BACKGROUND: Seventy-one cases of scar-related and spontaneous endometriosis of the skin and superficial soft tissue were studied, with a focus on atypical features and types of müllerian differentiation. All patients were women, whose ages ranged from 22 to 65 years (median, 32 years). METHODS: Histological, immunohistochemical, and electronmicroscopic studies were performed. Clinical information was ascertained via a questionnaire solicited by the referring physicians. RESULTS: All types of metaplastic changes of müllerian epithelium were found, including tubal (61%), oxyphilic (15%), hobnail (10%), mucinous (4%), and papillary syncytial (3%) metaplasia. Atypical features included reactive atypia (23%) and atypical mitoses in glandular epithelium (6%). Stromal changes included smooth muscle metaplasia (31%), decidualization (<1%), stromal endometriosis (<1%), and elastosis (<1%). Other features recognized included lipoblast-like cells (15%), some with intranuclear inclusions; atypical/degenerative myocytes (10%); spiral arteries (4%); and perineurial invasion (<1%). CD56 staining identified large granular lymphocytes in 15 of 20 studied specimens. Ultrastructurally, these cells showed cytoplasmic granules, some with a delimiting membrane. LIMITATIONS: This study utilizes tissue specimens that mainly were received as consultations; therefore some inherent selection bias exists. Specimens were randomly selected for CD56 immunostaining, leading also to potential sampling error. CONCLUSIONS: All types of müllerian metaplasia can be encountered in cutaneous endometriosis. In addition, so-called atypical features described in endometriosis affecting other anatomic sites may be seen in the skin. Some features may represent a diagnostic pitfall.

Sikl's Department of Pathology Charles University Medical Faculty Hospital Pilsen Czech Republic

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