The ultrastructure of the calcium carbonate balance organs of the inner ear: an ultra-high resolution electron microscopy study.

The balance organs of the inner ear of vertebrates, found as single, large growths of aragonite ('otoliths') in fish and small clumped masses ('otoconia') of either aragonite (amphibians) or calcite (mammals), have long been regarded as polycrystalline and single crystals respectively. The use of ultra-high resolution electron microscopy and electron diffraction to study comparatively crushed samples of these biominerals and samples of geological calcium carbonates, as examples of pure inorganic crystals, reveals that the biological structures are composed of microcrystals joined together by organic matrices to form composite crystals. Such structures either grow to a finite, controlled size (otoconia) or have daily growth patterns (otoliths). Mechanisms of growth are proposed to link these seemingly different patterns varying only in the number of nucleation sites and the degree of biological as against chemical control over the growth.