Hematic barriers were studied in femoral and sciatic nerves and their peripheral branches in male albino rabbits by means of fluorescence microscopy and ultra-freezing and drying technics. The fluorescent tracer used was a diaminoacridine, acriflavine neutral, which is non-toxic at the 20 mg/kg dose administered intravenously, has a low molecular weight (259.7), and has a unique quality of chemically binding in vivo with nuclear DNA and RNA. This latter property of acriflavine neutral permitted the observation of fluorescent nuclei where the substance penetrated and no fluorescence where it did not penetrate. The interface between the two regions was identified as a locus of a hematic barrier.
Three hematic barrier loci were observed in spinal nerves and their branches: (1) in the plasma membranes of the cells forming inner lamellae of the “perineurial epithelium,” (2) in the luminal face of the plasma membrane of endothelialcellsforming the wall of endoneurial capillaries, and (3) in the “perineurial epithelial” sheath which surrounds endoneurial precapillaries. These barriers appeared to be dependent on normal living physiologic processes for their proper maintenance. In addition, they were very similar to some of the hematic barriers associated with the central nervous system.