Unmyelinated nerve fiber estimation by immunocytochemistry. Correlation with electron microscopy.

Electron microscopy (EM) is currently required for quantitation of unmyelinated nerve fiber (UMNF) densities. Electron microscopy is time-consuming, costly, and generally only considers a fraction of an entire nerve. Anti-PGP 9.5, which recognizes a neuron-associated antigen, may be used in glutaraldehyde-fixed, paraffin-embedded human sural nerve biopsies to identify unmyelinated axons. In nerves counterstained with Luxol fast blue, the correlation between EM-obtained UMNF densities and paraffin-obtained UMNF densities was excellent (p < 0.0001). In addition, myelinated nerve fiber (MNF) densities estimated by the same method from paraffin-embedded nerve gave excellent correlation with traditional morphometric estimates (p = 0.0026). PGP 9.5 immunocytochemistry enables the detection of minute axons (< 0.5 microns) and multiple axons per Schwann cell subunit, but does not allow for the preparation of accurate fiber size histograms or the analysis of UMNF pathology. Whether used for UMNF quantitation or as a qualitative review of the UMNF population of a nerve, this method is quicker and less expensive than traditional EM methodologies.