A new role for the chondrocyte in fracture repair: endochondral ossification includes direct bone formation by former chondrocytes

Application of Antioxidant Compounds in Bone Defect Repair

Bone defects caused by trauma, tumor resection, and infections are significant clinical challenges. Excessive reactive oxygen species (ROS) usually accumulate in the defect area, which may impair the function of cells involved in bone formation, posing a serious challenge for bone repair. Due to the potent ROS scavenging ability, as well as potential anti-inflammatory and immunomodulatory activities, antioxidants play an indispensable role in the maintenance and protection of bone health and have gained increasing attention in recent years. This narrative review aims to give an overview of the main research directions on the application of antioxidant compounds in bone defect repair over the past decade. In addition, the positive effects of various antioxidants and their biomaterial delivery systems in bone repair are summarized to provide new insights for exploring antioxidant-based strategies for bone defect repair.

The role of hypertrophic chondrocytes in regulation of the cartilage-to-bone transition in fracture healing

Endochondral bone formation is an important pathway in fracture healing, involving the formation of a cartilaginous soft callus and the process of cartilage-to-bone transition. Failure or delay in the cartilage-to-bone transition causes an impaired bony union such as nonunion or delayed union. During the healing process, multiple types of cells including chondrocytes, osteoprogenitors, osteoblasts, and endothelial cells coexist in the callus, and inevitably crosstalk with each other. Hypertrophic chondrocytes located between soft cartilaginous callus and bony hard callus mediate the crosstalk regulating cell-matrix degradation, vascularization, osteoclast recruitment, and osteoblast differentiation in autocrine and paracrine manners. Furthermore, hypertrophic chondrocytes can become osteoprogenitors and osteoblasts, and directly contribute to woven bone formation. In this review, we focus on the roles of hypertrophic chondrocytes in fracture healing and dissect the intermingled crosstalk in fracture callus during the cartilage-to-bone transition.

Altered developmental programs and oriented cell divisions lead to bulky bones during salamander limb regeneration

There are major differences in duration and scale at which limb development and regeneration proceed, raising the question to what extent regeneration is a recapitulation of development. We address this by analyzing skeletal elements using a combination of micro-CT imaging, molecular profiling and clonal cell tracing. We find that, in contrast to development, regenerative skeletal growth is accomplished based entirely on cartilage expansion prior to ossification, not limiting the transversal cartilage expansion and resulting in bulkier skeletal parts. The oriented extension of salamander cartilage and bone appear similar to the development of basicranial synchondroses in mammals, as we found no evidence for cartilage stem cell niches or growth plate-like structures during neither development nor regeneration. Both regenerative and developmental ossification in salamanders start from the cortical bone and proceeds inwards, showing the diversity of schemes for the synchrony of cortical and endochondral ossification among vertebrates.

Monocyte Chemotactic Proteins Mediate the Effects of Hyperglycemia in Chondrocytes: In Vitro Studies

Chemokines are secreted by a large variety of cells. They are involved in controlling cell trafficking, maturation, and differentiation. However, the specific responses and effects of chemokines on specific skeletal cell types under high glucose conditions have not been investigated. Chondrocytes play an important role in osteoarthritis and fracture healing. Delayed fracture healing is one of the major health complications caused by diabetes, so the goal of this study was to evaluate the response of several chemokines to high glucose conditions in chondrocyte cells and analyze their role in the catabolic effect of hyperglycemia. ATDC5 chondrocytes were cultured in normal and high glucose media, and mRNA expression levels of several chemokines and chondrocyte differentiation markers were quantified. Bindarit, a specific inhibitor of monocyte chemotactic proteins (MCPs), was used to determine the role of MCPs in mediating the effects of high glucose conditions in chondrocyte cells. High glucose treatment upregulated the expression of three Mcps, as well as the expression of matrix metalloproteinase 13 (Mmp13) and Osteocalcin (Oc). Furthermore, bindarit treatment downregulated Mmp13 and Oc but upregulated Collagen 2 (Col2) mRNA levels in chondrocytes treated with high glucose. Moreover, treatment of chondrocytes with ascorbic acid reduced the effect of high glucose conditions on the expression of chemokines and Mmps. These data together suggest that MCPs mediate the catabolic effect of high glucose in chondrocytes.

