CK14 immunostaining in N‐butyl‐N‐(4‐hydroxybutyl)nitrosamine‐induced rat bladder lesions. (a) Control animal. Normal bladder urothelium (400×). Note immunostaining restricted to scattered basal cells. (b) Nodular hyperplasia (nh, 200×). Note diffuse immunostaining. (c) Urothelial dysplasia (d, 200×). Note immunostaining in the full thickness of the urothelium. (d) Squamous metaplasia (sqm, 200×). Note intense immunostaining. (e) Urothelial papilloma (pap, 200×). (f) Papillary tumour of low malignant potential (ptlmp, 40×). (g) Low‐grade papillary carcinoma: C, carcinoma; LP, lamina propria; SM, lamina submucosa; M, lamina muscularis; S, lamina serosa (lgpc, 40×). The upper left corner inset is a high‐power image of the same lesion (400×). (h) High‐grade papillary carcinoma (hgpc, 400×). Note variably intense, diffuse staining of lesions e–h.