S8C34P20-2: Vessels are dilated; the dermis is edematous. Lymphocytes are present in the perivascular spaces of the dilated vessel on the left at the bottom of the field. In the reticular dermis among the collagen bundles, the infiltrates are composed of neutrophils and histiocytes with occasional eosinophils. There are fragments of nuclear debris (leukocytoclasia); some of the nuclear fragments have been phagocytyzed by the histiocytes (histiocytes containing the fragments of nuclear debris might be characterized as “bean-bag” cells) (Sweet’s syndrome, a neutrophilic collagenosis). There are scattered extravasated red blood cells.

S8C34P20-4: In this example of Sweet’s syndrome, the edema of the papillary dermis is extreme; focally, the edema has disrupted the fibrous frame-work (reticulum) of the papillary dermis. Fibrin deposits are prominent; they are associated with collections of extravasated red blood cells. The infiltrates are composed of neutrophils, eosinophils, and histiocytes. There are fragments of nuclear debris. Vessels are ectatic, but their walls are intact; there are no areas showing the pattern of a necrotizing vasculitis.

S8C34P20-5:  In this field, a portion of a lesion of pyoderma gangrenosum is represented. On the left above the green arrows, there is a pustule that, in part, is intra-epidermal. Above the yellow arrows, regenerating squamous epithelium is represented; a small sub-epidermal pustule is present below the epidermis. Blue arrows point to vessels showing prominent fibrinoid necrosis. Infiltrates of neutrophils have destroyed the fibrous frame-work of the reticular dermis. The necrotizing vasculitis (blue arrows) sets this lesion apart from what, at a histologic level, would, otherwise, be classic Sweet’s syndrome.