The Formation of the Epiphyseal Bone Plate Occurs via Combined Endochondral and Intramembranous-Like Ossification

The formation of the epiphyseal bone plate, the flat bony structure that provides strength and firmness to the growth plate cartilage, was studied in the present study by using light, confocal, and scanning electron microscopy. Results obtained evidenced that this bone tissue is generated by the replacement of the lower portion of the epiphyseal cartilage. However, this process differs considerably from the usual bone tissue formation through endochondral ossification. Osteoblasts deposit bone matrix on remnants of mineralized cartilage matrix that serve as a scaffold, but also on non-mineralized cartilage surfaces and as well as within the perivascular space. These processes occur simultaneously at sites located close to each other, so that, a core of the sheet of bone is established very quickly. Subsequently, thickening and reshaping occurs by appositional growth to generate a dense parallel-fibered bone structurally intermediate between woven and lamellar bone. All these processes occur in close relationship with a cartilage but most of the bone tissue is generated in a manner that may be considered as intramembranous-like. Overall, the findings here reported provide for the first time an accurate description of the tissues and events involved in the formation of the epiphyseal bone plate and gives insight into the complex cellular events underlying bone formation at different sites on the skeleton.

Pathogenesis of traumatic temporomandibular joint ankylosis: a narrative review

OBJECTIVE

To comprehensively review the literature and summarize the results from human and animal studies related to the possible causes and pathogenesis of traumatic temporomandibular joint ankylosis (TMJA).

MATERIALS AND METHODS

The Google Scholar, Embase, and Web of Science databases were used to search for articles related to traumatic TMJA from 2011 to 2020. All articles were screened according to the inclusion and exclusion criteria, collected, and analyzed.

RESULTS

Nineteen relevant articles were collected. These articles were classified into three groups: predisposing and etiological factors, cellular studies, and molecular studies.

CONCLUSION

The pathological mechanisms are similar between TMJA and nonunion hypertrophy. Aberrant structural and etiological factors as well as disordered cellular and molecular mechanisms might contribute to TMJA formation. Although preclinical and clinical data have provided new evidence on the pathogenesis of traumatic TMJA, the molecular mechanisms and biological events require further exploration.

A Second Career for Chondrocytes-Transformation into Osteoblasts

PURPOSE OF REVIEW

The goal of the review is to summarize the current knowledge on the process of chondrocyte-to-osteoblast transdifferentiation during endochondral bone formation and its potential implications in fracture healing and disease.

RECENT FINDINGS

Lineage tracing experiments confirmed the transdifferentiation of chondrocytes into osteoblasts. More recent studies lead to the discovery of molecules involved in this process, as well as to the hypothesis that these cells may re-enter a stem cell-like phase prior to their osteoblastic differentiation. This review recapitulates the current knowledge regarding chondrocyte transdifferentiating into osteoblasts, the developmental and postnatal events where transdifferentiation appears to be relevant, and the molecules implicated in this process.

Osteochondrosis in the Distal Femoral Physis of Pigs Starts With Vascular Failure

Articular osteochondrosis (OC) arises due to vascular failure and ischemic chondronecrosis. The aim of the study was to describe the histological and computed tomographic (CT) characteristics of changes in the distal femoral physis of pigs, to determine if they represented OC lesions and if the pathogenesis was the same as for articular OC. The material included 19 male Landrace pigs bred for predisposition to OC. One or 2 pigs were euthanized and CT-scanned at 2-week intervals from 82 to 180 days of age. Material from 10 pigs was available for histological validation. The CT scans revealed 31 lesions confirmed in 3 planes and 1 additional macroscopically visible lesion confirmed in 2 CT planes. Twelve of the lesions were histologically validated. All lesions were compatible with OC. Cartilage canal and eosinophilic streak morphological changes corresponded to failure of end arteries coursing from the epiphysis, toward the metaphysis. The location of lesions was compatible with failure at the point of vessel incorporation into bone. Vascular failure was associated with retention of viable hypertrophic chondrocytes and delayed ossification but not cartilage necrosis. Lesion width ranged from 1.1% to 45.6% of the physis. Several lesions were expected to resolve due to small size and evidence of CT-identifiable, reparative ossification. Angular limb deformity was not detected in any pig. The pathogenesis of physeal OC started with vascular failure that was morphologically identical to articular OC. The heritable predisposition may therefore be the same. The association between lesions and limb deformity should be studied further in older pigs in future.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Origin of Reparative Stem Cells in Fracture Healing

