Histological preparation of developing vestibular otoconia for scanning electron microscopy

Relationship between osteoporosis and benign paroxysmal positional vertigo based on evidence-based medicine and bioinformatics

It has been reported that osteoporosis is a possible risk factor of benign paroxysmal positional vertigo (BPPV).

PURPOSE

We analyzed the correlation between osteoporosis and BPPV and the possible mechanism by performing evidence-based medicine meta-analysis and bioinformatics analysis.

METHODS

Initially, English articles related to osteoporosis and BPPV were obtained through PubMed and EMBASE databases. Stata12.0 software was used for meta-analysis to calculate the odd ratio (OR) and 95% confidence interval (CI) of outcome indicators, and the heterogeneity was evaluated by subgroup analysis, publication bias evaluation, and sensitivity analysis. In addition, microarray datasets related to BPPV and osteoporosis were obtained from gene expression omnibus (GEO) database to screen differentially expressed genes. At last, a mouse model of osteoporosis was established by bilateral oophorectomy for validation. RT-qPCR and Western blot analysis were performed to determine expression of related factors in mouse tissues.

RESULTS

Osteoporosis was suggested as an important risk factor for BPPV through meta-analysis of these 12 articles. It was found that PPP2CA was upregulated in BPPV and low bone mineral density (BMD) samples. Moreover, PPP2CA induced dephosphorylation of BCL2, which may be involved in BPPV through regulation of BMD. Through this mechanism, silencing of PPP2CA could elevate the incidence of BPPV by promoting bone remodeling and reducing the density of otoconia around the macula.

CONCLUSIONS

PPP2CA reduces BMD expression by inducing dephosphorylation of BCL2, which may be one of the mechanisms responsible for the onset of BPPV in osteoporosis.

Functional derivatives of human dentin matrix protein 1 modulate morphology of calcium carbonate crystals

Dentin matrix protein 1 (DMP1) is an acidic, extracellular matrix protein essential for biomineralization of calcium phosphate, in bone and dentin. It is proteolytically processed into two fragments, 44K and 56K. Recently, the presence of DMP1 was noticed in inner ear, specifically in otoconia, which are calcium carbonate biominerals involved in sensing of balance. In this study, the solution structure and biomineralization activity of otoconial 44K and 56K fragments toward calcium carbonate were investigated. The results of analytical ultracentrifugation, circular dichroism, and gel filtration indicated that DMP1 fragments are disordered in solution. Notably, 56K formed oligomers in the presence of calcium ions. It was also observed that both fragments influenced the crystal growth by in vitro biomineralization assay and scanning electron microscopy. In addition, they sequester the calcium ions during the calcite formation. Calcium carbonate crystals precipitated in vitro changed their size and shape in the presence of DMP1 fragments. Oligomerization propensity of 56K may significantly enhance this function. Our study indicates that intrinsically disordered DMP1 has a previously unknown regulatory function for biomineralization of otoconia.

In vitro calcite crystal morphology is modulated by otoconial proteins otolin-1 and otoconin-90

Otoconia are formed embryonically and are instrumental in detecting linear acceleration and gravity. Degeneration and fragmentation of otoconia in elderly patients leads to imbalance resulting in higher frequency of falls that are positively correlated with the incidence of bone fractures and death. In this work we investigate the roles otoconial proteins Otolin-1 and Otoconin 90 (OC90) perform in the formation of otoconia. We demonstrate by rotary shadowing and atomic force microscopy (AFM) experiments that Otolin-1 forms homomeric protein complexes and self-assembled networks supporting the hypothesis that Otolin-1 serves as a scaffold protein of otoconia. Our calcium carbonate crystal growth data demonstrate that Otolin-1 and OC90 modulate in vitro calcite crystal morphology but neither protein is sufficient to produce the shape of otoconia. Coadministration of these proteins produces synergistic effects on crystal morphology that contribute to morphology resembling otoconia.

Development of otolith receptors in Japanese quail

This study examined the morphological development of the otolith vestibular receptors in quail. Here, we describe epithelial growth, hair cell density, stereocilia polarization, and afferent nerve innervation during development. The otolith maculae epithelial areas increased exponentially throughout embryonic development reaching asymptotic values near posthatch day P7. Increases in hair cell density were dependent upon macular location; striolar hair cells developed first followed by hair cells in extrastriola regions. Stereocilia polarization was initiated early, with defining reversal zones forming at E8. Less than half of all immature hair cells observed had nonpolarized internal kinocilia with the remaining exhibiting planar polarity. Immunohistochemistry and neural tracing techniques were employed to examine the shape and location of the striolar regions. Initial innervation of the maculae was by small fibers with terminal growth cones at E6, followed by collateral branches with apparent bouton terminals at E8. Calyceal terminal formation began at E10; however, no mature calyces were observed until E12, when all fibers appeared to be dimorphs. Calyx afferents innervating only Type I hair cells did not develop until E14. Finally, the topographic organization of afferent macular innervation in the adult quail utricle was quantified. Calyx and dimorph afferents were primarily confined to the striolar regions, while bouton fibers were located in the extrastriola and Type II band. Calyx fibers were the least complex, followed by dimorph units. Bouton fibers had large innervation fields, with arborous branches and many terminal boutons.

