Proteins Involved in Otoconia Formation and Maintenance

The vestibule of the inner ear senses head motion for spatial orientation and bodily balance. In vertebrates, the vestibular system consists of three fluid filled semicircular canals, which detect rotational acceleration, and two gravity receptor organs, the utricle and saccule, which respond to linear acceleration and gravity (Figure 1). The utricle and saccule are also referred to as the otolithic organs because they contain bio-crystals called otoconia (otolith in fish). These crystals are partially embedded in a honeycomb layer atop a fibrous meshwork, which are the otoconial complex altogether. This complex rests on the stereociliary bundles of hair cells in the utricular and saccular sensory epithelium (aka macula). When there is head motion, the otoconial complex is displaced against the macula, leading to deflection of the hair bundles. This mechanical stimulus is converted into electrical signals by the macular hair cells and transmitted into the central nervous system (CNS) through the afferent vestibular nerve. In the CNS, these electrical signals, combined with other proprioceptive inputs, are interpreted as position and motion data, which then initiate a series of corresponding neuronal responses to maintain the balance of the body. Electrophysiological and behavioral studies show that the size and density of these tiny biominerals determine the amount of stimulus input to the CNS (Anniko et al. 1988; Jones et al. 1999; Jones et al. 2004; Kozel et al. 1998; Simmler et al. 2000a; Trune and Lim 1983; Zhao et al. 2008b).