MUC5AC and cyst: Therein, the immunolabeling was found in some cyst-like structures and confined to the apical plasma membrane of either non-characteristic columnar cells (Fig. 3A–C, C2, black arrowhead) or cells with evident mucous cell differentiation, i.e. with a flattened nucleus located at the basal portion and cytoplasm filled with mucin granules (Fig. 3C, C1, blue arrowhead).