Impaired vibration of auditory ossicles in osteopetrotic mice

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.

Evaluation of acoustic changes in and the healing outcomes of rat eardrums with pars tensa and pars flaccida perforations

Objectives

To systematically explore the differences in acoustic changes and healing outcomes of tympanic membranes (TMs) with pars flaccida perforation (PFP) and pars tensa perforation (PTP).

Methods

We created PFPs and PTPs of various sizes in Sprague-Dawley rats, and evaluated TM umbo velocity and hearing function using laser Doppler vibrometry and auditory brainstem response (ABR) measurement before and immediately after perforation. Two weeks later, hearing was reevaluated and TMs were investigated by immunohistochemical staining.

Results

Small PFPs and PTPs did not significantly affect umbo velocity and hearing function. Large PFPs increased umbo velocity loss at low frequency (1.5 kHz) and elevated ABR thresholds within 1-2 kHz. Large PTP caused significant velocity loss at low frequencies from 1.5 to 3.5 kHz and threshold elevations at full frequencies (1-2 kHz). Two weeks after the perforation, the hearing function of rats with healed PFPs recovered completely. However, high-frequency hearing loss (16-32 kHz) persisted in rats with healed PTPs. Morphological staining revealed that no increase in the thickness and obvious increase in collagen I level of regenerated par flaccida; regenerated pars tensa exhibited obvious increase in thickness and increased collagen I, while the collagen II regeneration was limited with discontinuous and disordered structure in regenerated pars tensa.

Conclusion

The hearing loss caused by large PFP limits at low frequencies while large PTP can lead to hearing loss at wide range frequencies. PFP and PTP have different functional outcomes after spontaneous healing, which is determined by the discrepant structure reconstruction and collagen regeneration.

Postnatal expression and possible function of RANK and RANKL in the murine inner ear

The bone encasing the inner ear, known as the otic capsule, is unique because it remodels little postnatally compared to other bones in the body. Previous studies established that osteoprotegerin (OPG) in the inner ear inhibits otic capsule remodeling. OPG acts as a decoy receptor of receptor activator of nuclear factor κB ligand (RANKL) to disrupt the interaction between RANKL and RANK, the primary regulators of bone metabolism. Here we studied the expression and function of RANK and RANKL in the murine cochlea. Using a combination of in situ hybridization, real-time quantitative RT-PCR, and western blot, we demonstrate that Rankl and Rank genes and their protein products are expressed in the intracochlear soft tissues and the otic capsule in a developmentally regulated manner. Using a culture of neonatal murine cochlear neurons, we show that the interaction between RANK and RANKL inhibits neurite outgrowth in these neurons, and is associated with upregulation of NOGO-A expression. Taken together, our results suggest that, in addition to regulating otic capsule bone remodeling, RANK and RANKL expressed by intracochlear soft tissues may also regulate spiral ganglion neuron function by affecting neurite outgrowth.

Auditory Brainstem Deficits from Early Treatment with a CSF1R Inhibitor Largely Recover with Microglial Repopulation

Signaling between neurons and glia is necessary for the formation of functional neural circuits. A role for microglia in the maturation of connections in the medial nucleus of the trapezoid body (MNTB) was previously demonstrated by postnatal microglial elimination using a colony stimulating factor 1 receptor (CSF1R). Defective pruning of calyces of Held and significant reduction of the mature astrocyte marker glial fibrillary acidic protein (GFAP) were observed after hearing onset. Here, we investigated the time course required for microglia to populate the mouse MNTB after cessation of CSF1R inhibitor treatment. We then examined whether defects seen after microglial depletion were rectified by microglial repopulation. We found that microglia returned to control levels at four weeks of age (18 d postcessation of treatment). Calyceal innervation of MNTB neurons was comparable to control levels at four weeks and GFAP expression recovered by seven weeks. We further investigated the effects of microglia elimination and repopulation on auditory function using auditory brainstem recordings (ABRs). Temporary microglial depletion significantly elevated auditory thresholds in response to 4. 8, and 12 kHz at four weeks. Treatment significantly affected latencies, interpeak latencies, and amplitudes of all the ABR peaks in response to many of the frequencies tested. These effects largely recovered by seven weeks. These findings highlight the functions of microglia in the formation of auditory neural circuits early in development. Further, the results suggest that microglia retain their developmental functions beyond the period of circuit refinement.

