Formation and fate of the otoconia: Scanning and transmission electron microscopy

Although mammalian otoconia are known to be composed of calcium carbonate in calcite form, their morphogenesis, maintenance, and fate are not well understood. More information on these problem areas would aid considerably in better understanding various clinical conditions, such as cupulolithiasis and otolith degeneration. This study was intended to clarify the fine morphology of the otolith in normal and adverse conditions in laboratory animals with the use of the scanning and transmission electron microscopes. It was confirmed by this study that the mammalian otoconium is composed of an organic matrix and minerals (calcium carbonate). When the minerals are removed by decalcification, or chelation, a well arranged organic matrix, and even a nucleus, can be found in the crystal. The matrix of the crystal is identical to the gelatin matrix of the otolithic membrane. This finding supports the possibility that a normal protein matrix is a prerequisite for normal otoconia formation, and that the exchange of calcium ions can occur without altering the crystal structure. The vestibular dark cells, which are thought to be endolymph-secreting cells, appear to be capable of removing calcium from the otoconia that are attached to the dark cell surfaces. Although this evidence is only circumstantial, its consistency is impressive. On the basis of the foregoing, it is tempting to speculate that the dark cells participate in the removal of the dislodged otoliths, but further study is required to ascertain this point.