A comprehensive examination of the literature has revealed no reference to the presence of pavement epithelium of primary origin in the mammary gland. Numerous reports, however, of metaplasia and its associated changes prove its frequency and include descriptions of its occurrence in several organs of the body, among which are the nose, frontal sinus, middle ear, lungs, esophagus, stomach, gallbladder, pancreatic duct, renal pelvis, bladder, urethra, uterus and ovary. It is doubtful that metaplasia has actually been found in many of the reported cases, owing to the possibility of epithelium growing over from adjacent lesions of the skin or of the mucous membrane. Naturally, all fistulas warrant suspicion. The greatest offenders in the production of metaplasia have been reported as chronic inflammatory processes.
Consideration of a subject so complex as metaplasia and its associated processes demands brief mention of the striking figures in a literature which is muddled with a