Identification of Cellular Voids in the Human Otic Capsule

The otic capsule consists of dense highly mineralized compact bone. Inner ear osteoprotegerin (OPG) effectively inhibits perilabyrinthine remodeling and otic capsular bone turnover is very low compared to other bone. Consequently, degenerative changes like dead osteocytes and microcracks accumulate around the inner ear. Osteocytes are connected via canaliculi and need a certain connectivity to sustain life. Consequently, stochastic osteocyte apoptosis may disrupt the osteocytic network in unsustainable patterns leading to widespread cell death. When studying bulk-stained undecalcified human temporal bone, large clusters of dead osteocytes have been observed. Such "cellular voids" may disrupt the perilabyrinthine OPG mediated remodeling inhibition possibly leading to local remodeling. In the common ear disease otosclerosis pathological bone remodeling foci are found exclusively in the otic capsule. We believe the pathogenesis of otosclerosis is linked to the unique bony dynamics of perilabyrinthine bone and cellular voids may represent a starting point for otosclerotic remodeling. This study aims to identify and characterize cellular voids of the human otic capsule. This would allow future cellular void quantification and comparison of void and otosclerotic distribution to further elucidate the yet unknown pathogenesis of otosclerosis.

Distribution of microcrack surface density in the human otic capsule

BACKGROUND

The bony otic capsule is comprised of highly mineralized and dense compact bone. It is rarely remodelled and degenerative changes, therefore, accumulate around the inner ear. It is also a predilection site for the pathological remodelling seen in otosclerosis. Morphometric studies have documented increased numbers of dead osteocytes and microcracks in the human otic capsule. Microcracks may disrupt the lacuno-canalicular network and cause osteocyte apoptosis ultimately breaking up the perilabyrinthine bone signalling pathways and dynamics. This may be important to understand the pathogenesis of remodelling diseases like otosclerosis.

AIMS/OBJECTIVES

This study describes the spatial and regional distribution of microcrack surface density in relation to the inner ear and compares it to that previously recorded for otosclerosis.

MATERIAL AND METHODS

Forty-two temporal bones and five ribs were used. All samples were undecalcified, bulk stained in basic fuchsin and plastic embedded. Unbiased stereology was used to estimate the true surface density of microcracks (mm²/mm³) in perilabyrinthine bone.

RESULTS

The surface density of microcracks accumulates around the inner ear spaces, particularly in the lateral window regions, and increases with age.

CONCLUSIONS AND SIGNIFICANCE

This study documents the spatial and temporal association between microfractures and otosclerosis in the otic capsule.

Increased cochlear otic capsule thickness and intracortical canal porosity in the oim mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI or brittle bone disease) is a group of genetic disorders of the connective tissues caused mainly by mutations in the genes encoding collagen type I. Clinical manifestations of OI include skeletal fragility, bone deformities, and severe functional disabilities, such as hearing loss. Progressive hearing loss, usually beginning in childhood, affects approximately 70% of people with OI with more than half of the cases involving the inner ear. There is no cure for OI nor a treatment to ameliorate its corresponding hearing loss, and very little is known about the properties of OI ears. In this study, we investigate the morphology of the otic capsule and the cochlea in the inner ear of the oim mouse model of OI. High-resolution 3D images of 8-week old oim and WT inner ears were acquired using synchrotron microtomography. Volumetric morphometric measurements were conducted for the otic capsule, its intracortical canal network and osteocyte lacunae, and for the cochlear spiral ducts. Our results show that the morphology of the cochlea is preserved in the oim ears at 8 weeks of age but the otic capsule has a greater cortical thickness and altered intracortical bone porosity, with a larger number and volume density of highly branched canals in the oim otic capsule. These results portray a state of compromised bone quality in the otic capsule of the oim mice that may contribute to their hearing loss.

