S8C11P1-1: The definition of a vasculitis often emphasizes the character of the infiltrate; priority is placed on the presence, in the vascular domain, of both infiltrates of neutrophils and fragments of nuclear debris (leukocytoclasia). In addition, necrosis is often mentioned in definitions, but not rigidly required in practice. The vessel in the center of the field satisfies somewhat different criteria for the recognition of a vasculitis; it shows a central thrombus (the lumen is occluded). Fibrinoid material is present in the wall of the vessel and in its adventitia (acidophilic fibrils form a mesh [a deposit] in the adventitia of the vessel, to the right and below the center of the field). If we then require the presence of an infiltrate of inflammatory cells - and are not too demanding - the migratory histiocytes in the adventitia of the vessels would qualify the process as a necrotizing histiocytic vasculitis. It would be difficult to identify a single cell in the infiltrate as a neutrophil. There are fragments of nuclear debris; some observers would extrapolate from this evidence to conclude that all the neutrophils in the infiltrate have disintegrated. A more reasonable approach would be to cite the population of histiocytes as the source of the nuclear debris (thrombotic thrombocytopenic purpura).

S8C11P1-2: In this field, neutrophils have been caught in the act; they have infiltrated the wall of a vessel at the interface between the papillary dermis and the reticular dermis. A few neutrophils have extended beyond the adventitia of the vessel into the neighboring dermis. The papillary dermis is edematous, a sign of abnormal vascular permeability. In the proper clinical setting, this pattern is easily assigned to the category of vasculitis. On the other hand, there is no evidence of necrosis of the wall of the vessel and very little evidence of leukocytoclasia. This is a neutrophilic angiitis, but it is uncertain that, with time, the lesion would assume the qualities of classic leukocytoclastic angiitis. This could be an early manifestation of an infectious process, or of a neutrophilic dermatitis of insufficient severity to be characterized as Sweet’s syndrome; neutrophilic changes of this type have even been described in some examples of “urticaria.” There are a few fragments of nuclear debris at the dermal-epidermal interface. In the same area, there are vacuolar changes at the dermal-epidermal interface.

S8C11P1-3: An area of necrosis, as evidence of ischemia, is saucer-shaped from left to right (with the area on the right being one extremity of the zone of necrosis). The viable epithelium has regenerated along the basement membrane zone. The extremity of the regenerating epithelium is moving in a direction from right to left (the extremity is marked by red arrows). In this example, a large vessel was occluded; the dermis is relatively free of inflammatory cells. The nuclei of cells in the dermis are pyknotic; The dermis also is necrotic.