The aims of this project are to study the spontaneous and experimentally induced aural cholesteatoma in the Mongolian gerbil. The spontaneous gerbilline cholesteatoma is the only non-human spontaneous cholesteatoma ever described; the primary purpose of this study is to establish this cholesteatoma as a model with which to study the invasive and erosive characteristics of human aural cholesteatoma. Gerbils with spontaneous aural cholesteatomas will be detected by far field auditory nerve evoked (responses; those with threshold shifts will be studied histologically, with transmission electron microscopy and cytocochleography, in order to determine the natural history of this entity. Similar cholesteatomas will be induced by ear canal ligation in another group of gerbils. This group will be studied histologically to determine whether the induced cholesteatoma is, as we suspect, identical to the spontaneous gerbilline cholesteatoma. In selected animals, impermeable and semi-permeable barriers will be surgically interposed between the advancing front of the cholesteatoma and the bone of the middle ear that would be eroded by contact with the cholesteatoma. From this experiment we hope to determine whether cholesteatoma induced bone erosion is due to pressure, enzymatic action (e.g., proteases), cellular action (e.g., osteoclasts) or a combination of these factors. An understanding of the pathophysiology of the erosive qualities of the gerbilline cholesteatoma would lead to a better understanding of the human disease and possibly lead to means of preventing and treating this prevalent disease.