CCDC154 Mutant Caused Abnormal Remodeling of the Otic Capsule and Hearing Loss in Mice

Osteopetrosis is a rare inherited bone disease characterized by dysfunction of osteoclasts, causing impaired bone resorption and remodeling, which ultimately leads to increased bone mass and density. Hearing loss is one of the most common complications of osteopetrosis. However, the etiology and pathogenesis of auditory damage still need to be explored. In this study, we found that a spontaneous mutation of coiled-coil domain-containing 154 (CCDC154) gene, a new osteopetrosis-related gene, induced congenital deafness in mice. Homozygous mutant mice showed moderate to severe hearing loss, while heterozygous or wild-type (WT) littermates displayed normal hearing. Pathological observation showed that abnormal bony remodeling of the otic capsule, characterized by increased vascularization and multiple cavitary lesions, was found in homozygous mutant mice. Normal structure of the organ of Corti and no substantial hair cell or spiral ganglion neuron loss was observed in homozygous mutant mice. Our results indicate that mutation of the osteopetrosis-related gene CCDC154 can induce syndromic hereditary deafness in mice. Bony remodeling disorders of the auditory ossicles and otic capsule are involved in the hearing loss caused by CDCC154 mutation.

Three-Dimensional Visualisation of Skeletal Cavities

Bones contain spaces within them. The extraction and the analysis of those cavities are crucial in the study of bone tissue function and can inform about pathologies or past traumatic events. The use of medical imaging techniques allows a non-invasive visualisation of skeletal cavities opening a new frontier in medical inspection and diagnosis. Here, we report the application of a new mesh-based approach for the isolation of skeletal cavities of different size and geometrical structure. We apply a mesh-based approach to extract (i) the main virtual cavities inside the human skull, (ii) a complete human endocast, (iii) the inner vasculature of the malleus bone and (iv) the medullary of a human femur. The detailed description of the mesh-based isolation method and its pioneristic application to four different case-studies show the potential of this approach in medical visualisation.

Synchrotron radiation imaging revealing the sub-micron structure of the auditory ossicles

PURPOSE

Synchrotron-based X-ray Phase Contrast Imaging (SR X-PCI) allows, thanks to a highly coherent and powerful X-ray beam, the imaging of surface and cross-sectional tissue properties with high absorption-contrast. The objective of this study is to investigate the sub-micron structure of the ossicular chain. The understanding of its morphological properties at sub-micron scale will help to refine the understanding of its structural properties. The investigation of intact, non-decalcified and unstained ossicular bones allows to study the spatial relationship between surface properties, internal structure and tomographical slides.

MAIN RESULTS

The tomography datasets with a pixel size of 0.65 μm were reconstructed and 3D volume rendering models of all specimens were analyzed. Based on surface models, the surfaces of the articulations, the insertion of the tensor tympani and stapedial muscle tendons and the nutritional foramina, where the vessels penetrate the ossicles, were visualized. Moreover, a branched network of inner channels could be represented and its connection to the nutritional foramen was demonstrated. Looking at the tomographic structure of the three ossicles a mineralization pattern for every auditory bone was described, indicating a considerable variation throughout the bones.

CONCLUSIONS

This study investigates the submicron-structure of the auditory ossicles at a pixel size of 0.65 μm, which is to the best of our knowledge the highest resolution reported in the investigation of the human auditory system so far. The provided data helps in the further understanding of the anatomical conformation of the ossicular chain.

