Ultrastructural characteristics of mesenchyme in the middle ear.

Prior to pneumatization of the middle ear, the mesenchyme present undergoes marked changes until it is transformed via a precollagenous step into the tunica propria of the mucosa. The ultrastructural characteristics of the middle ear mesenchyme in the rat reinforce previous light microscopic studies on human temporal bones. The possible interaction between middle ear mesenchyme and its epithelium in pneumatization and in the etiology of cholesteatoma is discussed.