PURPOSE OF REVIEW

The identity and functional roles of stem cell population(s) that contribute to fracture repair remains unclear. This review provides a brief history of mesenchymal stem cell (MSCs) and provides an updated view of the many stem/progenitor cell populations contributing to fracture repair.

RECENT FINDINGS

Functional studies show MSCs are not the multipotential stem cell population that form cartilage and bone during fracture repair. Rather, multiple studies have confirmed the periosteum is the primary source of stem/progenitor cells for fracture repair. Newer work is also identifying other stem/progenitor cells that may also contribute to healing. Although the heterogenous periosteal cells migrate to the fracture site and contribute directly to callus formation, other cell populations are involved. Pericytes and bone marrow stromal cells are now thought of as key secretory centers that mostly coordinate the repair process. Other populations of stem/progenitor cells from the muscle and transdifferentiated chondroctyes may also contribute to repair, and their functional role is an area of active research.

Postnatal expression of chondrogenic and osteogenic regulatory factor mRNA in the rat condylar cartilage

OBJECTIVES

The condylar cartilage is a key site of growth and development of the mandible. The aim of this research was to determine the mRNA expression levels of a number of chondrogenic and osteogenic regulatory factors in the condylar cartilage of the postnatal rat.

MATERIALS AND METHODS

Condyles were extracted from 40 rats aged 4, 10, 21 or 90 days with 10 rats assigned to each age group. The condyles from one rat from each age group was fixed and decalcified in 10% EDTA for histology. Using cryogenic grinding combined with QIAzol reagent total RNA was purified from pooled samples collected for each age group. Each pool contained six condyles (N = 3). mRNA expression levels for 28 genes were determined using qPCR.

RESULTS

Histological analysis revealed distinct morphological differences in the condyle tissue of the 4, 10, 21 and 90 day old postnatal rats. Expression of all examined genes was detected. High levels of mRNA for Alpl, Bglap, Col1a1, Col2a1, Runx2, Sox9 and Sp7 but not Msx1 were detected. Fgf1 and Fgf2 were expressed at a similar level. No significant difference (defined as ± fold-regulation > 2 and P < 0.05) in the gene mRNA expression levels was found when days 10, 21 or 90 were compared to day 4.

CONCLUSIONS

Apparent morphological changes of the rat condylar cartilage are not reflected in a change in the expression levels of the chondrogenic and osteogenic regulatory factor mRNA investigated in this study.

Ca2+/Mg2+ homeostasis‑related TRPM7 channel mediates chondrocyte hypertrophy via regulation of the PI3K‑Akt signaling pathway

