Initiation and stages of mineralization in tympanosclerosis.
The ultrastructure of tympanosclerotic tissue is described with particular reference to the structure of matrix vesicles and calcospherules, often of intricate structure, and to their role in the induction of calcification. The vesicles formed by degenerating cells within the collagenous matrix or epithelium of the tympanosclerotic tissues are the primary site of calcification. The released spherules appear scattered throughout the collagenous layers, and form the organic basis of calcospherules. Through fusion larger aggregates are formed which lead to the development of macroscopically recognizable calcified plaques in the middle ear cleft. The entire process is triggered off by the inflammatory processes of otitis media. Compared with other processes of excessive fibrogenesis and calcification, tympanosclerosis fits well into the overall scheme of induced calcification in both normal and pathological conditions which is similar in all mineralizing tissues composed of collagen and protoglycons. The cause of the prolific fibrogenesis and mineralization has remained doubtful. Clearly inflammatory and degenerative processes in otitis media may trigger off the release of a factor (or factors), and of matrix vesicles acting as fibrogenetic stimulants and primary sites of calcification. Katchburian's working hypothesis of mineralization (Katchburian, 1973) has universal appeal and can be confirmed on the basis of our observations on tympanosclerosis. Tympanosclerosis may be added to the 'crystal deposition diseases' of McCarthy.