Otoconia: Mimicking a calcite-based functional material of the human body. From basic research to medical aspects

Abstract

Otoconia (calcite-based biominerals) are part of the sensory system in the inner ear of vertebrates, acting as gravity receptors responding to linear accelerations. Biomimetic otoconia are grown by doublediffusion into gelatine-gel matrices, and represent the first example of successful imitation of a biomineral, not only in outer shape but also in composite structure and hierarchical inner architecture. Biomimetic and biogenic (human) otoconia are investigated by X-ray methods, chemical analytics, ESEM, and TEM. Shape development (morphogenesis) as well as (partial) dissolution of the calcite component of the composite underline the hierarchical inner architecture built of more dense rhombohedral branches (with plane endfaces) and a rounded, more porous belly area. Atomistic simulations are performed in order to get insight into very first nucleation steps. Based on the detailed observations made up to now, first assumptions for the function of otoconia are developed, including the questions of density distribution within the volume of the specimen, the surrounding endolymph, as well as anchoring and interconnections of otoconia. A final point concerns the degeneration of otoconia which is caused by complexing agents and/or changes in ion concentrations (and pH) of the endolymph.

Effect of calcium ions on structure and stability of the C1q‐like domain of otolin‐1 from human and zebrafish

“…The ultrastructure of otoconia is a matter of debate. In contrast, the effects of demineralization by EDTA, acids, deionized water or aminoglycoside drugs, as well as observations of otoconia isolated during inner ear surgery, point to the fact that otoconia are continuous crystals, in which chemically resistant 'branch' regions and a more porous, chemically pliable 'belly' region can be distinguished [6,9,10]. In contrast, the effects of demineralization by EDTA, acids, deionized water or aminoglycoside drugs, as well as observations of otoconia isolated during inner ear surgery, point to the fact that otoconia are continuous crystals, in which chemically resistant 'branch' regions and a more porous, chemically pliable 'belly' region can be distinguished [6,9,10].…”

Functional derivatives of human dentin matrix protein 1 modulate morphology of calcium carbonate crystals

“…Linear acceleration causes the otoconia, which are denser than the surrounding endolymph, to move relatively to hair cells. This creates a signal, which together with visual stimuli from the eyes is interpreted as a movement . In contrast to bones, which are continuously resorbed and synthesized throughout life, otoconia are formed during the embryonic development, mature during the perinatal period, and do not regenerate later in life.…”

The sense of balance in humans: Structural features of otoconia and their response to linear acceleration

“…On the other hand, the anisotropic growth of otoconia (branches grow quickly and the belly region grows with a significant delay), together with the resulting composite nanostructure can be taken as a hint to call the strict -3m description into question. In fact, detailed investigations of the outer shape of otoconia lead to the conclusion, that the rhombohedral faces are different in size, thereby destroying the centrosymmetry, which, at the same time, affects the density distribution within the volume [ 16 ]. This observation is taken as a first suggestion, that deviations from centrosymmetry are prerequisites for the function of otoconia belonging to the otoconial complex.…”