The CaV1.2 L-type calcium channel regulates bone homeostasis in the middle and inner ear

Bone remodeling of the auditory ossicles and the otic capsule is highly restricted and tightly controlled by the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa-Β ligand (RANKL)/receptor activator of nuclear factor kappa-Β (RANK) system. In these bony structures, a pathological decrease in OPG expression stimulates osteoclast differentiation and excessive resorption followed by accrual of sclerotic bone, ultimately resulting in the development of otosclerosis, a leading cause of deafness in adults. Understanding the signaling pathways involved in maintaining OPG expression in the ear would shed light on the pathophysiology of otosclerosis and other ear bone-related diseases. We and others previously demonstrated that Ca2+ signaling through the L-type CaV1.2 Ca2+ channel positively regulates OPG expression and secretion in long bone osteoblasts and their precursor cells in vitro and in vivo. Whether CaV1.2 regulates OPG expression in ear bones has not been investigated. We drove expression of a gain-of-function CaV1.2 mutant channel (CaV1.2TS) using Col2a1-Cre, which we found to target osteochondral/osteoblast progenitors in the auditory ossicles and the otic capsule. Col2a1-Cre;CaV1.2TS mice displayed osteopetrosis of these bones shown by μCT 3D reconstruction, histological analysis, and lack of bone sculpting, findings similar to phenotypes seen in mice with an osteoclast defect. Consistent with those observations, we found that Col2a1-Cre;CaV1.2TS mutant mice showed reduced osteoclasts in the otic capsule, upregulated mRNA expression of Opg and Opg/Rankl ratio, and increased mRNA expression of osteoblast differentiation marker genes in the otic capsule, suggesting both an anti-catabolic and anabolic effect of CaV1.2TS mutant channel contributed to the observed morphological changes of the ear bones. Further, we found that Col2a1-Cre;CaV1.2TS mice experienced hearing loss and displayed defects of body balance in behavior tests, confirming that the CaV1.2-dependent Ca2+ influx affects bone structure in the ear and consequent hearing and vestibular functions. Together, these data support our hypothesis that Ca2+ influx through CaV1.2TS promotes OPG expression from osteoblasts, thereby affecting bone modeling/remodeling in the auditory ossicles and the otic capsule. These data provide insight into potential pathological mechanisms underlying perturbed OPG expression and otosclerosis.

Innervation of the tibial epiphysis through the intercondylar foramen

The periosteum and mineralized bone are innervated by nerves that sense pain. These include both myelinated and unmyelinated neurons with either free nerve endings or bearing nociceptors. Parasympathetic and sympathetic autonomic nerves also innervate bone. However, little is known about the route sensory nerves take leaving the epiphyses of long bones at the adult knee joint. Here, we used transgenic mice that express fluorescent Venus protein in Schwann cells (Sox10-Venus mice) to visualize myelinated and unmyelinated nerves in the tibial epiphysis. Immunofluorescence to detect a pan-neuronal marker and the sensory neuron markers calcitonin gene-related peptide (CGRP) and tropomyosin receptor kinase A (TrkA) also revealed Schwann cell-associated sensory neurons. Foramina in the intercondylar area of the tibia were conserved between rodents and primates. Venus-labeled fibers were detected within bone marrow of the proximal epiphysis, exited through foramina along with blood vessels in the intercondylar area of the tibia, and joined Venus-labeled fibers of the synovial membrane and meniscus. These data suggest that innervation of the subchondral plate and trabecular bone within the tibial epiphysis carries pain signals from the knee joint to the brain through intercondylar foramina.

Trans-pairing between osteoclasts and osteoblasts shapes the cranial base during development

Bone growth is linked to expansion of nearby organs, as is the case for the cranial base and the brain. Here, we focused on development of the mouse clivus, a sloping surface of the basioccipital bone, to define mechanisms underlying morphological changes in bone in response to brain enlargement. Histological analysis indicated that both endocranial and ectocranial cortical bone layers in the basioccipital carry the osteoclast surface dorsally and the osteoblast surface ventrally. Finite element analysis of mechanical stress on the clivus revealed that compressive and tensile stresses appeared mainly on respective dorsal and ventral surfaces of the basioccipital bone. Osteoclastic bone resorption occurred primarily in the compression area, whereas areas of bone formation largely coincided with the tension area. These data collectively suggest that compressive and tensile stresses govern respective localization of osteoclasts and osteoblasts. Developmental analysis of the basioccipital bone revealed the clivus to be angled in early postnatal wild-type mice, whereas its slope was less prominent in Tnfsf11^−/− mice, which lack osteoclasts. We propose that osteoclast-osteoblast “trans-pairing” across cortical bone is primarily induced by mechanical stress from growing organs and regulates shape and size of bones that encase the brain.

