The atypical mucinous epithelium lining the mucinous cystadenomas of the appendix may have papillary, villous, or villoglandular configuration. This low-power view shows cross section of an appendix with a dilated lumen containing wispy mucin and complete replacement of the normal appendiceal mucosa with a villous adenoma. The muscularis mucosa is intact. Over time, the accumulated mucin may cause pressure atrophy of the mucosa leaving behind a mucin-filled cystic structure. The wall may become hyalinized and fibrotic. Mucin may dissect the fibrotic wall.