Estimation of volume referent bone turnover in the otic capsule after sequential point labeling

Signaling by way of type IB and II bone morphogenetic protein receptors regulates bone formation in otospongiosis

HYPOTHESIS

The main goal of this study was to perform an immunohistologic analysis of bone morphogenetic protein receptors (BMPR) in otospongiosis.

BACKGROUND

BMP-2, -4, and -7 play an essential role in bone formation and repair. They do so as well in otosclerosis. It has been shown that these BMPs are traceable in osteocytes and osteoclasts in the active phase of otosclerosis (otospongiosis). The role of the different BMP receptors in otosclerotic bone transformation has not been previously analyzed.

METHODS

The posterior parts of the stapes footplates, collected during partial stapedectomies in 35 patients with clinical otosclerosis, were analyzed for histologic otosclerotic lesions after hematoxylin staining. Immunohistochemical analysis was performed using polyclonal immunoglobulin G antibodies for BMPR-IA, -IB, and -II, as well as biotinylated secondary antibodies, avidin-biotin-peroxidase complex reaction, and alkaline phosphatase staining with nitroblue-tetrazolium-chloride.

RESULTS

Seventeen of 35 (49%) specimens contained otosclerosis, but only 5 of these exhibited an otospongiotic phase. The abundant osteoblasts and osteoclasts in these cases showed distinct immunochemical staining for BMP-2, -4, and -7. In two cases, there could also be found an immense positive staining for BMPR-IB and modest staining for BMPR-II, whereas BMPR-1A always remained negative.

CONCLUSION

It was demonstrated for the first time that in otospongiosis, the actions of the BMPs are mediated through BMPR-IB and BMPR-II. To determine this role in detail, further investigations, especially for the phosphorylated Smad proteins within the BMP dependent mediator cascade, will be necessary.

Effects of early auditory experience on the spoken language of deaf children at 3 years of age

OBJECTIVE

By age 3, typically developing children have achieved extensive vocabulary and syntax skills that facilitate both cognitive and social development. Substantial delays in spoken language acquisition have been documented for children with severe to profound deafness, even those with auditory oral training and early hearing aid use. This study documents the spoken language skills achieved by orally educated 3-yr-olds whose profound hearing loss was identified and hearing aids fitted between 1 and 30 mo of age and who received a cochlear implant between 12 and 38 mo of age. The purpose of the analysis was to examine the effects of age, duration, and type of early auditory experience on spoken language competence at age 3.5 yr.

DESIGN

The spoken language skills of 76 children who had used a cochlear implant for at least 7 mo were evaluated via standardized 30-minute language sample analysis, a parent-completed vocabulary checklist, and a teacher language-rating scale. The children were recruited from and enrolled in oral education programs or therapy practices across the United States. Inclusion criteria included presumed deaf since birth, English the primary language of the home, no other known conditions that interfere with speech/language development, enrolled in programs using oral education methods, and no known problems with the cochlear implant lasting more than 30 days.

RESULTS

Strong correlations were obtained among all language measures. Therefore, principal components analysis was used to derive a single Language Factor score for each child. A number of possible predictors of language outcome were examined, including age at identification and intervention with a hearing aid, duration of use of a hearing aid, pre-implant pure-tone average (PTA) threshold with a hearing aid, PTA threshold with a cochlear implant, and duration of use of a cochlear implant/age at implantation (the last two variables were practically identical because all children were tested between 40 and 44 mo of age). Examination of the independent influence of these predictors through multiple regression analysis revealed that pre-implant-aided PTA threshold and duration of cochlear implant use (i.e., age at implant) accounted for 58% of the variance in Language Factor scores. A significant negative coefficient associated with pre-implant-aided threshold indicated that children with poorer hearing before implantation exhibited poorer language skills at age 3.5 yr. Likewise, a strong positive coefficient associated with duration of implant use indicated that children who had used their implant for a longer period of time (i.e., who were implanted at an earlier age) exhibited better language at age 3.5 yr. Age at identification and amplification was unrelated to language outcome, as was aided threshold with the cochlear implant. A significant quadratic trend in the relation between duration of implant use and language score revealed a steady increase in language skill (at age 3.5 yr) for each additional month of use of a cochlear implant after the first 12 mo of implant use. The advantage to language of longer implant use became more pronounced over time.

