S8C16P3-1: Medium-sized vessels tend to be involved in polyarteritis nodosa. Vessels of the skin may be involved as a manifestation of classic generalized disease. In some examples, as in this case, the involvement is limited to the skin (limited or cutaneous polyarteritis nodosa). Blue arrows point to a muscular vessel containing a thrombus. At the bottom of the field, the muscular wall of the vessel is interrupted by an infiltrate of inflammatory cells.

S8C16P3-3: At higher magnification, the zone of fibrinoid necrosis is acellular. Inflammatory cells form palisades along the wall of the vessel at the interface between the zone of necrosis  and the adventitia. To the right of the vessel, the zone of fibrinoid extends into the adventitia.

S8C16P3-4: In cross-section, the rounded outlines of the infiltrates provide a clue as to the nature of the stucture that has been involved by the infiltrates; the rounded outlines would be in keeping with a muscular vessel that has been cut in cross-section. The infiltrates are mixed; they are composed of lymphocytes, histiocytes, neutrophils, and eosinophils (polyarteritis nodosa).

S8C16P3-5: This lesion is a change of pace, but is representative of a process in which neutrophils and necrosis are a feature. This is a section of a “neurotic excoriation” as seen under polarized light. Collagen bundles are bright white (they are birefringent). There is an irregular defect just below the plane defined by the row of green arrows. This defect is the site of a separation between viable and necrotic tissue (the tissue above the row of green arrows is a sequestrum); it consists of  keratin, necrotic epidermis, necrotic papillary dermis, and a thin zone of necrotic reticular dermis (i.e., the bright bundles above the defect provide a marker for a contribution of the reticular dermis to the sequestrum). The present of these collagen bundles in the sequestrum is a clear measure of the level of the defect; birefringent bundles of this type are not a component of the papillary dermis. “Neurotic excoriation” is an inaccurate characterization of the process; although, the designation may have clinical relevance. The process is, in sequence, a response to external trauma, an activation of inflammatory mediators, an infiltration of the zone of damage (in response to trauma) with neutrophils, a release of enzymes, and a digestion of tissue to include the tissue damaged by trauma (this often includes a thin portion of the reticular dermis). Vascular changes, including thrombosis and necrosis, are common adjacent to the site of damage; they probably have a role in defining the area to be infiltrated by neutrophils.