Demonstration of intravital microfissures in undecalcified plastic-embedded temporal bones with the prestaining technique

The Spatial Distribution of Cellular Voids in the Human Otic Capsule: An Unbiased Quantification of Osteocyte-Depleted Areas

OBJECTIVE

This study aimed to describe the spatial distribution of osteocyte-depleted areas, so-called cellular voids, in the human otic capsule and compare it with that of otosclerosis.

BACKGROUND

Systematic histological studies of the bony otic capsule have revealed an osteoprotegerin (OPG)-mediated inhibition of normal bone remodeling around the inner ear. The resulting accumulation of bony degeneration and dead osteocytes has been thoroughly documented, and the spatial distribution of dead osteocytes and matrix microcracks resembles that of the human ear disease otosclerosis. Clusters of dead osteocytes that may interfere with osteocyte connectivity and thereby the OPG signaling pathway have been described in human temporal bones. It is possible that these cellular voids create disruptions in the antiresorptive OPG signal that may give rise to local pathological remodeling.

METHODS

Recently, a method of detecting cellular voids was developed. This study uses unbiased stereology to document the spatial distribution of cellular voids in bulk-stained undecalcified human temporal bone.

RESULTS

Cellular voids accumulate around the inner ear and increase in number and size with age. Furthermore, cellular voids are more frequently found in the anterior and lateral regions of the otic capsule, which are known predilection sites of otosclerosis.

CONCLUSION

This colocalization of cellular voids and otosclerosis suggests a causal relationship between focal degeneration and otosclerotic remodeling.

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.

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.

The real response of bone to exercise

This review presents findings made in studies of large mammalian bones, especially from racehorse training experiments (2-8 years old, third metacarpal, tarsal) and human autopsy orthopaedic femoral implant retrievals and other human biopsy and autopsy cases. Samples were cleaned to analyse mineralized matrix in three dimensions, or poly methyl-methacrylate embedded and micromilled to delete topography and study the superficial c. 0.5-microm two-dimensional section using quantitative backscattered electron imaging. With experimental implant studies in rabbits, observations were also made in vivo using confocal microscopy. Cracks in both calcified cartilage and bone may be removed by infilling with calcified matrix. This may be a general repair mechanism for calcified connective tissue crack repair. The fraction of the organ volume occupied by any form of bone tissue in equine distal third metacarpal extremities was increased in the more exercised groups by bone deposited within former marrow adipocytic space. Where deposited upon prior lamellar bone surfaces, this occurred without the intervention of prior resorption and without the formation of a hypermineralized cement line. Exercise inhibited osteoclastic resorption at external anatomical growth modelling sites where it normally occurs. Addition is not coupled to time-wasting resorption: both internally and externally, it occurs both by layering on existing cancellous surfaces and by creation of new immature scaffold, with de novo incorporation of a rich, capillary blood vessel supply. The real response within bone organs subjected to mechanical overload exercise within normal physiological limits is to make more, and to lose less, bone.