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The panniculitides comprise a group of heterogeneous inflammatory diseases that involve the subcutaneous fat. Histopathological study is required for the specific diagnosis of these disorders, because different panniculitides usually show the same clinical appearance – which typically consists of erythematous nodules on the lower extremities. However, the histopathological study of panniculitis is difficult because of the poor correlation between clinical and pathological findings and the changing microscopic appearances as the lesions evolve. Furthermore, large scalpel incisional biopsies are required in order to evaluate panniculitis fully. From the histopathological point of view, all panniculitides are somewhat mixed, because the inflammatory infiltrate involves both the septa and lobules. However, differentiation between a predominantly septal and a predominantly lobular panniculitis is usually straightforward at scanning magnification on the basis of which subcutaneous structures are the more intensely involved by the inflammatory infiltrate.
Keywords subcutaneous fat, subcutis, fat lobule, adipocytes, erythematous nodules, septal and lobular panniculitis, erythema nodosum, erythema induratum