The morphologic events of blister formation in porphyria cutanea tarda.

In porphyria cutanea tarda (PCT), the submicroscopic event causing the formation of spontaneous blisters is the appearance of membrane-limited vacuoles in the superficial dermis. These vacuoles are apparently formed through cytolysis of dermal cells and swelling of the pseudopodia of the basal cells protruding into the dermis, probably as a consequence of photochemically-induced lysosomal damage. The number and size of these vacuoles progressively increase near the cleavage area of the blisters, where their limiting membranes merge and break, giving rise to the cavity of the blister, which is formed under the basal lamina. The presence of coated vesicles along the limiting membranes as well as inside the vacuoles suggests their involvement in the transport of fluids from the damaged vessels, thus contributing to the progressive volume increase of vacuoles themselves.