood-peripheral nerve barrier in rat subcutaneous nerves. Samples were taken from the subcutaneous fascia and skin 5 and 30 min after the injection. In the specimens taken 5 min after the injection, the fluorescence had diffused into the dermal tissue. The intraneural, epineural, and perineural blood vessels contained fluorescent material. The entry of fluorescent material from subcutaneus tissue into the nerves was prevented by the perineurium. The distribution of the tracer was uniform throughout molecular weight range tested (376-150,000). Both dermal and intravascular fluorescence were clearly weaker in the specimens taken 30 rain following the injection, particularly ifa tracer with a MW lower than 40,000 was used. After bilateral ligature of the renal blood vessels the fluorescence remained more intense, even after 30 rain, and it was independent of the molecular diameter of the tracer injected. The location of the fluorescence was not changed by the operation and the intraneural capillaries remained intact.
The results indicate that the intraneural capillaries are impermeable to the fluorescent tracers tested. In contrast, all the marker substances passed freely through cutaneous capillaries. The perineurium prevented the diffusion of fluorescent tracers from the surrounding tissue into the nerve.