Eosinophilic cell change of the endometrium: a possible relationship to mucinous differentiation

Eosinophilic cell change is one of the most common endometrial metaplasias occurring in both non-neoplastic and neoplastic endometrium. Its phenotypic characteristics have not still been fully clarified. We examined expression of mucin core proteins in a total of 95 distinct histological areas of endometrial specimens comprising 39 benign nonhyperplastic endometria, 14 endometrial hyperplasias, and 42 endometrial carcinomas. Eosinophilic cell change was very common, seen in 27 endometrial areas (28%); mucinous metaplasia (28%) and ciliated (tubal) change (31%), were also frequently seen. Eosinophilic cell change was more frequently seen in endometrial hyperplasia and carcinoma than in benign nonhyperplastic endometrium. In endometrial carcinomas, eosinophilic cell change was frequently associated with mucinous metaplasia and the two types of metaplastic cells were occasionally intermingled in a single neoplastic gland. A total of 23 (85%) of 27 eosinophilic cell changes and 18 (72%) of 25 mucinous metaplasias showed MUC5AC expression. These frequencies of MUC5AC expression did not differ significantly among benign non-hyperplastic endometrium, endometrial hyperplasia and endometrial carcinoma. Totally, 15 (52%) of 29 ciliated (tubal) changes and two (100%) of two surface syncytial changes, which showed cytoplasmic eosinophilia at least focally, also expressed MUC5AC. Most of the endometrial changes characterized by cytoplasmic eosinophilia may be subtypes of immature mucinous metaplasia which express a mucin core protein but are not fully glycosylated.