Comparative histology of the tympanic membrane and its relationship to cholesteatoma.

The purpose of this study was to determine whether anatomic differences in the tympanic membranes of various species could explain differences in the propensity to form aural cholesteatomas and retraction pockets. Tympanic membranes from humans, dogs, cats, rabbits, guinea pigs, rats, gerbils, and mice were examined histologically. The pars flaccida and pars tensa varied greatly among the species studied. The guinea pig's pars flaccida was very small and had a thin lamina propria. In contrast, the lamina propria of the rabbit and cat pars flaccida were thick. The amount of collagen, elastin, mast cells, and macrophages varied widely. The human and gerbilline tympanic membranes were anatomically dissimilar; for example, the human pars flaccida and pars tensa contained more and denser collagen than did those of the gerbil. The presence of macrophages or mast cells did not correlate with the propensity to develop cholesteatomas. Therefore, anatomic differences among these species do not explain why some develop aural cholesteatomas and others do not.