Spontaneous cutaneous umbilical endometriosis: report of a new case with immunohistochemical study and literature review.

Endometriosis (i.e., the presence of functional endometrial tissue outside the uterine cavity) may rarely affect the skin either spontaneously or secondary to abdomino-pelvic surgery. Spontaneous cutaneous endometriosis appears most commonly on the umbilicus (Villar nodule) and manifests with a red-brown nodule that characteristically shows cyclic variations (tenderness or bleeding) with the menstrual cycle. We present herein a new case of spontaneous cutaneous endometriosis of the umbilicus that was studied immunohistochemically and review the salient clinicopathologic features of this rare condition.