Mucocoeles of the appendix. Their underlying epithelia, behaviour and associated tumours.

MUCOCOELE is a descriptive term for mucus accumulation in the distended lumen of an organ, and does not indicate the underlying pathology. This paper describes four distinct pathological entities identified in a series of 65 mucocoeles of the appendix. The first category, designated as retention cyst, is due to simple obstruction of the lumen often associated with post inflammatory fibrosis or a faecalith. The diagnosis is restricted to those cases in which there is no evidence of any epithelial abnormality in the mucosal lining of the appendix other than flattening. The second category, designated as metaplastic polyp, is characterised by mucosal glands whose deeper aspect resembles normal appendicular epithelium, but towards the lumen the glands become dilated and lined by goblet cells and cells with pink staining cytoplasm with pale vesicular nuclei and only an occasional mitotic figure. Papillary infolding gives a characteristic serrated appearance (Fig. 1). The third category is designated mucinous cystadenoma. These tumours are identical to tubular and tubulo-villous adenomas seen in the colon. They exhibit a glandular or pure villous histology, and the neoplastic epithelium shows mild, moderate or severe dysplasia. Mucin production is usually very prominent and may lead to a flattened epithelium (Fig. 2).