Morphometric investigation on osteocytes in human auditory ossicles

Characterizing bone density pattern and porosity in the human ossicular chain using synchrotron microtomography

The auditory ossicles amplify and transmit sound from the environment to the inner ear. The distribution of bone mineral density is crucial for the proper functioning of sound transmission as the ossicles are suspended in an air-filled chamber. However, little is known about the distribution of bone mineral density along the human ossicular chain and within individual ossicles. To investigate this, we analyzed fresh-frozen human specimens using synchrotron-based phase-contrast microtomography. In addition, we analyzed the volume and porosity of the ossicles. The porosity for the auditory ossicles lies, on average, between 1.92% and 9.85%. The average volume for the mallei is 13.85 ± 2.15 mm3, for the incudes 17.62 ± 4.05 mm3 and 1.24 ± 0.29 mm3 for the stapedes. The bone density distribution showed a similar pattern through all samples. In particular, we found high bone mineralization spots on the anterior crus of the stapes, its footplate, and along areas that are crucial for the transmission of sound. We could also see a correlation between low bone mineral density and holey areas where the bone is only very thin or missing. Our study identified a similar pattern of bone density distribution within all samples: regions exposed to lower forces generally show higher bone density. Further, we observed that the stapes shows high bone mineral density along the anterior crus and its footplate, which may indicate its importance in transmitting sound waves to the inner ear.

Isolated Fracture of the Malleus: An Overlooked Cause of Conductive Hearing Loss?

OBJECTIVE

To report the largest case series of isolated malleus fractures with systematic review to characterize the disease's presentation and natural history, and provide suggestions for management.

DATA SOURCES

PubMed, Embase, Cochrane Library.

REVIEW METHODS

Retrospective cohort study was performed on 12 patients with isolated malleus fractures. History, physical exam, pre- and post-treatment audiograms, and imaging were obtained. Systematic review of the literature was performed.

RESULTS

Including the cases herein, 58 isolated malleus fractures were identified, the majority of which were published in the 21st century. Mean time to presentation after injury was 34.4 months. Most common etiology was external auditory canal (EAC) manipulation. Physical exam and imaging did not identify any abnormality at presentation in 16% and 21% of cases, respectively. The majority of fractures involved the manubrium. Air-bone gap (ABG) at initial presentation ranged from 16 to 26 dB, and was greater at higher frequencies. Thirty-six cases underwent surgery. ABG improvement was greater at all frequencies for those who underwent surgery. Final ABG was significantly less than initial ABG at nearly every frequency for those who underwent surgery (p < 0.05), while not at any frequency for those who were observed.

CONCLUSIONS

Isolated malleus fractures may occur more often than historical data suggests, and are perhaps underdiagnosed. Abrupt removal of a finger from the EAC with pain and hearing loss is nearly pathognomonic. Conductive hearing loss with ABG greater at higher frequencies is most often observed. Observation is unlikely to produce spontaneous improvements in hearing, while surgery demonstrates reliable decreases in ABG. Laryngoscope, 134:1032-1041, 2024.

Micropetrosis: Osteocyte Lacunar Mineralization in Aging and Disease

PURPOSE OF REVIEW

As the importance of osteocytes for bone mineral homeostasis is increasingly recognized, there is growing interest in osteocyte cell death as a relevant indicator in various physiological and pathological conditions. Micropetrosis is an established term used to describe osteocyte lacunae that are filled with minerals following osteocyte death. While the early reports of micropetrosis were purely descriptive, there is now an increasing body of literature showing quantitative data on micropetrosis in various conditions such as aging, osteoporosis, immobilization, and diabetes, and in osteoporosis treatment (denosumab and bisphosphonates). This review summarizes quantitative findings on micropetrosis, with a particular emphasis on the recent advances in the field.

RECENT FINDINGS

There is growing evidence that micropetrosis is more common in older, osteoporotic, and immobilized individuals, as well as in individuals with type 1 or type 2 diabetes. Denosumab and bisphosphonates seem to affect lacunar mineralization differently, where specifically bisphosphonates have been shown to prolong osteocyte viability and reduce micropetrosis. Despite continuous proceedings in the field of osteocyte-lacunar-network characteristics, more studies are necessary to further clarify the mechanisms of lacunar mineralization, the inter-site variability of micropetrosis accumulation, the relevance of micropetrosis in various diseases and conditions, and whether micropetrosis could be an indicator of bone fragility or a target for treatment.

Mammalian middle ear mechanics: A review

The middle ear is part of the ear in all terrestrial vertebrates. It provides an interface between two media, air and fluid. How does it work? In mammals, the middle ear is traditionally described as increasing gain due to Helmholtz's hydraulic analogy and the lever action of the malleus-incus complex: in effect, an impedance transformer. The conical shape of the eardrum and a frequency-dependent synovial joint function for the ossicles suggest a greater complexity of function than the traditional view. Here we review acoustico-mechanical measurements of middle ear function and the development of middle ear models based on these measurements. We observe that an impedance-matching mechanism (reducing reflection) rather than an impedance transformer (providing gain) best explains experimental findings. We conclude by considering some outstanding questions about middle ear function, recognizing that we are still learning how the middle ear works.

