Parakeratosis in some epidermal tumors: fine structural and immunohistochemical similarity between parakeratosis and "Hornmark" -like structure.

Both parakeratosis and "Hornmark" -like structure are basic histopathologic changes found in a variety of skin lesions. In order to investigate more closely the architecture and components of these two structures, three cases of epidermal tumor which showed both parakeratosis and "Hornmark" -like structure were studied by using routine histology and histochemistry, immunofluorescence technique, and electron microscopy. In the intercellular space of the stratum corneum in these two structures, diastase-resistant PAS-positive substance was found. This substance also showed positive immunofluorescence for immunoglobulins, complement, and fibrinogen. Electron microscopically, this substance was moderately electron-dense and fine granular, and was located in the intercellular space and in part in the cytoplasm of the horny cells. On the basis of these findings, this substance was judged more likely to be derived from the components of plasma protein than a result of the degeneration of epidermal cells. Only quantitative difference was observed between these two structure. This substance was considered to play an important role on the increased cohesiveness of the horny cells. However, whether this is a product of antigen-antibody reactions, or the result of the process similar to blood coagulation, remains to be elucidated.