Benign epithelial structures on the surface of appendices.

Reported here are five cases of rare and hitherto unreported adenomatoid epithelial structures which occurred under the peritoneal surface of appendix vermiformis. These epithelial structures consisted of mature, small non-branching, cystically dilated adenomatoid spaces lined by one or two layers of flat cells without atypia or mitoses. In two cases these structures underwent focal squamous cell metaplasia. It is hypothesized that these epithelial structures arise from the peritoneum. They should be especially distinguished from metastatic adenocarcinoma.