Chondrocytes are specialized cells that form cartilage tissue, and are able to respond to their osmotic environment and exercise important roles in endochondral ossification via undergoing proliferation, hypertrophy and apoptosis. The transient receptor melastatin potential 7 (TRPM7) cation channel can modulate the intracellular and extracellular levels of Ca2+ and Mg2+, and therefore the cellular osmotic environment. However, the molecular pathways involved in TRPM7‑mediated signal transduction have yet to be elucidated. In the present study, the expression and functionality of TRPM7 were investigated during chondrocyte proliferation and hypertrophy. The ATDC5 mouse cell line was employed and cellular viability was evaluated using the MTT assay, whereas hypertrophy was monitored via evaluating the expression of chondrogenic marker genes and the activity of alkaline phosphatase (ALP). Gene expression of TRPM7 appeared slightly upregulated during the proliferative stages of chondrocyte development, and significantly upregulated during the hypertrophic stages, suggesting the importance of Ca2+/Mg2+ homeostasis for chondrocyte growth. Low extracellular Ca2+/Mg2+ levels significantly reduced the expression of type X collagen, Indian hedgehog homolog (Ihh) and matrix metalloproteinase (MMP)‑13 genes, as well as ALP activity; however, cell viability remained unaffected. Conversely, the gene expression levels of TRPM7 appeared upregulated in ATDC5 cells under low extracellular Ca2+ or Mg2+ conditions. Silencing TRPM7 expression during the chondrocyte differentiation period also reduced type X collagen, Ihh and MMP‑13 gene expression, and ALP activity. Furthermore, the phosphatidylinositol‑4,5‑bisphosphate 3‑kinase (PI3K)‑Akt signaling pathway was activated following TRPM7 overexpression, and inhibited following TRPM7 silencing. Notably, the actions of TRPM7 on chondrocyte hypertrophy were abolished through the inhibition of PI3K‑Akt signaling. The present results suggested that TRPM7 may be involved in Ca2+/Mg2+ homeostasis during chondrocyte hypertrophy, and contribute to endochondral ossification via interacting with the PI3K‑Akt signaling pathway.

Loss of scleraxis in mice leads to geometric and structural changes in cortical bone, as well as asymmetry in fracture healing

Scleraxis (Scx) is a known regulator of tendon development, and recent work has identified the role of Scx in bone modeling. However, the role of Scx in fracture healing has not yet been explored. This study was conducted to identify the role of Scx in cortical bone development and fracture healing. Scx green fluorescent protein-labeled (ScxGFP) reporter and Scx-knockout (Scx-mutant) mice were used to assess bone morphometry and the effects of fracture healing on Scx localization and gene expression, as well as callus healing response. Botulinum toxin (BTX) was used to investigate muscle unloading effects on callus shape. Scx-mutant long bones had structural and mechanical defects. Scx gene expression was elevated and bmp4 was decreased at 24 h after fracture. ScxGFP⁺ cells were localized throughout the healing callus after fracture. Scx-mutant mice demonstrated disrupted callus healing and asymmetry. Asymmetry of Scx-mutant callus was not due to muscle unloading. Wild-type littermates (age matched) served as controls. This is the first study to explore the role of Scx in cortical bone mechanics and fracture healing. Deletion of Scx during development led to altered long bone properties and callus healing. This study also demonstrated that Scx may play a role in the periosteal response during fracture healing.-McKenzie, J. A., Buettmann, E., Abraham, A. C., Gardner, M. J., Silva, M. J., Killian, M. L. Loss of scleraxis in mice leads to geometric and structural changes in cortical bone, as well as asymmetry in fracture healing.

Lessons on skeletal cell plasticity from studying jawbone regeneration in zebrafish

Three major mesenchymal cell types have important roles in determining the shapes of vertebrate animals: bone-producing osteoblasts, cartilage-producing chondrocytes, and fat-producing adipocytes. Although often considered discrete cell types, accumulating evidence is revealing mesenchymal cells of intermediate identities and interconversion of cell types. Such plasticity is particularly evident during adult skeletal repair. In this Review, we highlight recent work in zebrafish showing a role for hybrid cartilage-bone cells in large-scale regeneration of the adult jawbone, as well as their origins in the periosteum. An emerging theme is that the unique mechanical and signaling environment of the adult wound causes skeletal cell differentiation to diverge from the discrete lineages seen during development, which may aid in rapid and extensive regeneration of bone.

