Distribution of anionic sites on the perineurium

The distribution of anionic sites on the perineurial basal lamina (BL) and plasmalemma of dorsal root ganglia and sciatic nerves was determined using cationic ferritin (CF) and cationic gold (CCG). The probes were applied to the tissue before and after resin embedding and visualised by electron microscopy. There were no apparent differences in charge distribution between the 2 tissues. At physiological pH a strong anionic charge was distributed evenly over the BL as demonstrated by pre-embedding labelling with CF; the plasmalemma was only moderately anionic. A similar application of CCG at pH 2.0 revealed a quasi-regular distribution of anionic sites (presumably due to acidic carbohydrate moieties) on the BL, whilst CCG-labelling of L. R. White sections indicated a differential distribution of these moieties on the BL of the inner and outer perineurial lamellae. Cationic ferritin (12 nm diameter) crossed the BL and entered perineurial caveolae, but CCG (effective diameter of 15 nm) did not, suggesting that the BL is a size-restrictive filter. These results are discussed with regard to the ultrastructure and function of the BL of other tissues and the possible role of perineurial BL charge as a determinant of perineurial permeability.