Mast cell alterations in chronic psoriasis vulgaris: response to low-strength anthralin treatment. A transmission electron microscopic study.

Mast cell degranulation (MCD) was studied in lesions of chronic psoriasis vulgaris before and during topical treatment with low-strength anthralin. Before the treatment, two forms (A and B) of mast cells with Type I MCD were distinguished in the lesions, in addition to mast cells showing Type II MCD. In Type I MCD, electron-dense mast cell granules in form A mast cells, and electron-dense mast cell granules and vacuoles containing granule matrix in form B mast cells, were released as intact structures by the mechanism of diacytosis. Distinct gaps of the mast cell plasma membranes were observed. Around blood vessels, beneath the epidermal-dermal junction and in the intercellular space of strata basale and spinosum, the mast cell granules appeared partly as intact structures and partly in more or less disintegrated form. In Type II MCD the granule matrix was released into the extracellular compartment by the mechanism of exocytosis. During treatment with low-strength anthralin, the mast cell changes underwent regression. In macular psoriasis only form A mast cells of Type I MCD were demonstrated, and the released intact mast cell granules were restricted to the immediate neighbourhood of the mast cells. There were no mast cell granules in the epidermis. At the sites with clinically complete clearance of psoriatic lesions, the mast cells displayed no degranulation but distinct gaps were still found in the mast cell plasma membranes. Low-strength anthralin's mode of action in psoriasis is suggested to involve regression of a series of systems, including prevention of mast cell degranulation, thereby inhibiting release of histamine, proteinase and other mast cell mediators sustaining the psoriatic process.