CONCLUSIONS

Longer use of a cochlear implant in infancy and very early childhood dramatically affects the amount of spoken language exhibited by 3-yr-old, profoundly deaf children. In this sample, the amount of pre-implant intervention with a hearing aid was not related to language outcome at 3.5 yr of age. Rather, it was cochlear implantation at a younger age that served to promote spoken language competence. The previously identified language-facilitating factors of early identification of hearing impairment and early educational intervention may not be sufficient for optimizing spoken language of profoundly deaf children unless it leads to early cochlear implantation.

Current research in otosclerosis

PURPOSE OF REVIEW

The aim of this article is to summarize and put into historical perspective current advances in research in otosclerosis, a disorder of the human temporal bone with a hereditary predisposition that is among the most common causes of acquired hearing loss.

RECENT FINDINGS

Genetic studies have revealed that otosclerosis is heterogeneous, with evidence for defects in at least seven genes associated with six distinct chromosomal loci. Measurements of high levels of osteoprotegerin expression in the normal otic capsule and soft tissues of the cochlea provide the first molecular insight as to why the normal otic capsule remodels minimally, if at all. Osteoprotegerin knockout mice provide the best available animal model to date to study abnormal otic capsule remodeling that closely resembles otosclerosis. There is mounting evidence that the measles virus plays an important role in pathogenesis of otosclerosis although the mechanisms by which the virus results in otosclerosis remain unknown. Quantitative measures of angiogenesis can reliably distinguish between clinical and histological otosclerosis. Advances in the emerging field of osteoimmunology will likely impact and benefit from the research in otosclerosis.

SUMMARY

Insights into molecular mechanisms that inhibit extensive remodeling in the normal otic capsule, and understanding of how these mechanisms are dysregulated in otosclerosis will allow future design of rational treatment strategies for otosclerosis.

Osteoprotegrin knockout mice demonstrate abnormal remodeling of the otic capsule and progressive hearing loss

OBJECTIVES

The otic capsule, when compared with other bones in the body, is unique in that it undergoes no significant remodeling of bone after development. We previously demonstrated that osteoprotegerin (OPG), which inhibits formation and function of osteoclasts, is produced at high levels in the inner ear of normal mice and secreted into the perilymph from where it diffuses into the surrounding otic capsule bone through a lacunocanalicular system. To test our hypothesis that the high level of OPG may be important in the inhibition of otic capsule remodeling, we studied the light microscopic histology of the otic capsule in OPG knockout mice for evidence of abnormal remodeling of bone. We also tested the hearing in OPG knockout mice to determine whether OPG and its influence on surrounding bone is important for auditory function.

METHODS

Temporal bone histopathology and pathophysiology were compared in homozygous OPG knockout mice and C57BL/6 (B6) mice, the background strain for the knockouts. Auditory function in age-matched animals from each group was evaluated at approximately 4-week intervals from 8 to 21 weeks using frequency-specific auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). After each of the last three evaluations, the cochleae from one mouse of each group were harvested, processed, and examined by light microscopy.

