: Rittinghausen, S.; Kaspareit, J.

Abstract
A spontaneous cystic keratinizing epithelioma in the lung of a 704-days-old female Hsd:Sprague-Dawley rat is described. The rat has a white subpleural discoloration on 1 diaphragmatic lobe at necrospy. Microscopically, a central keratin mass, surrounded by a multilayered squamous epithelium of varying thickness, was found. In the periphery of the tumor, epithelial nests projected multifocally into the adjacent alveolar spaces. Immunohistochemical labeling of nuclei by PNAC (proliferating cell nuclear antigen) revealed proliferative activity in 1 or 2 peripheral cell layers in most locations. To the best of the authors' knowledge, this is the first report dealing with the morphological and immunohistochemical features of a spontaneous cystic keratinizing epithelioma in a rat.