Cutaneous Nerve Stimulation by Psoralen-Ultraviolet A Therapy: An Ultrastructural Study

Abstract

Proliferation of intra-epidermal and dermal free nerve endings was studied in 7 psoriatic white patients before and after psoralen-ultraviolet A (PUVA) treatment. Many free nerve endings were observed in contact with epidermal basal cells. Two distinct types of nerve terminals were found in the epidermis: One was a naked axon terminal surrounded by keratinocytes instead of Schwann cell sheath. The second, a bulbous terminal partly covered by Schwann cell sheath. Melanocyte-nerve contacts were also found in 3 of 7 psoriatic patients. In the dermis, sometimes immediately below the epidermis, there were many free nerve endings which were partly denuded. Polyaxonal Schwann cells containing several unmyelinated nerves were in the vicinity of the epidermis. An increase of amorphous material was observed around these Schwann cells. Although a total of three intra-epidermal axon terminals were found in pretreatment psoriatic lesions in 2 cases out of 7, we found a total of 24 nerves in the same 7 cases after PUVA therapy, including at least one intra-epidermal nerve in each case. PUVA therapy is suggested to stimulate the cutaneous nerves and cause a neural invasion into the epidermis. The majority of melanocytes after treatment showed signs of hyperactivity. Contacts of nerve endings to melanocytes are possibly related to this biological hyperactivation of melanocytes.