The otoconia of the guinea pig utricle: Internal structure, surface exposure, and interactions with the filament matrix

A unique feature of the vertebrate gravity receptor organs, the saccule and utricle, is the mass of biomineral structures, the otoconia, overlying a g elatinous matrix also called "otoconial membrane" on the surface of the sen sory epithelium. In mammals, otoconia are deposits of calcium carbonate in the form of composite calcite crystals. We used quick-freezing, deep etchin g to examine the otoconial mass of the guinea pig utricle. The deep-etching step exposed large expenses of intact and fractured otoconia, showing the fine structure and relationship between their internal crystal structure, t heir surface components, and the filament matrix in which they are embedded , Each otoconium has a compact central core meshwork of filaments and a com posite outer shell of ordered crystallites and macromolecular aggregates, A distinct network of 20-nm beaded filaments covers the surface of the otoco nia, The otoconia are interconnected and secured to the gelatinous matrix b y surface adhesion and by confinement within a loose interotoconial filamen t matrix. The gelatinous matrix is a dense network made of yet another type of filament, 22 nm in diameter, which are cross-linked by shorter filament s, characteristically 11 nm in diameter. Our freeze-etching data provide a structural framework for considering the molecular nature of the components of the otoconial complex, their mechanical properties, and the degree of b iological versus chemical control of otoconia biosynthesis, (C) 2000 Academ ic Press.