Abstracts of the 8th Meeting of the Italian Peripheral Nerve Study Group: 70

Abstract

The availability of antibodies against the pan‐axonal marker protein‐gene‐protein 9.5 (PGP) allowed to definitely demonstrate the existence of small‐diameter nerve fibers within the vital layers of the epidermis and prompted to their quantification in sensory neuropathies. However, morphological features of intra‐epidermal nerve fibers (IENF) have not been fully characterized. We performed immunohistochemical and confocal immunofluorescence studies in skin biopsies taken in 15 healthy subjects at the proximal thigh or the distal leg using anti‐PGP, anti‐unique b‐tubulin (TuJ1), anti‐non‐phosphorylated microtubule‐associated protein‐1B (MAP‐1B), anti‐70 and 200 kDa neurofilament (NF), anti‐phosphorylated neurofilament‐312 (SMI), anti‐peripheral myelin protein‐22 (PMP22), anti‐basic myelin protein (MBP), and anti‐myelin associated glycoprotein (MAG) antibodies. IENF density was quantified under light microscope using PGP, TuJ1, MAP‐1B, NF, and SMI immunostaining. In each site, the density of TuJ1‐ and MAP‐1B‐positive IENF was similar to that of PGP‐positive IENF, whereas that of NF‐positive IENF was significantly lower. Only sporadic SMI‐positive IENF were found. Double confocal staining studies confirmed that TuJ1 and MAP‐1B diffusely co‐localized with PGP in IENF. These findings suggest that IENF cytoskeleton is predominantly composed by tubules, whereas neurofilaments are less represented. No IENF showed PMP22, MBP, or MAG labeling. All dermal nerve fibers presented PMP22 immunoreactivity, which labeled Schwann cell membrane, whereas only a few of them were MBP‐ or MAG‐positive, confirming that most fibers are unmyelinated. Double staining confocal microscope studies revealed that PMP22 labeling in dermal nerves stopped at the dermal‐epidermal junction, whereas TuJ1‐positive axons crossed it and run between keratinocytes. This definitely demonstrates that IENF are naked axons, arising from sub‐papillary unmyelinated nerves which loose the Schwann cell at the dermal‐epidermal junction. The presence of naked nerve fibers within the epidermis suggests that functional interactions with the resident cells, even in sensory transduction, might exist.