Anatomy and histology of normal subcutaneous fat, necrosis of adipocytes, and classification of the panniculitides.

The panniculitides comprise a group of heterogeneous inflammatory diseases that involve the subcutaneous fat. Histopathologic study is required for the specific diagnosis of these disorders, because different panniculitides usually show the same clinical appearance, which consists of erythematous nodules on the lower extremities. The histopathologic study of panniculitis is difficult, however, because of an inadequate clinicopathologic correlation and the changing evolutive nature of the lesions. Some cutaneous lymphomas may simulate panniculitis, both from clinical and histopathologic points of view, and for that reason are included in this article despite the fact that they are not inflammatory processes but authentic lymphocytic neoplasms involving subcutaneous tissue.