RESULTS

Osteoprotegerin knockout mice demonstrated abnormal remodeling of bone within the otic capsule with multiple foci showing osteoclastic bone resorption and formation of new bone. Such changes were not seen in the age-matched B6 controls. The active bone remodeling process in the knockout animals showed many similarities to otosclerosis seen in human temporal bones. Over the time period that we monitored, auditory function was significantly and progressively compromised in the knockout animals relative to B6 controls. At the earliest age of test (8 wk), the loss was apparent as a mild, high-frequency reduction in sensitivity by ABR. In contrast, DPOAE losses in the knockouts were substantial even at 8 weeks, and by 21 weeks, these losses exceeded our equipment limits. Results of ABR testing showed hearing sensitivity changes in the animals of the background strain were confined largely to the high frequencies, whereas OPG knockouts demonstrated substantial low-frequency shifts in addition to those at high frequencies.

CONCLUSIONS

The histopathological and pathophysiological findings in OPG knockout mice support the hypothesis that OPG is important in the inhibition of bone remodeling within the otic capsule and the maintenance of normal auditory function. This mouse may provide a valuable animal model of human otosclerosis.

Quantification of Angiogenesis in Otosclerosis

OBJECTIVES/HYPOTHESIS

The determinants of clinical versus histologic otosclerosis are unknown, but angiogenesis is associated with active disease. We hypothesized that quantification of angiogenesis in otosclerotic human temporal bones could reveal significant differences between clinical and histologic cases.

STUDY DESIGN

We reviewed all otosclerosis specimens meeting criteria from the temporal bone collection of the Massachusetts Eye and Ear Infirmary and 10 normal controls.

METHODS

Digital images were taken at predilection sites, followed by computer-assisted analysis. Canalicular area (CA), the aggregate of vascular spaces within bone, microvessel density (MVD), area, and depth were the main measures. Evidence of a direct connection between local vessels and the vasculature of the otosclerotic focus was also recorded for each specimen.

RESULTS

The average area (mm) and depth (number of sections containing otosclerosis) of clinical lesions was significantly greater than histologic lesions. Total microvessel counts were significantly greater in clinical versus histologic lesions, and both clinical and histologic lesions contained significantly greater numbers of microvessels than the normal otic capsule. CA was also significantly higher in clinical lesions. MVD was slightly but not significantly higher in clinical lesions. Importantly, a direct connection between named vessels and the otosclerotic vasculature was significantly more frequent in clinical lesions.

CONCLUSIONS

Computer-assisted quantification revealed significantly greater measures of angiogenesis in clinical versus histologic otosclerosis. Direct connection to adjacent vessels may support angiogenesis in this disease. Sustained angiogenesis may be an important determinant of clinical otosclerosis.

Osteoprotegerin in the Inner Ear May Inhibit Bone Remodeling in the Otic Capsule

OBJECTIVES

To elucidate factors that may be responsible for the inhibition of remodeling of bone within the otic capsule.

METHODS

Expression of osteoprotegerin (OPG), receptor activator of nuclear factor kappa B (RANK), and RANK ligand (RANKL) were assayed in samples of bone obtained from the otic capsule, calvarium, and femur, and from the soft tissue within the cochlea using semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) in mice. Immunostaining was used for histologic localization of the gene products. An enzyme-linked immunosorbent assay (ELISA) was used to quantify the amount of OPG within perilymph, serum, and cerebrospinal fluid. The micro-anatomy of the interface between the otic capsule and the fluid spaces of the cochlea was investigated by brightfield and phase-contrast microscopy and by three-dimensional reconstruction in the mouse and human.

RESULTS

OPG, a powerful inhibitor of bone remodeling, was expressed at extremely high levels within the soft tissue of the cochlea and was present in the perilymph at very high concentrations. The OPG produced within the inner ear may diffuse into the surrounding otic capsule, where it may be responsible for inhibition of bone turnover. Our anatomic studies revealed an extensive system of interconnected canaliculi within the otic capsule that had direct openings into the fluid spaces of the inner ear, thus providing a possible anatomic route for the diffusion of OPG from the inner ear into the surrounding bone.

CONCLUSION

OPG, a potent inhibitor of osteoclast formation and function, is expressed at high levels within the inner ear and is secreted into the perilymph and the surrounding bone and may serve to inhibit active bone remodeling within the otic capsule, especially immediately adjacent to the cochlea. By this means, the cochlear soft tissue may control the nature of the surrounding petrous bone.