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.

Dissection of the Auditory Bulla in Postnatal Mice: Isolation of the Middle Ear Bones and Histological Analysis

In most mammals, auditory ossicles in the middle ear, including the malleus, incus and stapes, are the smallest bones. In mice, a bony structure called the auditory bulla houses the ossicles, whereas the auditory capsule encloses the inner ear, namely the cochlea and semicircular canals. Murine ossicles are essential for hearing and thus of great interest to researchers in the field of otolaryngology, but their metabolism, development, and evolution are highly relevant to other fields. Altered bone metabolism can affect hearing function in adult mice, and various gene-deficient mice show changes in morphogenesis of auditory ossicles in utero. Although murine auditory ossicles are tiny, their manipulation is feasible if one understands their anatomical orientation and 3D structure. Here, we describe how to dissect the auditory bulla and capsule of postnatal mice and then isolate individual ossicles by removing part of the bulla. We also discuss how to embed the bulla and capsule in different orientations to generate paraffin or frozen sections suitable for preparation of longitudinal, horizontal, or frontal sections of the malleus. Finally, we enumerate anatomical differences between mouse and human auditory ossicles. These methods would be useful in analyzing pathological, developmental and evolutionary aspects of auditory ossicles and the middle ear in mice.

Osteogenic capillaries orchestrate growth plate-independent ossification of the malleus

Endochondral ossification is a developmental process by which cartilage is replaced by bone. Terminally differentiated hypertrophic chondrocytes are calcified, vascularized, and removed by chondroclasts before bone matrix is laid down by osteoblasts. In mammals, the malleus is one of three auditory ossicles that transmit vibrations of the tympanic membrane to the inner ear. The malleus is formed from a cartilaginous precursor without growth plate involvement, but little is known about how bones of this type undergo endochondral ossification. Here, we demonstrate that in the processus brevis of the malleus, clusters of osteoblasts surrounding the capillary loop produce bone matrix, causing the volume of the capillary lumen to decrease rapidly in post-weaning mice. Synchrotron X-ray tomographic microscopy revealed a concentric, cylindrical arrangement of osteocyte lacunae along capillaries, indicative of pericapillary bone formation. Moreover, we report that overexpression of Fosl1, which encodes a component of the AP-1 transcription factor complex, in osteoblasts significantly blocked malleal capillary narrowing. These data suggest that osteoblast/endothelial cell interactions control growth plate-free endochondral ossification through ‘osteogenic capillaries’ in a Fosl1-regulated manner.

Summary: The endochondral ossification of the malleus, an ossicle of the mouse inner ear, occurs around capillaries and is mediated by the AP-1 transcription factor Fosl1.

The ontology of craniofacial development and malformation for translational craniofacial research

We introduce the Ontology of Craniofacial Development and Malformation (OCDM) as a mechanism for representing knowledge about craniofacial development and malformation, and for using that knowledge to facilitate integrating craniofacial data obtained via multiple techniques from multiple labs and at multiple levels of granularity. The OCDM is a project of the NIDCR-sponsored FaceBase Consortium, whose goal is to promote and enable research into the genetic and epigenetic causes of specific craniofacial abnormalities through the provision of publicly accessible, integrated craniofacial data. However, the OCDM should be usable for integrating any web-accessible craniofacial data, not just those data available through FaceBase. The OCDM is based on the Foundational Model of Anatomy (FMA), our comprehensive ontology of canonical human adult anatomy, and includes modules to represent adult and developmental craniofacial anatomy in both human and mouse, mappings between homologous structures in human and mouse, and associated malformations. We describe these modules, as well as prototype uses of the OCDM for integrating craniofacial data. By using the terms from the OCDM to annotate data, and by combining queries over the ontology with those over annotated data, it becomes possible to create "intelligent" queries that can, for example, find gene expression data obtained from mouse structures that are precursors to homologous human structures involved in malformations such as cleft lip. We suggest that the OCDM can be useful not only for integrating craniofacial data, but also for expressing new knowledge gained from analyzing the integrated data.

