Biological changes of human cutaneous nerves caused by ultraviolet irradiation: an ultrastructural study.

Three white male volunteers were irradiated by long wave ultraviolet (UV-A) and by solar simulating radiation (SSR). An acute change of cutaneous nerves was found following a single exposure of UV-A irradiation. Non-myelinated Schwann cells and perineural cells in the papillary and reticular dermis were degenerated. Axoplasms appeared to be electron-dense, but were less affected than Schwann cells. After long-term repeated exposure, many free nerve endings were found in the dermo-epidermal junction above the basal lamina and some of them made a terminal enlargement. In one instance an axon made a swelling. Dermal free nerve endings also seemed to be increased in number and some of them were situated immediately beneath the melanocytes which were active in melanogenesis. Multiplication of basal lamina of the Schwann cells and perineural cells was observed. Amorphous material was precipitated around the non-myelinated Schwann cells. After a single exposure of SSR irradiation the degeneration of Schwann cells and axons in the dermo-epidermal junction was less severe than after UV-A and changes were minor in the reticular dermis. After repeated exposure, intra-epidermal proliferation of free nerve endings was detected. Dermal nerves were slightly affected. No intra-epidermal free nerve endings were observed in controls. The intra-epidermal proliferation of free nerve endings was confirmed following repeated UV exposures. Melanocyte-nerve association is suggested to be the cause of stimulating melanocyte activity.