Structural Relationship between Calcite-Gelatine Composites and Biogenic (Human) Otoconia

Biogenic otoconia (ear dust) are composite materials of calcite with about 2 wt.-% proteins showing an average longitudinal size of about 10 mu m. The tiny biomineral particles are situated in the inner ear (in the maculae) and act as sensors for gravity and linear acceleration. Our comparative study of calcitegelatine composites (grown by double diffusion) and human otoconia is based on decalcification experiments, scanning electron microscopy, TEM and X-ray investigations in order to obtain a complete picture of the 3D structure and morphogenesis of the materials. Otoconia as calciteprotein composites display a cylindrical body with terminal rhombohedral faces intersecting at the pointed ends. As evidenced by TEM on focused ion beam cuts, both the artificial composites and human otoconia show a particular distribution of areas with different volume densities leading to a dumbbell-shape of the more dense parts consisting of rhombohedral branches (with end faces) and a less ordered, less dense area (the belly region). The peculiar inner architecture of otoconia with its dumbbell-shaped mass/density distribution is assumed to be necessary for optimal sensing of linear accelerations.