[Ultrastructure of parapsoriasis lesions. Parapsoriasis en plaques and parakeratosis variegata as prelymphoma; differences from pityriasis lichenoides].

The morphological alterations of involved skin in three different types of parapsoriasis were investigated in 9 patients by electron microscopy. Pityriasis lichenoides chronica (PLC) is characterized by a lymphohistiocytic dermal infiltrate and by epidermotropic histiocytic cells, which penetrate up to the horny layer. In parapsoriasis en plaques (PeP) and in parakeratosis variegata (PV) the dermal infiltrate is mainly composed of lymphocytoid cells, some of which, particularly in PV, reveal the features of Sézary-Cells (11% and 30% respectively). The epidermis is predominantly invaded by partly atypical lymphoid cells. In some instances membrane contacts between Langerhans cells, keratinocytes and atypical lymphoid cells can be observed. The increased number of epidermotropic cells and the increase of atypical lymphoid cells in the dermal infiltrate are the main ultrastructural features of the malignant transformation of PV. Finally, an important difference between PLC and the other two types of parapsoriasis is the fact that in PLC epidermotropic cells are mostly of histiocytic origin, whereas in PeP and PV they are mainly lymphocytes. The electron microscopic findings support the opinion that PLC should not be considered as a type of parapsoriasis and that PeP and PV probably correspond to prelymphoma.