An Analysis of Otolith Aging, a Contributing Cause of Vestibular Dysfunction
The vestibular system provides the sensory inputs needed for balance and orientation. Residing in the inner ear, it consists of distinct physical structures that are responsive to rotation (the semicircular canals) and linear acceleration (the otoliths). Like all complex biological systems, the vesibular system becomes dysfunctional with age. This dysfunction leads to loss of balance and increased incidence of falls, a potentially life-threatening event when taking place in an already frail older population.
Today's open access paper is an interesting look at how the otolith ages into dysfunction, evaluating the response to acceleration in mice of various ages. An otolith is a small calcified structure suspended in a viscous medium that presses upon sensory hair cells when it is shifted by the movement of the body. The strength of the sensory response depends on both the degree of acceleration and the weight of the otolith. If the otolith's weight changes, then sensory dysfunction will result - and otolith density declines with advancing age. There are, however, other changes in the aging of the otolith and its surroundings that could negatively impact the sensory response to acceleration, and which may or man not be more important. Considering only otolith weight is an oversimplification, and it is worth reading the discussion section of the paper for a more nuanced view.
Effects of aging on otolith morphology and functions in mice
Increased fall risk caused by vestibular system impairment is a significant problem associated with aging. A vestibule is composed of linear acceleration-sensing otoliths and rotation-sensing semicircular canals. Otoliths, composed of utricle and saccule, detect linear accelerations. Otolithic organs partially play a role in falls due to aging. Aging possibly changes the morphology and functions of otoliths. However, the specific associations between aging and otolith changes remain unknown. Therefore, this study aimed to clarify these associations in mice.
Young C56BL/6N (8 week old) and old (108-117 weeks old) mice were used in a micro-computed tomography (μCT) experiment for morphological analysis and a linear acceleration experiment for functional analysis. Young C56BL/6N (8 week old) and middle-aged (50 week old) mice were used in electron microscopy experiments for morphological analysis. μCT revealed no significant differences in the otolith volume but significant differences in the otolith density between young and old mice. μCT and electron microscopy revealed significant differences in the structure of striola at the center of the otolith. Significant differences were also observed in the amplitude of the eye movement during the vestibulo-ocular reflex induced by linear acceleration, indicating that the otolith function was worse in old mice than in young mice.
This study demonstrated the decline in otolith function with age caused by age-related morphological changes. Specifically, when otolith density decreased, inertial force acting on the hair cells decreased, and when the structure of striola collapsed, the function of cross-striolar inhibition decreased, thereby causing a decline in the overall otolith function.