Differences between the peripheral and the central nervous system in permeability to sodium fluorescein

Publisher Website
Google Scholar
Abstract
Sodium fluorescein (SF) was used as a very small tracer (MW 376; 5 .ANG. diameter) to examine diffusion barriers in mouse peripheral nerves and to compare them to those in other regions of the nervous system. Immobilization of the tracer was accomplished by rapid freezing, followed by freeze-drying and vacuum embedding in paraffin. The localization of the SF was determined in tissue sections using fluorescence microscopy. Even at the highest doses of i.v. tracer, no extravasation was detected in the cerebral cortex. SF penetrated very rapidly into peripheral ganglia and into the epineurium and perineurium of large peripheral nerves. The penetration of SF into the endoneurium of large nerves was much more restricted with tracer detectable within the endoneurium only at high doses and long survival times. In the endoneurium, the level of SF fluorescence was much lower within nerve fascicles than in the epineurium and the perineurium, and a sharp gradient in fluorescence intensity persisted at the inner border of the perineurium. The extent of extravasation into the endoneurium varied markedly between different fascicles of the same nerve and between different nerves in the same animal. Injection of high doses of SF adjacent to the nerve indicated relatively little movement of SF across the perineurium. The accumulation of tracer within the endoneurium apparently was from direct extravasation of SF from the endoneural blood vessels. Small nerve branches (< 100 .mu. in diameter) showed an earlier and more extensive penetration of SF into the endoneurium than large nerves like the sciatic, hypoglossal or ventral tail nerve. This may be due to a diffusion of SF along the extracellular space of the endoneurium from nerve terminals where the perineurial barrier is open-ended. After i.v. injection of a solution containing green fluorescent SF and red fluorescent Evans Blue (Evans Blue-serum albumin complex, EBA = MW 69,000) the distribution of SF was directly compared at various sites and sacrifice times to that of EBA, a much larger tracer. SF appeared more rapidly and extensively than EBA in the various compartments in ganglia and peripheral nerve. The distribution of EBA was the same as when this tracer is injected alone. No change in vascular permeability is apparently associated with i.v. injection of SF. Since SF is of very small size, freely diffusible, nontoxic and detectable at very low concentrations, it should be a useful complement to existing tracers. When tissues are processed according to the procedure, a very sensitive and reliable localization of this tracer is possible for studies in the nervous system concerning various pathological conditions associated with permeability alterations.