Panel 2

Objective

This report reviews the literature to identify the advances in our understanding of the middle ear (ME)–Eustachian tube (ET) system during the past 4 years and, on that basis, to determine whether the short-term goals elaborated in the last report were achieved and propose updated goals to guide future otitis media (OM) research.

Data Sources

Databases searched included PubMed, Web of Science (1945-present), Medline (1950 to present), Biosis Previews (1969-present), and the Zoological Record (1978 to present). The initial literature search covered the time interval from January 2007 to June 2011, with a supplementary search completed in February 2012.

Review Methods

The panel topic was subdivided; each contributor performed a literature search and provided a preliminary report. Those reports were consolidated and discussed when the panel met on June 9, 2011. At that meeting, the progress was evaluated and new short-term goals proposed.

Conclusions

Progress was made on 16 of the 19 short-term goals proposed in 2007. Significant advances were made in the characterization of ME gas exchange pathways, modeling ET function, and preliminary testing of treatments for ET dysfunction.

Implications for Practice

In the future, imaging technologies should be developed to noninvasively assess ME/ET structure and physiology with respect to their role in OM pathogenesis. The new data derived from form/function experiments should be integrated into the finite element models and used to develop specific hypotheses concerning OM pathogenesis and persistence. Finally, rigorous studies of treatments, medical or surgical, of ET dysfunction should be undertaken.

Spontaneous optic nerve compression in the osteopetrotic (op/op) mouse: a novel model of myelination failure

We report a focal disturbance in myelination of the optic nerve in the osteopetrotic (op/op) mouse, which results from a spontaneous compression of the nerve resulting from stenosis of the optic canal. The growth of the op/op optic nerve was significantly affected, being maximally suppressed at postnatal day 30 (P30; 33% of age matched control). Myelination of the nerve in the optic canal was significantly delayed at P15, and myelin was almost completely absent at P30. The size of nerves and myelination were conserved both in the intracranial and intraorbital segments at P30, suggesting that the axons in the compressed site are spared in all animals at P30. Interestingly, we observed recovery both in the nerve size and the density of myelinated axons at 7 months in almost half of the optic nerves examined, although some nerves lost axons and became atrophic. In vivo and ex vivo electrophysiological examinations of P30 op/op mice showed that nerve conduction was significantly delayed but not blocked with partial recovery in some mice by 7 months. Transcardial perfusion of FITC-labeled albumin suggested that local ischemia was at least in part the cause of this myelination failure. These results suggest that the primary abnormality is dysmyelination of the optic nerve in early development. This noninvasive model system will be a valuable tool to study the effects of nerve compression on the function and survival of oligodendrocyte progenitor cells/oligodendrocytes and axons and to explore the mechanism of redistribution of oligodendrocyte progenitor cells with compensatory myelination.

Of mice, moles and guinea pigs: functional morphology of the middle ear in living mammals