Human auditory ossicles as an alternative optimal source of ancient DNA

DNA recovery from ancient human remains has revolutionized our ability to reconstruct the genetic landscape of the past. Ancient DNA research has benefited from the identification of skeletal elements, such as the cochlear part of the osseous inner ear, that provides optimal contexts for DNA preservation; however, the rich genetic information obtained from the cochlea must be counterbalanced against the loss of morphological information caused by its sampling. Motivated by similarities in developmental processes and histological properties between the cochlea and auditory ossicles, we evaluate the ossicles as an alternative source of ancient DNA. We show that ossicles perform comparably to the cochlea in terms of DNA recovery, finding no substantial reduction in data quantity and minimal differences in data quality across preservation conditions. Ossicles can be sampled from intact skulls or disarticulated petrous bones without damage to surrounding bone, and we argue that they should be used when available to reduce damage to human remains. Our results identify another optimal skeletal element for ancient DNA analysis and add to a growing toolkit of sampling methods that help to better preserve skeletal remains for future research while maximizing the likelihood that ancient DNA analysis will produce useable results.

Human auditory ossicles as an alternative optimal source of ancient DNA

DNA recovery from ancient human remains has revolutionized our ability to reconstruct the genetic landscape of the past. Ancient DNA research has benefited from the identification of skeletal elements, such as the cochlear part of the osseous inner ear, that provides optimal contexts for DNA preservation; however, the rich genetic information obtained from the cochlea must be counterbalanced against the loss of morphological information caused by its sampling. Motivated by similarities in developmental processes and histological properties between the cochlea and auditory ossicles, we evaluate the ossicles as an alternative source of ancient DNA. We show that ossicles perform comparably to the cochlea in terms of DNA recovery, finding no substantial reduction in data quantity and minimal differences in data quality across preservation conditions. Ossicles can be sampled from intact skulls or disarticulated petrous bones without damage to surrounding bone, and we argue that they should be used when available to reduce damage to human remains. Our results identify another optimal skeletal element for ancient DNA analysis and add to a growing toolkit of sampling methods that help to better preserve skeletal remains for future research while maximizing the likelihood that ancient DNA analysis will produce useable results.

Angles of Axes of Incudes

Incus angles of axes (the angle between "short process axis" and the "long process axis") are more open in humans than chimpanzees: 64.0 versus 55.7 degrees (Quam et al.: J Anat 225 (2014) 167-196). However, Flohr et al. (Anat Rec 293 (2010) 2094-2106) raise concern about interobserver agreement of the axes. The concern is important as phylogenetic relationships of mammals are inferred from the incus (and malleus and stapes). We sought to check (1) interobserver agreement; and (2) if the angles of the axes of incudes (incuses) exhibit bilateral symmetry, which is expected if the axes are genetically determined. We studied incudes from 41 modern adult crania with clinically normal temporal bones. Angles of axes were determined on rectilinear digital photographs of incudes in standard lateral orientation. Two observers independently drew the axes and measured the axes. Interobserver agreement was within 4 degrees for 24 of 34 left-sided incudes and for 27 of 35 right-sided incudes. The mean of the two observers' angle determinations were used. Left incudes' median was 67 degrees, range 60-73; right 67.5 degrees, range 58-77. Bilateral symmetry of angles of axes was found: r = 0.55, N = 31, 95% CI 0.24-0.75. Angles of axes of modern human incudes are probably genetically determined features, but are of doubtful physiologic or evolutionary advantage in modern humans. Interobserver agreement of angles of modern human axes is concerning and must be specified in reports. Consideration should be given to a convention to designate axes in ambiguous cases. Anat Rec, 302:1615-1619, 2019. © 2019 American Association for Anatomy.

Early bone tissue aging in human auditory ossicles is accompanied by excessive hypermineralization, osteocyte death and micropetrosis

Within the mineralized bone, osteocytes form a multifunctional mechanosensitive network orchestrating bone remodelling. A preserved osteocyte population is a crucial determinant of bone quality. In human auditory ossicles, the early decrease in osteocyte numbers but maintained integrity remains an unexplained phenomenon that might serve for sound transmission from air to the labyrinth. Here we analysed the frequency, size and composition of osteocyte lacunae in the auditory ossicles of 22 individuals from early postnatal period to old age. Mineralization of the bone matrix was determined using backscattered electron imaging. No signs of bone remodelling were observed above the age of 1 year. We detected characteristics of early bone tissue aging, such as decrease in osteocytes, lower total lacunar density and lacunar area, as well as high matrix mineralization accompanied by distinct accumulation of micropetrotic lacunae and decreased indentation depths. The majority of these changes took place in the first months and years of life, while afterwards only minor reorganization was present. With osteocyte apoptosis potentially being a consequence of low mechanical stimuli, the early loss of osteocytes without initiation of bone remodelling indicates an adaptive response conserving the architecture of the auditory ossicles and ensuring stable sound transmission throughout life.

