Erythropoietic protoprophyria--submicroscopic events during the acute photosensitivity flare.

The immediate response of erythropoietic protoporphyria (EPP) skin to long-wave ultraviolet radiation (UVR) was studied with the electron microscope. The main finding was severe vascular injury. This was confined to the superficial vessels of the dermis and consisted of endothelial cell degeneration and a pronounced leakage of vascular contents. In contrast, the epidermis showed no abnormalities. Short-wave UV irradiation of EPP skin resulted in epidermal changes typical for the usual sunburn reaction and spared the dermal blood vessels. The following conclusions are drawn: (i) Endothelial cells are the primary cellular target for the photodynamic reaction in EPP. (ii) The fibrillar material, characteristic for chronic EPP lesions, originates from the vessels and vascular contents. (iii) The multilayered basement membranes observed in such lesions reflect multiple consecutive reparative processes that follow endothelial injuries.