Ihha induces hybrid cartilage-bone cells during zebrafish jawbone regeneration

The healing of bone often involves a cartilage intermediate, yet how such cartilage is induced and utilized during repair is not fully understood. By studying a model of large-scale bone regeneration in the lower jaw of adult zebrafish, we show that chondrocytes are crucial for generating thick bone during repair. During jawbone regeneration, we find that chondrocytes co-express genes associated with osteoblast differentiation and produce extensive mineralization, which is in marked contrast to the behavior of chondrocytes during facial skeletal development. We also identify the likely source of repair chondrocytes as a population of Runx2(+)/Sp7(-) cells that emanate from the periosteum, a tissue that normally contributes only osteoblasts during homeostasis. Analysis of Indian hedgehog homolog a (ihha) mutants shows that the ability of periosteal cells to generate cartilage in response to injury depends on a repair-specific role of Ihha in the induction as opposed to the proliferation of chondrocytes. The large-scale regeneration of the zebrafish jawbone thus employs a cartilage differentiation program distinct from that seen during development, with the bone-forming potential of repair chondrocytes potentially due to their derivation from osteogenic cells in the periosteum.

The multifaceted role of the vasculature in endochondral fracture repair

Fracture healing is critically dependent upon an adequate vascular supply. The normal rate for fracture delayed or non-union is estimated to be between 10 and 15%, and annual fracture numbers are approximately 15 million cases per year. However, when there is decreased vascular perfusion to the fracture, incidence of impaired healing rises dramatically to 46%. Reduction in the blood supply to the fracture can be the result of traumatic injuries that physically disrupt the vasculature and damage supportive soft tissue, the result of anatomical location (i.e., distal tibia), or attributed to physiological conditions such as age, diabetes, or smoking. The role of the vasculature during repair is multifaceted and changes during the course of healing. In this article, we review recent insights into the role of the vasculature during fracture repair. Taken together these data highlight the need for an updated model for endochondral repair to facilitate improved therapeutic approaches to promote bone healing.

Estrogen stimuli promote osteoblastic differentiation via the subtilisin-like proprotein convertase PACE4 in MC3T3-E1 cells

Estrogenic compounds include endogenous estrogens such as estradiol as well as soybean isoflavones, such as daidzein and its metabolite equol, which are known phytoestrogens that prevent osteoporosis in postmenopausal women. Indeed, mineralization of MC3T3-E1 cells, a murine osteoblastic cell line, was significantly decreased in medium containing fetal bovine serum treated with charcoal-dextran to deplete endogenous estrogens, but estradiol and these soybean isoflavones dose-dependently restored the differentiation of MC3T3-E1 cells; equol was tenfold more effective than daidzein. These differentiation-promoting effects were inhibited by the addition of fulvestrant, which is a selective downregulator of estrogen receptors. Analysis of the expression pattern of bone-related genes by reverse transcription PCR (RT-PCR)/quantitative real-time PCR (qRT-PCR), which focused on responsiveness to the estrogen stimuli, revealed that the transcription of PACE4, a subtilisin-like proprotein convertase, was tightly linked with the differentiation of MC3T3-E1 cells induced by estrogen stimuli. Moreover, treatment with RNAi of PACE4 in MC3T3-E1 cells resulted in a drastic decrease of mineralization in the presence of estrogen stimuli. These results strongly suggest that PACE4 participates in bone formation at least in osteoblast differentiation, and estrogen receptor-mediated stimuli induce osteoblast differentiation through the upregulation of PACE4 expression.

Endochondral bone formation in gelatin methacrylamide hydrogel with embedded cartilage-derived matrix particles

The natural process of endochondral bone formation in the growing skeletal system is increasingly inspiring the field of bone tissue engineering. However, in order to create relevant-size bone grafts, a cell carrier is required that ensures a high diffusion rate and facilitates matrix formation, balanced by its degradation. Therefore, we set out to engineer endochondral bone in gelatin methacrylamide (GelMA) hydrogels with embedded multipotent stromal cells (MSCs) and cartilage-derived matrix (CDM) particles. CDM particles were found to stimulate the formation of a cartilage template by MSCs in the GelMA hydrogel in vitro. In a subcutaneous rat model, this template was subsequently remodeled into mineralized bone tissue, including bone-marrow cavities. The GelMA was almost fully degraded during this process. There was no significant difference in the degree of calcification in GelMA with or without CDM particles: 42.5 ± 2.5% vs. 39.5 ± 8.3% (mean ± standard deviation), respectively. Interestingly, in an osteochondral setting, the presence of chondrocytes in one half of the constructs fully impeded bone formation in the other half by MSCs. This work offers a new avenue for the engineering of relevant-size bone grafts, by the formation of endochondral bone within a degradable hydrogel.

