Otolith

An otolith, also known as an otoconium or statoconium, is a calcium carbonate (CaCO3) structure in the inner ear of vertebrates, including fish and mammals. These structures are essential for the perception of gravity, linear acceleration, and sound. Otoliths are located within the utricle and saccule of the vestibular system, which is responsible for balance and spatial orientation.

There are three otoliths in each inner ear: the utricle contains the utricular otolith (sometimes called the lapillus in fish), and the saccule contains the saccular otolith (sometimes called the sagitta in fish). A third, smaller otolith, the lagena (sometimes called the asteriscus in fish), exists in some species.

The otoliths are much denser than the surrounding endolymph fluid within the inner ear. When the head tilts or accelerates linearly, the otoliths, due to their inertia, lag behind the movement of the head. This lag causes them to deflect the stereocilia (sensory hair cells) of the macula, a specialized sensory epithelium located beneath the otoliths. The deflection of the stereocilia generates neural signals that are transmitted to the brainstem and ultimately to the brain, allowing the organism to perceive changes in head position and movement.

The size and shape of otoliths vary among species and even within individuals. In fish, otoliths grow continuously throughout their life, and their composition reflects the chemical environment in which the fish lived. This makes them valuable tools for studying fish migration patterns, age determination, and habitat use. Analysis of otolith microstructure and elemental composition can provide insights into the life history of fish and the environmental conditions they have experienced. In mammals, otoliths do not grow throughout life but still play a vital role in vestibular function.

Dysfunction of the otolith organs can lead to balance disorders, such as benign paroxysmal positional vertigo (BPPV), where otoliths become dislodged and enter the semicircular canals, causing inappropriate stimulation of the vestibular system and resulting in vertigo. Other vestibular disorders can also be related to otolith damage or dysfunction.