Systematic Review of Ossicular Chain Anatomy: Strategic Planning for Development of Novel Middle Ear Prostheses

Objective

To systematically review the anatomy of the ossicular chain.

Data Sources

Google Scholar, PubMed, and otologic textbooks.

Review Methods

A systematic literature search was performed on January 26, 2015. Search terms used to discover articles consisted of combinations of 2 keywords. One keyword from both groups was used: [ ossicular, ossicle, malleus, incus, stapes] and [ morphology, morphometric, anatomy, variation, physiology], yielding more than 50,000 hits. Articles were then screened by title and abstract if they did not contain information relevant to human ossicular chain anatomy. In addition to this search, references of selected articles were studied as well as suggested relevant articles from publication databases. Standard otologic textbooks were screened using the search criteria.

Results

Thirty-three sources were selected for use in this review. From these studies, data on the composition, physiology, morphology, and morphometrics were acquired. In addition, any correlations or lack of correlations between features of the ossicular chain and other features of the ossicular chain or patient were noted, with bilateral symmetry between ossicles being the only important correlation reported.

Conclusion

There was significant variation in all dimensions of each ossicle between individuals, given that degree of variation, custom fitting, or custom manufacturing of prostheses for each patient could optimize prosthesis fit. From published data, an accurate 3-dimensional model of the malleus, incus, and stapes can be created, which can then be further modified for each patient’s individual anatomy.

Characterization of primary osteocyte-like cells from rat mandibles

OBJECTIVE

The mandible is continuously undergoing remodeling as a result of mechanobiologic factors, such as chewing forces, tooth loss, orthodontic forces, and periodontitis. The effects of mechanical stress and biologic signals in bone homeostasis have been the focus of many investigations. However, much of this research utilized osteocytes derived from long bones, but little is known about the mandible-derived osteocytes. This study tests a protocol to isolate and grow osteocytes from rat mandible.

STUDY DESIGN

Rat mandibles were harvested, sectioned into small pieces, and subjected to a sequence chemical treatment and enzymatic digestion. The treated tissues were cultured for a few weeks while cells emerged. Cells were sorted by using the osteocyte marker podoplanin, an early marker for osteocyte differentiation. The cells were then characterized according to morphology, biochemical markers (osteocalcin, podoplanin, and sclerostin), and alkaline phosphatase activity and compared with an isotype cell line MLO-Y4 cells.

RESULTS

The mandibular osteocytic cells had stellate shape and were positive for osteocalcin, podoplanin, and sclerostin and lower alkaline phosphatase activity compared with MLO-Y4 osteocyte-like cells.

CONCLUSIONS

The protocol to isolate osteocyte-like cells will allow the investigators to investigate the mechanobiologic differences in biomechanical response between these mandibular and long bone osteocyte-like cells under various conditions.

Bone quality and biomechanical function: a lesson from human ossicles

In humans, the middle ear contains a chain of three ossicles with a major highly specific mechanical property (transmission of vibrations) and modeling that stops rapidly after birth. Their bone quality has been rarely studied either in noninflammatory ossicles or in those from ears with chronic inflammation. Our primary goal was to assess bone microarchitecture, morphology and variables reflecting bone quality from incuses, in comparison with those from human femoral cortical bone as controls. Secondly, the impact of chronic inflammation on quality of ossicles was documented. The study was performed on 15 noninflammatory incuses from 15 patients (35±32 years, range: 2-91). Comparisons were performed with 13 inflammatory incuses from 13 patients (55±20 years, range: 1-79) with chronic inflammation of the middle ear, essentially cholesteatoma. Microarchitecture and bone mineral density (BMD) were assessed by microcomputed tomography. Microhardness was measured by microindentation. Mineral and organic characteristics were investigated by Fourier transform infrared microspectroscopy. Noninflammatory incuses were composed of a compact, well mineralized bone without bone marrow and with sparse vessels. Remodeling activity was rarely observed. Woven or lamellar textures and numerous osteocytes were observed. In inflammatory incuses, architecture was degraded, organic tissue was abundant and bone cavities contained fibrocellular tissue and adipocytes. BMD of noninflammatory incuses was significantly higher than BMD from both control bones (4 embedded cortical femoral bone samples; age: 72±15 years, range: 50-85) and inflammatory incuses. Noninflammatory incuses were less hard than both control bone (8 cortical femoral bone samples; age: 49±18 years, range: 24-74) and inflammatory incuses. All incuses were more mineralized and less mature than controls. In conclusion, bone quality of incuses (dense, well mineralized, hard) is well adapted to their function of sound transmission. In inflammatory condition, incuses were degraded, thus explaining the decline of hearing. Moreover, microhardness was found higher than in noninflammatory incuses. Compared to bone with remodeling, the mineralization index in incuses does not explain variation of microhardness. Interestingly, a linear multiple regression model indicated that a combination of two variables, i.e., crystallinity index (crystal size/perfection) and carbonation (incorporation of carbonate ions in apatite) explains 26% of the increase in microhardness variability. Because the low remodeling level of ossicles could not prevent the reversibility of their degradation which impacts audition quality, an early management of ear inflammation in chronic otitis is recommended.