Stem cell-derived endochondral cartilage stimulates bone healing by tissue transformation

Although bone has great capacity for repair, there are a number of clinical situations (fracture non-unions, spinal fusions, revision arthroplasty, segmental defects) in which auto- or allografts attempt to augment bone regeneration by promoting osteogenesis. Critical failures associated with current grafting therapies include osteonecrosis and limited integration between graft and host tissue. We speculated that the underlying problem with current bone grafting techniques is that they promote bone regeneration through direct osteogenesis. Here we hypothesized that using cartilage to promote endochondral bone regeneration would leverage normal developmental and repair sequences to produce a well-vascularized regenerate that integrates with the host tissue. In this study, we use a translational murine model of a segmental tibia defect to test the clinical utility of bone regeneration from a cartilage graft. We further test the mechanism by which cartilage promotes bone regeneration using in vivo lineage tracing and in vitro culture experiments. Our data show that cartilage grafts support regeneration of a vascularized and integrated bone tissue in vivo, and subsequently propose a translational tissue engineering platform using chondrogenesis of mesenchymal stem cells (MSCs). Interestingly, lineage tracing experiments show the regenerate was graft derived, suggesting transformation of the chondrocytes into bone. In vitro culture data show that cartilage explants mineralize with the addition of bone morphogenetic protein (BMP) or by exposure to human vascular endothelial cell (HUVEC)-conditioned medium, indicating that endothelial cells directly promote ossification. This study provides preclinical data for endochondral bone repair that has potential to significantly improve patient outcomes in a variety of musculoskeletal diseases and injuries. Further, in contrast to the dogmatic view that hypertrophic chondrocytes undergo apoptosis before bone formation, our data suggest cartilage can transform into bone by activating the pluripotent transcription factor Oct4A. Together these data represent a paradigm shift describing the mechanism of endochondral bone repair and open the door for novel regenerative strategies based on improved biology.

Inducing ossification in an engineered 3D scaffold-free living cartilage template

Large and complex bone defects or voids cannot rely on natural bone healing process for recovery. They require natural or engineered grafts to facilitate osteo-progenitor cell recruitment and development. In this study, we have employed an in vitro macro-sized 3D cell-based platform for investigation and application of osteogenesis. The model is based on a porous construct made of engineered living cartilaginous tissue named living hyaline cartilaginous graft (LhCG). It is scaffold-free and is solely made up of living chondrocytes and their extra cellular matrix (ECM). To evaluate the efficiency of LhCG as a viable platform for bone formation, osteoblast and human mesenchymal stem cell (hMSC) were seeded respectively into LhCG constructs, establishing a co-culture system consisting of osteo-progenitors and chondrocytes. The results showed that LhCG could support both osteoblast and hMSC maturation and differentiation to the osteogenic lineage respectively. Successful osteogenesis is also observed after subcutaneous implantation in nude mice model suggesting that bone formation could be achieved both in vitro and in vivo. Additionally, with exposure to osteogenic medium, LhCG construct without any further cell seeding expressed similar levels of osteogenic phenotype markers as the ones with hMSC seeded on. It suggests the existence of an osteoprogenitor sub-population residing within LhCG chondrocytes. Hence, it is demonstrated that LhCG, as a cartilage template, could serve as a dynamic platform to support osteogenesis and its intrinsic phenotypic flexibility may also permit a wide range of applications for stem cell research and processing.