Microcrack surface density in the human otic capsule: An unbiased stereological quantification

Bone is continuously remodeled to repair and strengthen degenerative bone with accumulating dead osteocytes and microfractures. Inner ear osteoprotegerin (OPG)-mediated inhibition of otic capsular bone remodeling causes excessive perilabyrinthine bone degeneration. Consequently, microcracks accumulate around the inner ear. Microcracks cause osteocyte apoptosis and may disrupt the canalicular network connecting osteocytes. Despite their linear microscopic appearance, microcracks are three-dimensional disruption planes and represent surface areas inside a tissue space. With an elevated microcrack burden the number of disconnected osteocytes is expected to increase. This may prove relevant to ongoing research in otic focal pathologies like otosclerosis. Therefore, an unbiased quantification of the microcrack surface density (mm² /mm³ ) in the human otic capsule is essential. In this study unbiased stereology was applied to undecalcified bulk stained human temporal bones to demonstrate its feasibility in describing the three-dimensional reality behind two dimensional observations of microcracks. A total of 28 human temporal bones and five ribs were bulk stained in basic fuchsin, serially sectioned and hand-ground to a thickness of 80-120 μm. Both horizontal and vertical sections were produced and compared. This study showed that surface density of microcracks was significantly higher around the inner ear compared to ribs. Furthermore, no significant difference in microcrack surface density between horizontal and vertical sections in the temporal bone was demonstrated.

Prevalence, size and distribution of microdamage in the human otic capsule

CONCLUSIONS

Age-dependent microdamage (MDx) accumulates excessively in human perilabyrinthine bone, where the bone turnover is almost absent. This may have pathological implications for bone-specific disorders such as otosclerosis. The role of MDx accumulation is discussed from an osteodynamic perspective.

OBJECTIVES

Bone remodelling is highly inhibited within the otic capsule compared with the rest of the skeleton. Consequently excessive accumulation of age-dependent capsular MDx is expected. This study describes the prevalence, size and topographical distribution of MDx in the human otic capsule.

METHODS

A total of 241 undecalcified human temporal bones were examined. Bulk staining and the cutting-grinding technique were used to separate in vivo MDx from microcrack artefacts induced post mortem by the milling procedure. Quantitative data were obtained by fluorescence microscopy by counting and measuring and by the use of stereology.

RESULTS

Microcracks accumulated continuously and extensively in the human otic capsule throughout life. Both the number and total length of MDx were higher close to the inner ear space as compared with the capsular periphery. The mean length of the MDx remained constant with age. There was no statistically significant sex difference.

Otosclerosis: a perilabyrinthine threshold phenomenon

This paper is a review of our most recent findings concerning the osteo-dynamics of the bony otic capsule and pathogenesis of otosclerosis. By exploring the spatial relationship between normal perilabyrinthine bone remodeling, the viability and spatial distribution of labyrinthine osteocytes, and the location of otosclerosis, a unique spatial pattern emerged. Bone remodeling is highly inhibited around the inner ear space. Most likely, inner ear anti-resorptive signals enter the bony otic capsule through the lacuno-canalicular porosity. The patency of this signaling pathway depends on the viability of individual osteocytes. In the young otic capsule the density of viable osteocytes is high and centripetally distributed. This arrangement may sustain a life-long osseus pathway for anti-resorptive signals even within a bone where a considerable loss of viable osteocytes must be expected, as demonstrated by a centripetal accumulation of dead osteocytes with age. The spatial distribution of dead osteocytes follows the same general pattern as otosclerosis. We suggest that clustering of dead osteocytes may impede the transmission of anti-resorptive signals locally, leaving such ghost regions susceptible to focal bone remodeling as in human otosclerosis. The preserved network of viable osteocytes around the depleted ghost regions may contain the process and distort the structure of bone remodeling into an abnormal otosclerotic pattern.