A review of osteocyte function and the emerging importance of sclerostin

➤ Osteocytes, derived from osteoblasts, reside within bone and communicate extensively with other bone cell populations to regulate bone metabolism. The mature osteocyte expresses the protein sclerostin, a negative regulator of bone mass.➤ In normal physiologic states, the protein sclerostin acts on osteoblasts at the surface of bone and is differentially expressed in response to mechanical loading, inflammatory molecules such as prostaglandin E2, and hormones such as parathyroid hormone and estrogen.➤ Pathologically, sclerostin dysregulation has been observed in osteoporosis-related fractures, failure of implant osseous integration, metastatic bone disease, and select genetic diseases of bone mass.➤ An antibody that targets sclerostin, decreasing endogenous levels of sclerostin while increasing bone mineral density, is currently in phase-III clinical trials.➤ The osteocyte has emerged as a versatile, indispensable bone cell. Its location within bone, extensive dendritic network, and close communication with systemic circulation and other bone cells produce many opportunities to treat a variety of orthopaedic conditions.

Evolution of the auditory ossicles in extant hominids: metric variation in African apes and humans

The auditory ossicles in primates have proven to be a reliable source of phylogenetic information. Nevertheless, to date, very little data have been published on the metric dimensions of the ear ossicles in African apes and humans. The present study relies on the largest samples of African ape ear ossicles studied to date to address questions of taxonomic differences and the evolutionary transformation of the ossicles in gorillas, chimpanzees and humans. Both African ape taxa show a malleus that is characterized by a long and slender manubrium and relatively short corpus, whereas humans show the opposite constellation of a short and thick manubrium and relatively long corpus. These changes in the manubrium are plausibly linked with changes in the size of the tympanic membrane. The main difference between the incus in African apes and humans seems to be related to changes in the functional length. Compared with chimpanzees, human incudes are larger in nearly all dimensions, except articular facet height, and show a more open angle between the axes. The gorilla incus resembles humans more closely in its metric dimensions, including functional length, perhaps as a result of the dramatically larger body size compared with chimpanzees. The differences between the stapedes of humans and African apes are primarily size-related, with humans being larger in nearly all dimensions. Nevertheless, some distinctions between the African apes were found in the obturator foramen and head height. Although correlations between metric variables in different ossicles were generally lower than those between variables in the same bone, variables of the malleus/incus complex appear to be more strongly correlated than those of the incus/stapes complex, perhaps reflecting the different embryological and evolutionary origins of the ossicles. The middle ear lever ratio for the African apes is similar to other haplorhines, but humans show the lowest lever ratio within primates. Very low levels of sexual dimorphism were found in the ossicles within each taxon, but some relationship with body size and several dimensions of the ear bones was found. Several of the metric distinctions in the incus and stapes imply a slightly different articulation of the ossicular chain within the tympanic cavity in African apes compared with humans. The limited auditory implications of these metric differences in the ossicles are also discussed. Finally, the results of this study suggest that several plesiomorphic features for apes may be retained in the ear bones of the early hominin taxa Australopithecus and Paranthropus as well as in the Neandertals.

Basic biology of skeletal aging: role of stress response pathways

Although a decline in bone formation and loss of bone mass are common features of human aging, the molecular mechanisms mediating these effects have remained unclear. Evidence from pharmacological and genetic studies in mice has provided support for a deleterious effect of oxidative stress in bone and has strengthened the idea that an increase in reactive oxygen species (ROS) with advancing age represents a pathophysiological mechanism underlying age-related bone loss. Mesenchymal stem cells and osteocytes are long-lived cells and, therefore, are more susceptible than other types of bone cells to the molecular changes caused by aging, including increased levels of ROS and decreased autophagy. However, short-lived cells like osteoblast progenitors and mature osteoblasts and osteoclasts are also affected by the altered aged environment characterized by lower levels of sex steroids, increased endogenous glucocorticoids, and higher oxidized lipids. This article reviews current knowledge on the effects of the aging process on bone, with particular emphasis on the role of ROS and autophagy in cells of the osteoblast lineage in mice.

Early hominin auditory ossicles from South Africa

The middle ear ossicles are only rarely preserved in fossil hominins. Here, we report the discovery of a complete ossicular chain (malleus, incus, and stapes) of Paranthropus robustus as well as additional ear ossicles from Australopithecus africanus. The malleus in both early hominin taxa is clearly human-like in the proportions of the manubrium and corpus, whereas the incus and stapes resemble African and Asian great apes more closely. A deep phylogenetic origin is proposed for the derived malleus morphology, and this may represent one of the earliest human-like features to appear in the fossil record. The anatomical differences found in the early hominin incus and stapes, along with other aspects of the outer, middle, and inner ear, are consistent with the suggestion of different auditory capacities in these early hominin taxa compared with modern humans.

