Development of otoconia.

The development of the majority of otoconia in the macula utriculi occurs on the fifteenth and sixteenth gestational days (CBA/CBA strain of mice). The epithelial cell layer shows extensive protrusion of some cells, cytoplasmic blebbing from the surface of both hair cells and supporting cells, and electron dense inclusion bodies and vesicles surrounded by a double membrane in the apical part of supporting cells. This may reflect the formation of otoconia of varying sizes. Otoconia have an organic matrix that often shows a fibrillar substructure early in development. The majority of the organic aggregations that probably become the future otoconia are surrounded by a limiting membrane. The hexagonal shape of the otoconia can be observed as early as the sixteenth to seventeenth gestational days. Some otoconia show a nucleus-like focus in the center of the organic substance. Elemental mapping of calcium on the sixteenth gestational day shows an accumulation of calcium not only in the otoconia but also in the apical part of the epithelia cell layer below the otoconia. The endolymphatic space does not contain much calcium. These findings indicate that during the incorporation of calcium into otoconia, calcium is likely to emanate largely from the surface cells of the utricle.