Osteoprotegerin expression and sensitivity in otosclerosis with different histological activity

Otosclerosis is a complex bone dystrophy of the human otic capsule leading to conductive and sensorineural hearing loss. Since otosclerosis may, at least in part, be considered as an autoimmune-inflammatory disease, disturbed balance of TNF-alpha and osteoprotegerin (OPG) expression has been implicated in the pathological bone remodeling. It has been supposed that active otosclerosis is characterized by decreased or missing local OPG production with invariable OPG sensitivity of the otosclerotic foci. Ankylotic stapes footplates (n = 41) removed by stapedectomy were processed to histological examination, OPG-specific RT-PCR, tissue culturing and alkaline-phosphatase (AP) activity assessment, respectively. OPG concentration of serum specimens (n = 41) was measured by ELISA. Cortical bone fragments harvested from the external ear canal were used as negative controls of otosclerosis. Among 41 ankylotic stapes footplates, 22 active and 19 inactive otosclerosis cases were histologically diagnosed. OPG expression was significantly lower (p < 0.001) in active otosclerosis compared to inactive cases. Osteoclast cultures originated from active otosclerotic foci showed a considerable susceptibility against external OPG dosage, which resulted in a significant decrease of AP activity (p < 0.001). In contrast, OPG serum levels were in the normal range (5-100 ng/ml) indicating a non-systemic bone resorption. In conclusion, secondary decreased local OPG production might play an important role in the pathogenesis of otosclerotic bone remodeling disorder. As to previous and current results, decreased OPG sensitivity of lesion-forming cells should be excluded. These observations may indicate the potential role of recombinant OPG treatment in early stages of otosclerosis.

The spatial distribution of otosclerosis: a quantitative study using design-based stereology

CONCLUSION

This study documents that otosclerotic bone remodeling is distributed centripetally around the inner ear space whereas normal bone remodeling is distributed centrifugally. We suggest that this inverse relation reflects the unique osteo-dynamic setting of the otic capsule: since perilabyrinthine bone remodeling is extremely low, osteocyte deficiency and microcracks accumulate in excess toward the inner ear space with age. This may disrupt the osseus functional network, impede propagation of anti-resorptive signals, and precipitate otosclerotic bone remodeling with a spatial preference for older bone.

OBJECTIVE

To quantify the spatial distribution of otosclerotic bone around the inner ear space in order to explore a possible spatial relation with normal capsular bone remodeling.

METHODS

Otosclerotic lesions in 53 undecalcified human temporal bones were identified and volume data were measured with the CAST-grid system and processed by dedicated software for advanced design-based stereology.

RESULTS

The maximum volume fraction of otosclerotic bone was observed in the innermost perilabyrinthine zones of the otic capsule. The volume fraction of otosclerotic bone declined gradually but significantly from the inner ear space towards the capsular periphery with a general perilabyrinthine centripetal distribution.

Three-dimensional reconstruction of the otosclerotic focus

CONCLUSIONS

The location and three-dimensional (3D) shapes of the otosclerotic foci suggest a general centripetal distribution of otosclerotic bone remodeling around the inner ear space, whereas the normal bone remodeling is distributed centrifugally. The existence of an inverse spatial relation between normal and otosclerotic bone remodeling suggests that inner ear mechanisms in control of bone remodeling may have a pathogenetic role in otosclerosis.

OBJECTIVES

To explore the 3D shape of otosclerotic lesions around the inner ear space by introducing the use of 3D reconstructions and to discuss the results in a new context of temporal bone dynamics and perilabyrinthine signaling pathways.

METHODS

Thirty-four otosclerotic lesions from 20 decalcified human temporal bones were rendered and visualized with the public 3D 'Reconstruct' software.

RESULTS

The majority of otosclerotic lesions were found close to the labyrinthine space at the well-established topographical sites of predilection with a smooth demarcation against the surrounding bone. However, in addition the virtual 3D technique revealed a new perilabyrinthine anisotropy of individual otosclerotic lesions, displaying a bulky end facing the inner ear space and a volumetric decline towards the capsular periphery.