Abstract
Human otoconia provide mechanical stimuli to deflect hair cells of the vestibular sensory epithelium for purposes of detecting linear acceleration and head tilts. During lifetime, the volume and number of otoconia are gradually reduced. In a process of degeneration morphological changes occur. Structural changes in human otoconia are assumed to cause vertigo and balance disorders such as benign paroxysmal positional vertigo (BPPV). The aim of this study was to investigate the main principles of morphological changes in human otoconia in dissolution experiments by exposure to hydrochloric acid, EDTA, demineralized water and completely purified water respectively. For comparison reasons artificial (biomimetic) otoconia (calcite gelatin nanocomposits) and natural calcite were used. Morphological changes were detected in time steps by the use of environmental scanning electron microscopy (ESEM). Under in vitro conditions three main dissolution mechanisms were identified as causing characteristic morphological changes of the specimen under consideration: pH drops in the acidic range, complex formation with calcium ions and changes of ion concentrations in the vicinity of otoconia. Shifts in pH cause a more uniform reduction of otoconia size (isotropic dissolution) whereas complexation reactions and changes of the ionic concentrations within the surrounding medium bring about preferred attacks at specific areas (anisotropic dissolution) of human and artificial otoconia. Owing to successive reduction of material, all the dissolution mechanisms finally produce fragments and remnants of otoconia. It can be assumed that the organic component of otoconia is not significantly attacked under the given conditions. Artificial otoconia serve as a suitable model system mimicking chemical attacks on biogenic specimens. The underlying principles of calcite dissolution under in vitro conditions may play a role in otoconia degeneration processes such as BPPV.
Highlights
Human otoconia represent biominerals, consisting mainly of calcite (.90 wt.-%) and a small amount of organic material (,5 wt.-%) [1,2,3,4]
The special materials feature of human otoconia is represented by an intergrowth of the calcite component with organic molecules, mainly glycoproteins and glycosaminoglycans, forming a composite system on the nanoscale, a so-called calcite based nanocomposite [3,4]
The aim of this study was to investigate the main principles of morphological changes in human otoconia in dissolution experiments by exposure to hydrochloric acid, EDTA, demineralized water and completely purified water respectively
Summary
Human otoconia represent biominerals, consisting mainly of calcite (.90 wt.-%) and a small amount of organic material (,5 wt.-%) [1,2,3,4]. Owing to their inertia they provide adequate sensory stimuli of the utricular and saccular maculae to detect linear acceleration and head tilts in relation to gravity [5,6]. The special materials feature of human otoconia is represented by an intergrowth of the calcite component with organic molecules, mainly glycoproteins and glycosaminoglycans, forming a composite system on the nanoscale, a so-called calcite based nanocomposite [3,4]. Otoconin 90 is the principal soluble otoconial protein whereas otolin represents the principal insoluble matrix protein which is present in fibrils interconnecting otoconia [7,8,9,10,11,12]
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