The middle ear apparatus varies considerably among living mammals. Body size, phylogeny and acoustic environment all play roles in shaping ear structure and function, but experimental studies aimed ultimately at improving our understanding of human hearing can sometimes overlook these important species differences. This review focuses on three groups of mammals, bringing together anatomical, zoological and physiological information in order to highlight unusual features of their middle ears and attempt to interpret their function. "Microtype" ears, found in species such as mice and bats, are associated with high-frequency hearing. The orbicular apophysis, the focus of some recent developmental studies on mouse ears, is characteristic of microtype mallei but is not found in humans or other "freely mobile" species. The apophysis increases ossicular inertia about the anatomical axis of rotation: its adaptive purpose in a high-frequency ear is still not clear. Subterranean mammals have convergently evolved a "freely mobile" ossicular morphology which appears to favour lower-frequency sound transmission. More unusual features found in some of these animals include acoustically coupled middle ear cavities, the loss of middle ear muscles and hypertrophied ossicles which are believed to subserve a form of inertial bone conduction. Middle ears of the rodent group Ctenohystrica (which includes guinea pigs and chinchillas, important models in hearing research) show some striking characteristics which together comprise a unique type of auditory apparatus requiring a classification of its own, referred to here as the "Ctenohystrica type". These characteristics include a distinctive malleus morphology, fusion of the malleus and incus, reduction or loss of the stapedius muscle, a synovial stapedio-vestibular articulation and, in chinchillas, enormously expanded middle ear cavities. These characteristics may be functionally linked and associated with the excellent low-frequency hearing found in these animals. The application of new experimental and imaging data into increasingly sophisticated models continues to improve our understanding of middle ear function. However, a more rigorous comparative approach and a better appreciation of the complex patterns of convergent and divergent evolution reflected in the middle ear structures of living mammals are also needed, in order to put findings from different species into the appropriate context. This article is part of a special issue entitled "MEMRO 2012".

Anti-apoptotic gene Bcl2 is required for stapes development and hearing

In this paper we describe novel and specific roles for the apoptotic regulators Bcl2 and Bim in hearing and stapes development. Bcl2 is anti-apoptotic while Bim is pro-apoptotic. Characterization of the auditory systems of mice deficient for these molecules revealed that Bcl2^−/− mice suffered severe hearing loss. This was conductive in nature and did not affect sensory cells of the inner ear, with cochlear hair cells and neurons present and functional. Bcl2^−/− mice were found to have a malformed, often monocrural, porous stapes (the small stirrup-shaped bone of the middle ear), but a normally shaped malleus and incus. The deformed stapes was discontinuous with the incus and sometimes fused to the temporal bones. The defect was completely rescued in Bcl2^−/−Bim^−/− mice and partially rescued in Bcl2^−/−Bim^+/− mice, which displayed high-frequency hearing loss and thickening of the stapes anterior crus. The Bcl2^−/− defect arose in utero before or during the cartilage stage of stapes development. These results implicate Bcl2 and Bim in regulating survival of second pharyngeal arch or neural crest cells that give rise to the stapes during embryonic development.

Bone cell interactions through Eph/ephrin: bone modeling, remodeling and associated diseases

Bones cannot properly form or be maintained without cell-cell interactions through ephrin ligands and Eph receptors. Cell culture analysis and evaluation of genetic mouse models and human diseases reveal various ephrins and Eph functions in the skeletal system. Migration, attachment and spreading of mesenchymal stem cells are regulated by ephrinB ligands and EphB receptors. ephrinB1 loss-of-function is associated with craniofrontonasal syndrome (CFNS) in humans and mice. In bone remodeling, ephrinB2 is postulated to act as a “coupling stimulator.” In that case, bidirectional signaling between osteoclastic ephrinB2 and osteoblastic EphB4 suppresses osteoclastic bone resorption and enhances osteoblastic bone formation, facilitating the transition between these two states. Parathyroid hormone (PTH) induces ephrinB2 in osteoblasts and enhances osteoblastic bone formation. In contrast to ephrinB2, ephrinA2 acts as a “coupling inhibitor,” since ephrinA2 reverse signaling into osteoclasts enhances osteoclastogenesis and EphA2 forward signaling into osteoblasts suppresses osteoblastic bone formation and mineralization. Furthermore, ephrins and Ephs likely modulate pathological conditions such as osteoarthritis, rheumatoid arthritis, multiple myeloma and osteosarcoma. This review focuses on ephrin/Eph-mediated cell-cell interactions in bone biology.