Microdisc gel electrophoresis in sodium dodecyl sulfate of organic material from rat otoconial complexes.

The gravity receptors of all vertebrates utilize a “test mass” consisting of a complex arrangement of mineral and organic substance that lies over the sensory receptor areas ( m a c u l a ~ ) . ~ ~ Carlstrom has shown that in most vertebrates, the mineral is a polymorph of calcium carbonate (vaterite, aragonite, calcite] in the form of minute, single crystals called otoconia (“ear dust”)? The otoconia contain a small amount of organic material that is continuous with intervening and underlying organic substance of the otoconial membrane, the organic and inorganic complements being referred to here collectively as an otoconial complex. In contrast, in certain fishes, a solitary otolith (”ear stone”) occurs or mixtures of otoliths and otoconia are present.’ The Agnatha, which otherwise lack mineralized tissue, have otoconia that, surprisingly, contain calcium phosphate (amorphous apatite, Reference 2). In the gravity receptors of man and other warm-blooded animals, only calcite is present and the otoconia have a basically similar morphology regardless of the species studied (FIGURES 1-3). They exhibit the characteristic threefold symmetry of calcite, and the rhombohedron is the basic structural unit. Moreover, the mammalian otoconia have the crystallographic properties of single crystals of calcite including the appropriate Miller indices (1011) (determined for human otoconia, see Reference 16).2.1e Thus, the otoconia of man and other mammals take their shape largely from the mineral they contain rather than from their organic fractions (FIGURE 41, even though both materials may be essential to their existence. It is unclear why individual crystals of mineral substance occur in an overwhelming number of vertebrate inner ears when other mineral deposits in the body are polycrystalline. or why one polymorph of calcium carbonate is used preferentially instead of another in particular classes of animals. From a chemical viewpoint, however, the fact that calcium ions combine with anions to form a mineral deposit in all normal vertebrate inner ears implies that phylogenetically there was a need to store calcium against periods of short supply. The fact that a well-ordered array of mineral material and organic substance has endured throughout the long evolutionary history of the vertebrate gravity receptors demonstrates that perfection of function was approached by this union. Required ions were stored in their ground-energy state [or entatic state)” through the participation of an appropriate “capture” protein material (see Reference 24). The end result was a reservoir of ions to buffer the surrounding fluid