Keratinizing cysts have also been called "squamous cysts" and "epidermal cysts." Keratinizing cysts are thought to be part of a continuum of keratinizing lesions seen in rats that also includes squamous metaplasia of the alveolar epithelium, cystic keratinizing epithelioma, and squamous cell carcinoma. Cystic keratinizing epitheliomas can be differentiated from keratinizing cysts by virtue of the thicker, more complex wall of epitheliomas, which has a rougher outer margin (cysts have a smoother outer margin) and may lack orderly maturation of the squamous epithelial cells and may have increased mitotic figures. Squamous cell carcinomas may appear similar to cystic keratinizing epitheliomas but show evidence of malignancy (invasion, cellular atypia, loss of cellular polarity, etc.). Congenital cystic spaces lined only by fibrous tissue have also been reported in the rat, and these are distinguishable from keratinizing cysts by the lack of squamous epithelium.
Boorman GA, Brockmann M, Carlton WW, Davis JM, Dungworth DL, Hahn FF, Mohr U, Reichhelm HB, Turusov VS, Wagner BM. 1996. Classification of cystic keratinizing squamous lesions of the rat lung: Report of a workshop. Toxicol Pathol 24:564-572.
Boorman GA, Eustis SL. 1990. Lung. In: Pathology of the Fischer Rat: Reference and Atlas (Boorman GA, Eustis SL, Elwell MR, Montgomery CA, MacKenzie WF, eds). Academic Press, San Diego, CA, 339-367.
Sells DM, Brix AE, Nyska A, Jokinen MP, Orzech DP, Walker NJ. 2007. Respiratory tract lesions in noninhalation studies. Toxicol Pathol 35:170-177.Full Text: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3433271/