Osteocyte apoptosis and absence of bone remodeling in human auditory ossicles and scleral ossicles of lower vertebrates: a mere coincidence or linked processes?

Considering the pivotal role as bone mechanosensors ascribed to osteocytes in bone adaptation to mechanical strains, the present study analyzed whether a correlation exists between osteocyte apoptosis and bone remodeling in peculiar bones, such as human auditory ossicles and scleral ossicles of lower vertebrates, which have been shown to undergo substantial osteocyte death and trivial or no bone turnover after cessation of growth. The investigation was performed with a morphological approach under LM (by means of an in situ end-labeling technique) and TEM. The results show that a large amount of osteocyte apoptosis takes place in both auditory and scleral ossicles after they reach their final size. Additionally, no morphological signs of bone remodeling were observed. These facts suggest that (1) bone remodeling is not necessarily triggered by osteocyte death, at least in these ossicles, and (2) bone remodeling does not need to mechanically adapt auditory and scleral ossicles since they appear to be continuously submitted to stereotyped stresses and strains; on the contrary, during the resorption phase, bone remodeling might severely impair the mechanical resistance of extremely small bony segments. Thus, osteocyte apoptosis could represent a programmed process devoted to make stable, when needed, bone structure and mechanical resistance.

Avascular necrosis associated with fracture of the femoral neck after hip resurfacing: histological assessment of femoral bone from retrieval specimens

The cause of fracture of the femoral neck after hip resurfacing is poorly understood. In order to evaluate the role of avascular necrosis we compared 19 femoral heads retrieved at revision for fracture of the femoral neck and 13 retrieved for other reasons. We developed a new technique of assessing avascular necrosis in the femoral head by determining the percentage of empty osteocyte lacunae present. Femoral heads retrieved as controls at total hip replacement for osteoarthritis and avascular necrosis had 9% (sd 4; n = 13) and 85% (sd 5; n = 10, p < 0.001) empty lacunae, respectively. In the fracture group the percentage of empty lacunae was 71% (sd 22); in the other group it was 21% (sd 13). The differences between the groups were highly significant (p < 0.001). We conclude that fracture after resurfacing of the hip is associated with a significantly greater percentage of empty osteocyte lacunae within the trabecular bone. This indicates established avascular necrosis and suggests that damage to the blood supply at the time of surgery is a potent risk factor for fracture of the femoral neck after hip resurfacing.

Contribution to morphological knowledge of the development of the human incudo-mallear joint

CONCLUSION

At the time of birth, the incudo-mallear joint is completely developed. OBJECTIVE. To study the development of the incudo-mallear joint in human embryos and fetuses. MATERIALS AND METHODS. In all, 46 temporal bones with ages between 9 mm and newborns were studied. The preparations were cut in a series and dyed using Martins' trichrome technique. RESULTS. The incudo-mallear joint acquires the characteristics of a saddle joint at 10 weeks of development. The cartilage that covers the articular surfaces is formed by different strata that develop successively: the superficial stratum at 14 weeks, the transitional between 15 and 19 weeks, and the radial from 20 weeks. The subchondral bone develops between weeks 25 and 28 by the mechanisms of apposition and extension of the periosteal and endosteal bones, but it is not until week 30 that it completely covers the articular surfaces, consisting of bone fascicles whereby the lines of force will be transmitted. The articular capsule is formed as from the inter-zone. The surface zone develops the capsular ligament, and the internal surface develops the synovial membrane. Even though it is not consistent, the primordium of the meniscus is seen at 18 weeks.

Anatomy of the distal incus in humans

The anatomy of the distal incus, including the lenticular process, was examined in histological sections from 270 normal cadaveric human temporal bones aged between less than 1 month and 100 years. All but nine of these sectioned specimens showed signs of a bony connection between the long process of the incus and the flattened plate of the lenticular process, and in 108 specimens a complete bony attachment was observed in a single 20 microm section. In these 108 ears, the bony lenticular process consisted of a proximal narrow "pedicle" connected to a distal flattened "plate" that forms the incudal component of the incudo-stapedial joint. A fibrous joint capsule extended from the stapes head to the pedicle of the lenticular process on all sides, where it was considerably thickened. Three-dimensional reconstructions made from serial 20 microm sections of four bones provided views from all directions that easily convey the anatomical features of this region. Morphometric measurements of the bony architecture of the distal incus in 103 temporal bones were made, including lengths and cross-sectional areas, estimates of the percentage of lacunae containing osteocytes, and the degree of bone resorption. These measurements, analyzed as a function of age, provided an anatomic description over a large age range that can serve as a normal baseline against which structural pathology can be compared. Although none of the bony dimensions showed significant age dependence, the estimated percentage of bony lacunae that contain osteocytes decreased significantly with age. The results have implications for the roles of specific components on the coupling of ossicular motion across the incudo-stapedial joint, and provide insights regarding bone resorption at the level of the distal incus, which occurs clinically in some patients with chronic otitis media or after stapedectomy.

