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

Abstract

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.

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

“…Both the average void size and number increased with age. This corresponds with the spatial and temporal distribution previously recorded for microcracks, dead osteocytes, and otosclerosis (6,7,14,22). The median cross-sectional area of cellular voids in female temporal bones was larger than in male temporal bones; however, the difference was not significant.…”

“…Stereology has been used to describe the distribution of living and dead osteocytes and microcracks around the inner ear (5,7). In this study, voids were measured and quantified using an unbiased point grid to estimate the volume fraction of cellular void volume per capsular bone volume (19,20).…”

“…Reference volumes of 200-μm-wide concentric zones around the inner ear previously calculated were used in this study (7). The volume fraction ( V v ) of void per 200-μm zone temporal bone volume was calculated using the following:…”

“…A potent antiresorptive osteoprotegerin (OPG) signal effectively suppresses remodeling in the otic capsule (1)(2)(3)(4). Consequently, dead osteocytes and fatigue microcracks accumulate around the inner ear (5)(6)(7). In otosclerosis, however, pathological remodeling occasionally forms despite this local inhibition.…”

“…Basic fuchsin bulk-stained undecalcified temporal bones have been used to document the accumulation of dead osteocytes and microfractures around the inner ear (5)(6)(7). Recently, the identification of cellular voids in undecalcified human temporal bones was validated (12) (Fig.…”