Histology of a typical preputial gland duct of Cyp1a1/1b1(−/−) double-knockout mice (following 12 weeks of oral BaP, 12.5 mg/kg/day), showing squamous cell carcinoma of the duct with invasion into the surrounding tissues. (A) normal preputial duct morphology with a stratified squamous cell epithelial lining of the larger ducts (white arrow). (B) cuboidal holocrine secretory acinar cells (white arrow). (C) a site of thickening of the squamous epithelium (white arrow), and increased keratin production can be seen (C & D black arrows). {D, E & F) excessive accumulation of keratin within ducts and resulting dilatation, and an epithelial morphology characteristic of squamous cell carcinoma. There is thickening of the lining of the tubular glands, with an abscess and excessive inflammation. The overall architecture is disorganized, and the amount of stroma between glands is increased. Bar = 100 microns for all images.