Apoptotic bodies convey activity capable of initiating osteoclastogenesis and localized bone destruction

INTRODUCTION

Osteocyte apoptosis co-localizes with sites of osteoclastic bone resorption in vivo, but to date, no causal molecular or signaling link has been identified between these two processes.

MATERIALS AND METHODS

Osteocyte apoptotic bodies (OABs) derived from the MLO-Y4 osteocyte-like cell line and primary murine osteocytes and apoptotic bodies (ABs) derived from primary murine osteoblasts were introduced onto the right parietal bone of murine calvariae, and osteoclastic bone resorption was examined 5 days after treatment. In addition, the ability of primary murine and cell line-derived OABs to support osteoclastogenesis was examined in vitro in co-culture with murine bone marrow hematopoietic progenitors in the absence of RANKL or macrophage-colony stimulating factor.

RESULTS

For the first time, we show that OABs are capable of initiating de novo osteoclastic bone resorption on quiescent bone surfaces in vivo. Furthermore, the addition of OABs to mononuclear osteoclast precursors (OPs) in vitro resulted in the maintenance of OP cell numbers and an increase in the proportion and activity of TRACP(+) cells. In contrast, application of ABs from osteoblasts showed no osteoclastogenic activity either in vivo or in vitro. The osteoclastogenic capacity of OABs was shown to be independent of the known osteoclastogenic factor RANKL but dependent on the induction of TNF-alpha production by OP.

CONCLUSIONS

These data point to a mechanism by which dying osteocytes might target bone destruction through the distribution of OAB-associated signals and give further physiological meaning to the apoptotic process in bone.

Scanning electron microscopic study of the human auditory ossicles

The human mallei, incudes and stapedes from 34 cadavers were examined using scanning electron microscope (SEM) to compare the bone surface type among different regions of auditory ossicles for males and females. On the malleus of both males and females, almost all of the surfaces showed a smooth fibrous appearance, characteristic of resting surface. Limited bone-forming or resorbing surfaces were identified on the malleus. As compared with the malleus, the percentage area of the resorbing surface and the vascular canal openings were higher on the incus and stapes, especially on the long process (Crus longum) of the incus and the neck of the stapes for both males and females. The percentage area occupied by the resorbing surface of the long process of the incus and the neck of the stapes correlated with that of the vascular canal openings. We consider that the malleus maintained the stable condition, while the long process of the incus and the neck of the stapes demonstrated marked bone resorption. We suppose that the bone erosion may be related to the vascularization in these regions. Though the percentage area of the resorbing surface and the vascular canal openings had the tendency to be high in females, we did not find any significant differences between the males and females. There was no significant correlation between the age and the area of resorbing surface or vascular canal openings.

Selective estrogen receptor modulator inhibits osteocyte apoptosis during abrupt estrogen withdrawal: implications for bone quality maintenance

Estrogens exert positive effects on the quantity and quality of bone, including the maintenance of osteocytes through the inhibition of their apoptosis. Ideally, selective estrogen receptor modulators (SERMs) confer all of the positive bone-associated effects of estrogens without any adverse effects. In a similar way to estrogen, the raloxifene analog LY 117018 has been shown to prevent bone loss in ovariectomized (OVX) rats. In this study, we investigated whether the osteocyte-sparing effect of 17beta-estradiol can be mimicked by the SERM LY 117018 in a rat model of OVX. Twenty-four juvenile female rats were divided into four treatment groups: sham-operated (SHAM), OVX, OVX + 17beta-estradiol (OVX+E(2)), and OVX + LY 117018 (OVX+SERM). At 7 or 14 days following the start of treatment, the radius and ulna were removed. The percentage of apoptotic osteocytes, determined using an in situ nick-translation method, was increased (2.5-fold at 7 days and sixfold at 14 days) in the OVX group compared with SHAM in both the radius and ulna. Treatment of OVX animals with either 17beta-estradiol at a dose rate of 0.125 mg/kg/day or LY 117018 at a dose rate of 3 mg/kg/day prevented these increases in osteocyte apoptosis similarly. These observations demonstrate that LY 117018 exerts a powerful inhibitory effect upon osteocyte apoptosis directly after estrogen loss, in a similar way to the known effect of 17beta-estradiol replacement. These results point to the potential benefits of SERMs on both the quantity and quality of bone in E(2)-depleted rats.

The biology of osteocytes

Osteocytes, the most abundant cell type in bone, remain the least characterized. Several theories have been proposed regarding their function, including osteolysis, sensing the strains produced in response to mechanical loading of bones, and producing signals that affect the function of osteoblasts and osteoclasts and hence, bone turnover. This review also discusses the role of osteocyte apoptosis in targeted bone remodeling and proposes that the occurrence of osteocyte apoptosis is consistent with the description of apoptosis as an essential homeostatic mechanism for the healthy maintenance of tissues.