Otosclerosis is a conformational disease

The otosclerotic focus is a result of the normal process of bone remodelling in the otic capsule - a place where there is little if any bone remodelling. There is still no single accepted theory for the pathogenesis of otosclerosis. Sources we used were relevant clinical and basic science publications from 1973 to 2001. The data from each publication were critically reviewed. In this paper we suggest a kind of molecular predisposition in otosclerosis, like molecular instability in some key protein with regulatory effect, cell receptor for example. Also it can be a destabilasing mutation of some repressor of the normal bone remodelling. Any triggering event like mechanic distress, vascular compromise, radiation, viral infection, etc. can start a process of conformational changes with subsequent induction of long-standing bone remodelling focuses. We conclude that many unusual features of otosclerosis can be explained if we consider this entity as a conformational disease.

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

Microfissures in the human otic capsule have been observed since the start of the century, but it was Otto Mayer, in 1930, who first realized that some of them were of intravital origin and not just processing artifacts. Since then, a small number of publications, based on decalcified temporal bones, have mostly confirmed his findings. With the introduction by Frost in the late 1950s of the undecalcified bone technique and the bulk staining technique for peripheral bones, a method was developed and refined for identifying even very small intravital microfissures (microdamage). Bulk staining of undecalcified otic capsules has not yet been used to verify the findings from the previous decalcified specimens. The present report presents our experience with the pertinent techniques, and suggests modifications and shortcuts pertinent to temporal bone research. Both large and tiny microfissures of intravital genesis are demonstrable within bulk-stained undecalcified human otic capsules. The importance of microfissures in the petrous bone in the causation of otosclerosis and perilymphatic leakage has long been discussed, and the present techniques may advance our understanding of these pathological conditions

Pathophysiology of otosclerosis

OBJECTIVE

To review current knowledge of the pathophysiology of otosclerosis and to review hypotheses for the amelioration of this disease.

DATA SOURCES

Review of the literature and experimental observations by the authors.

CONCLUSIONS

Otosclerosis is a localized disease of bone remodeling within the otic capsule of the human temporal bone. Unlike other similar bone diseases, it does not occur outside of the temporal bone. These lesions seem to begin by resorption of stable otic capsule bone in adults, followed by a reparative phase with bone deposition. There are clearly genetic factors that lead to this disease, but measles virus infection and autoimmunity also may play contributing roles. Surgical correction of the conductive hearing loss is highly effective, but nonsurgical intervention has not yet been shown to prevent or slow the disease. Of the factors that may inhibit this process, fluorides, cytokine inhibitors, and bisphosphonates, third-generation bisphosphonates appear to hold the most promise.

Predilection of otosclerotic foci related to the bone turnover in the otic capsule

Using multiple fluorochrome tagging of eight mongrel dogs and a newly established methodology of measuring bone turnover (BTO) in the otic capsule, it has previously been demonstrated that BTO in the canine otic capsule is highly reduced close to the perilymphatic spaces (PLS) compared to the normal level of BTO in the periphery. Reanalysis of these data shows that this inhibition of BTO is far more pronounced around the cochlea and vestibule than around the semicircular canals. Similar tendencies are seen for the numerical density and mean label area of the bone remodelling units. With increasing distance to the PLS, these patterns are weakened, but still recognizable. In otosclerosis, foci of abnormal bone deposition are particularly frequent around the oval and round windows and in the cochlear capsule, i.e. where inhibition of bone remodelling is most prominent. A surpassing (or failure) of this pronounced inhibition must precede the tumultuous but delimited osteogenesis of otosclerosis. Otosclerosis may be a deviation of the normal BTO process in an osteometabolically abnormal site, perhaps initiated as an osteogenetic response to abnormal stress exposure.