Electron microscopy of the effects of dithranol on healthy and on psoriatic skin

The fine structure of healthy and of psoriatic skin was studied after dithranol irritation. Following a minimum erythematogenic or a moderate-to-considerable irritant reaction of healthy skin with dithranol, all epidermal cell types were focally affected. The Langerhans' cells (LC) were most sensitive, reacting with strongly swollen mitochondria with broken cristae, and sometimes by forming branched and circular Birbeck granules. More often than is normal, the Birbeck granules showed continuity with the LC cytomembrane. Also, melanocytes were more sensitive than keratinocytes. Most of the more strongly affected keratinocytes became cytolytic with edemic cytoplasm, but scattered cells underwent dyskeratosis (apoptosis) resulting in colloid bodies in the upper dermis. High amounts of lipid droplets developed in the basal keratinocytes, LCs, melanocytes, and dermal cells. Lipid droplets also developed in the keratinocytes at the stratum granulosum/stratum corneum interface concomitantly with a decrease in keratohyalin. Ten days after challenge, keratohyalin granules were normal again, but otherwise many changes in epidermis persisted, with increased numbers of exocytic cells and LC-mononuclear cell contacts in the epidermis and immunocompetent cells crossing the dermal-epidermal junction. These findings indicate that an irritant reaction could predispose to sensitization via nonspecifically activated immunocompetent cells. A single half-hour contact treatment with dithranol caused negligible changes in the psoriatic skin, while 24 hours' occlusion caused moderate-to-massive changes in mitochondria of keratinocytes, resulting in giant perinuclear mitochondria with broken cristae. Dithranol affects all cell types in the skin and morphologically it cannot be concluded which effect is the important one in clearing the psoriatic lesions.