The antioxidant effect of estrogen and Selective Estrogen Receptor Modulators in the inhibition of osteocyte apoptosis in vitro

Withdrawal of estrogen represents the primary factor determining post menopausal bone loss and has been associated with negative indicators of bone quality including the apoptotic death of osteocytes in vivo. While hormone replacement therapy in the form of Estrogen or Selective Estrogen Receptor Modulators (SERMs) demonstrates clear estrogen receptor (ER)-mediated benefits to bone mass, less is known regarding the mechanism of action of these compounds in the maintenance of bone cell populations. We have investigated the potential antioxidant effects of estrogen, estrogen derivatives and the SERMs Raloxifene and LY117018 in the prevention of oxidative stress induced apoptosis in the osteocyte like cell line MLO-Y4. Treatment of MLO-Y4 with 0.3 mM H(2)O(2) induced apoptosis that was significantly inhibited (p< or =0.002) when the cells were pre-treated for 1 h with either 17beta-estradiol, Raloxifene or LY117018 (10 nM). The stereoisomer 17alpha-estradiol also prevented H(2)O(2) induced apoptosis in MLO-Y4. Importantly, pre-treatment of ER-negative HEK293 cells with either 1 microM, 100 nM or 10 nM 17beta-estradiol, Raloxifene or LY117018 significantly inhibited H(2)O(2) induced apoptosis in these cells (p< or =4.2x10(-5)) indicating an estrogen receptor-independent effect of these compounds. Comparisons of 17beta-estradiol and similar molecules containing the putative free radical scavenger C3-OH moiety on the steroid A-ring (17alpha-estradiol, 17alpha-ethinylestradiol; 10 nM) with structurally related molecules lacking the C3-OH grouping (Mestranol and Quinestrol; 10 nM) demonstrated that only compounds containing the C3-OH moiety showed anti-apoptotic behavior in these studies (p< or =0.0033). Similarly the identification of the presence of reactive oxygen species (ROS) in cells as evidenced by the free radical indicator 2'7'-dichlorodihydrofluorescein diacetate demonstrated that 17beta-estradiol, SERMs and related molecules with C3-OH moiety were capable of blocking ROS generated in cells by H(2)O(2) (p< or =0.002) while Mestranol and Quinestrol showed no such blockade. It is possible that the loss of osteocytes during estrogen insufficiency may occur through a failure to suppress the activity of naturally occurring or disease associated oxidant molecules. These data suggest that the osteocyte protective effects of estrogen and SERMs may operate through a common receptor-independent mechanism which may be related to the antioxidant activity of these molecules.

Differences in osteocyte and lacunar density between Black and White American women

We examined the differences in osteocyte and lacunar density between Black and White women, using previously obtained iliac bone biopsies from 34 healthy Black women, aged 21-70 years, and 94 White women, aged 20-73 years. For each subject, the density of osteocytes (Ot.N/B.Ar), empty lacunae (EL.N/B.Ar), and total lacunae (Tt.L.N/B.Ar) and the proportion of osteocyte-occupied lacunae (Ot.N/Tt.L.N) were separately measured in whole trabeculae, superficial bone (<25 microm from the bone surface), and deep bone (>45 microm from the bone surface). Compared with White women, Black women had higher values for osteocytes, empty lacunae, and total lacunae and lower values for percent occupied lacunae in superficial bone and whole trabeculae (P < 0.01 to <0.001). In deep bone there were more osteocytes and total lacunae in Black women, but the other measurements did not differ significantly between the two groups. As in White women, there were fewer osteocytes and total lacunae and more empty lacunae in deep than in superficial bone. The regressions of osteocyte and total lacunar density on age were not significant in Black women, but postmenopausal Black women had fewer osteocytes than premenopausal Black women, and percent occupied lacunae declined significantly with age in whole trabeculae and deep bone, which could only have resulted from osteocyte death. In contrast to White women, there was no inverse relationship between bone formation rate and osteocyte density in superficial bone and the observed bone formation rate was lower than predicted by osteocyte density. We conclude the following: (1) Cancellous bone is made with more osteocytes in Black than in White women, most likely because of diminished apoptosis of osteoblasts; this could contribute to increased bone strength in Black women. (2) In Black women, as in White women, there are fewer osteocytes and total lacunae and more empty lacunae in deep than in superficial bone. (3) There was moderate age-related loss of osteocytes in deep bone in Black women, indicating that osteocyte density depends more on the age of the bone than on the age of the subject. (4) The higher osteocyte density in Black women was not responsible for their lower bone formation rate.

Microdamage and apoptosis

Microdamage of healthy bone leads to targeted removal and repair of the damage. This process must involve the production of specific targeting signals. The identity of these signals is unknown but constitutes a legitimate research goal since it is this targeting process which appears to become impaired in ageing and disease. Here we discuss the potential role of the matrix bound osteocyte in the sensing and targeting of microdamage. In particular we will review current understanding concerning the apoptotic death of osteocytes at sites of microdamage and discuss the potential physiological significance of these findings in the light of knowledge of the significance of apoptosis in other cell systems.