Ultrastructural changes in otoconia of osteoporotic rats

The etiology of benign paroxysmal positional vertigo (BPPV) remains obscure in many cases and women are affected more often than men. A recent prospective study, performed in women >50 years of age suffering from recurrent BPPV, showed associated osteopenia or osteoporosis in a large percentage of these patients. These results suggested the possible relationship between recurrent BPPV and a decreased fixation of calcium in bone in women >50 years. To test this hypothesis, an experimental study was performed in adult female rats. Utricular otoconia of female rats in which osteopenia/osteoporosis was induced by bilateral ovariectomy (OVX) were compared to those of sham-operated adult females rats (SHAM), as control group. FIRST STUDY: The morphology of theutricles of OVX and SHAM rats was analyzed with scanning electron microscopy. In osteopenic/osteoporotic rats, the density of otoconia (i.e. the number of otoconia per unit area) was decreased (p = 0.036)and their size was increased (p = 0.036) compared to the control group. SECOND STUDY: To test the role of calcium turnover in such morphological changes, utricular otoconia of 2 other groups of OVX and SHAM rats, previously injected with calcein subcutaneously, were examined by conventional and epifluorescence microscopy. In epifluorescence microscopy, labeling with calcein showed no significant fluorescence in either group. This finding was interpreted as a lack of external calcium turnover into otoconia of adult female rats. The ultrastructural modifications of otoconia in osteopenic/osteoporotic female adult rats as well as the role of estrogenic receptors in the inner ear are discussed. The possible pathophysiological mechanisms which support the relationship between recurrent BPPV in women and the disturbance of the calcium metabolism of osteopenia/osteoporosis are debated.

Age-related changes on the morphology of the otoconia

OBJECTIVES/HYPOTHESIS

Although there are numerous reports on otoconial morphology using field-emission scanning electron microscopy (FESEM), there are few reports regarding the changes of otoconial morphology with aging. The aim of the current study was to investigate changes in otoconial morphology in rats according to age, using FESEM.

STUDY DESIGN

Laboratory study using experimental animals.

METHODS

We investigated age-related changes in otoconial morphology using FESEM in three groups of rats: young (1 wk old), middle-aged (6 mo old), and aged (23 mo old).

RESULTS

There was great size variation in utricular otoconia in the young and aged rats, but we found no clear regional separation of saccular otoconia in all groups based on size. In the oldest rats, the bodies of many otoconia in both maculae were pitted, fissured, penetrated, and eventually broken into several fragments. However, the terminal facets were smooth and the lines of intersection of facets were sharp, despite the degenerated bodies of the otoconia in this group. Giant otoconia were discovered frequently on the outer margin of the utricular maculae in aged rats. We directly observed weakened or broken linking filaments and otoconial fragments in the aged group.

CONCLUSION

The oldest rats showed the most degeneration of otoconia and linking filaments with otoconial fragments. This study of age-related morphologic changes in otoconia might help us understand the origin of idiopathic benign paroxysmal positional vertigo.

Morphometry of otoconia in the utricle and saccule of developing Japanese quail

The development of otoconia in the utricular and saccular maculae from initial embryonic formation to adult stages was examined in Japanese quails. Both the morphology and size of the otoconia were quantified at different developmental stages. It was observed that the otoconia were initially formed on embryologic stage E5 in the saccule and E6 in the utricle. Otolith mass areas increased in a sigmoidal growth pattern, with saccular otolith areas being smaller than the utricular mass areas. Saccular otolith masses reached adult values at embryonic stage E12 and utricular areas reached adult values at post-hatch day 7. Mature individual otoconia were characterized by a barrel shape with two trihedral faceted ends. However, initial formation of otoconia at E5 (saccular) and E6 (utricular) maculae was characterized by a double fluted morphology that consisted of an hourglass shape with extended fins forming trihedral angles of 120 degrees. Double fluted otoconia rapidly filled, so that by embryonic day 8 mature otoconia dominated the maculae for the remainder of development through adulthood. Thus, a progression from double fluted to mature forms was noted. Mature utricular otoconia in adult quails averaged 11 microm in length and 5 microm in width, with length/width ratios of approximately 2.5:1, for all size ranges. Saccular otoconia were smaller, having about 70% the size of utricular otoconia in both length and width. During development, the average size and range of individual otoconia increased nearly linearly for both otolith organs. In the utricular macula, large otoconia were concentrated in the lateral regions of the epithelium. In contrast, otoconia of various sizes were distributed uniformly across the surface of the saccular macula.