Isolated primary osteocytes express functional gap junctions in vitro

The osteocyte is the most abundant cell type in bone and is embedded in mineralized bone matrix. Osteocytes are still poorly characterized because of their location and the lack of primary osteocyte isolation methods. Data on the cell biology of osteocytes is especially limited. We have isolated primary osteocytes from rat cortical bone by applying repeated enzymatic digestion and decalcification. The isolated osteocytes expressed typical osteocytic morphology with cell-cell contacts via long protrusions after a 1-day culture. These cells were negative or faintly positive for alkaline phosphatase but expressed high levels of osteocalcin, PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome), and DMP1 (dentin matrix protein 1). These cells also revealed patchy membrane staining for connexin43. For studying the function of gap junctions in isolated osteocytes, we microinjected rhodamine-labeled dextran (MW: 10,000) and Lucifer yellow (MW: 457) and found that Lucifer yellow was rapidly transmitted to several surrounding cells, whereas dextran remained in the injected cells. Heptanol and 18alpha-glycyrrhetinic acid inhibited the transfer of Lucifer yellow. This clearly showed the existence of functional gap junctions in cultured osteocytes. Enveloped viruses, such as vesicular stomatitis virus and influenza A virus, were used for studying cell polarity. We were unable to demonstrate plasma membrane polarization with enveloped viruses in isolated primary osteocytes in culture. Our results suggest that osteocytes do not possess apical and basolateral plasma membrane domains as do osteoblasts, which are their precursors.

Theoretical and numerical study of a bone remodeling model: The effect of osteocyte cells distribution

It is well argued that osteocytes are mechanosensory cells and are involved in the regulation of bone remodeling. In previous works, the predictions from a simulation model have suggested that both the influencing distance of osteocytes and the magnitude of the mechanical loads determine the thickness of trabeculae whereas the number of osteocytes primarily affects the rate of bone remodeling. The question that remains not completely answered is: for the same number of osteocytes, what is the effect of different distributions on the remodeling process? Based on a particular regulatory bone remodeling model, the question is addressed, in part, by performing a stability analysis in connection with numerical simulations. The results allow us to demonstrate that, on one hand, we cannot reach a conclusion about the stability of the model for a nonuniform osteocyte distribution. This implies that there is no relationship between the different parameters conveying the stability of the model. On the other hand, we show that the osteocyte cell distribution has a significant influence on the bone morphology, which seems to be confirmed by simulations with real data obtained from rat tibia.

Relationships between osteocyte density and bone formation rate in human cancellous bone

Iliac cancellous osteocyte density decreases with age in deep bone but not in superficial bone, most likely because of remodeling. It has been suggested that osteocytes can inhibit bone remodeling. Accordingly, we examined the relationship between osteocyte density and bone formation rate in 92 healthy women. In superficial bone (<25 microm from the surface), we found a weak but significant (p < 0.03) inverse correlation between BFR/BS and Ot. N/B.Ar that was unaffected by menopause and independent of age. A weaker positive relationship with empty lacunar density improved significance. The data appear to suggest a negative feedback loop, but osteocytes explain only 10% of the variance in BFR/BS, and 97% of the variance in osteocyte density is explained by total lacunar density. This measure of initial osteocyte density during bone formation has a high coefficient of variation (20%) indicating large individual differences. We conclude that: (1) our data support the proposal that osteocytes can inhibit bone remodeling; (2) osteocyte density in superficial bone depends mainly on initial osteocyte density during bone formation and is maintained but not regulated by bone remodeling; and (3) the inverse relationship between BFR/BS and osteocyte density may reflect the homeostatic need to maintain calcium exchangeability in the lining cell-osteocyte syncytium.

NACOB presentation Keynote lecture. Cancellous bone biomechanics. North American Congress on Biomechanics

Cancellous bone is both a biological and a mechanical structure. The interaction between these two aspects of cancellous bone is sufficiently strong that understanding the mechanical properties of the tissue is not possible without consideration of the biology. This manuscript is a mathematical expansion of a portion of the first author's Keynote lecture at the 1998 NACOB presentation. The cellular activity of cancellous bone proceeds in part by the transport of metabolites between trabecular hard tissue and marrow. The anatomical observation is that human trabeculae are seldom internally served by a blood supply, suggesting that the transport mechanisms for trabecular survival are diffusion and a collection of mechanisms for active transport of metabolites independent of blood flow. It will be demonstrated that metabolite transport by diffusion can explain two notable empirical relationships for bone: (a) the close relationship between the bone surface and the bone volume, and (b) the exponential decline in the bone volume fraction during periods of mechanical disuse. A mathematical model is also developed showing how mechanical loading can effect bone volume fraction by increasing metabolite transport between the tissue compartments.