MRT letter: Contrast-enhanced computed tomographic imaging of soft callus formation in fracture healing

Formation of a cartilaginous soft callus at the site of a bone fracture is a pivotal stage in the healing process. Noninvasive, or even nondestructive, imaging of soft callus formation can be an important tool in experimental and pre-clinical studies of fracture repair. However, the low X-ray attenuation of cartilage renders the soft callus nearly invisible in radiographs. This study utilized a recently developed, cationic, iodinated contrast agent in conjunction with micro-computed tomography to identify cartilage in fracture calluses in the femora of C57BL/6J and C3H/HeJ mice. Fracture calluses were scanned before and after incubation in the contrast agent. The set of pre-incubation images was registered against and then subtracted from the set of post-incubation images, resulting in a three-dimensional map of the locations of cartilage in the callus, as labeled by the contrast agent. This map was then compared to histology from a previous study. The results showed that the locations where the contrast agent collected in relatively high concentrations were similar to those of the cartilage. The contrast agent also identified a significant difference between the two strains of mice in the percentage of the callus occupied by cartilage, indicating that this method of contrast-enhanced computed tomography may be an effective technique for nondestructive, early evaluation of fracture healing.

Osteoarthritic cartilage chondrocytes alter subchondral bone osteoblast differentiation via MAPK signalling pathway involving ERK1/2

Osteoarthritic subchondral bone is characterized by abnormal bone density and enhanced production of bone turnover markers, an indication of osteoblast dysfunction. Several studies have proposed that pathological changes in articular cartilage influence the subchondral bone changes, which are typical of the progression of osteoarthritis; however, direct evidence of this has yet to be reported. The aim of the present study was to investigate what effects articular cartilage cells, isolated from normal and osteoarthritic joints, may have on the subchondral bone osteoblast phenotype, and also the potential involvement of the mitogen activated protein kinase (MAPK) signalling pathway during this process. Our results suggest that chondrocytes isolated from a normal joint inhibited osteoblast differentiation, whereas chondrocytes isolated from an osteoarthritic joint enhanced osteoblast differentiation, both via a direct and indirect cell interaction mechanisms. Furthermore, the interaction of subchondral bone osteoblasts with osteoarthritic chondrocyte conditioned media appeared to significantly activate ERK1/2 phosphorylation. On the other hand, conditioned media from normal articular chondrocytes did not affect ERK1/2 phosphorylation. Inhibition of the MAPK-ERK1/2 pathways reversed the phenotype changes of subchondral bone osteoblast, which would otherwise be induced by the conditioned media from osteoarthritic chondrocytes. In conclusion, our findings provide evidence that osteoarthritic chondrocytes affect subchondral bone osteoblast metabolism via an ERK1/2 dependent pathway.

A novel model to study metaphyseal bone healing under defined biomechanical conditions

INTRODUCTION

Experimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimental model, which characterizes and controls the biomechanical condition in the fracture gap of a metaphyseal fracture.

MATERIALS AND METHODS

A partial osteotomy model in the distal femur of the sheep was developed. The osteotomy was located in the region of the trochlea groove. The osteotomy gap was 3 mm wide. The retro-patellar force acting on the joint in vivo causes a bending of the trochlea resulting in a narrowing of the osteotomy gap. To limit and control this interfragmentary movement, stainless steel plates of various thicknesses were implanted into the osteotomy gap. Forces acting on the trochlea were analyzed and a load-deflection curve of the model was determined in vitro. A pilot study on two sheep was performed using the new model with two different interfragmentary movements of 0.3 or 1 mm. Eight weeks, post-operatively, the sheep were sacrificed and undecalcified histology was performed.

RESULTS

The biomechanical analysis of the joint forces and the in vitro load-deflection behavior of the trochlea revealed that the forces acting on the trochlea were high enough to cause an interfragmentary movement of 1 mm in the osteotomy gap. This was confirmed by an X-ray of the sheep, which showed a closing of the proximal osteotomy gap under weight-bearing conditions. The histological section revealed no external callus formation. The sheep with the 0.3 mm interfragmentary movement showed almost complete bridging of the osteotomy gap with woven bone whereas the sheep with the 1 mm interfragmentary movement exhibited new bone formation only at the borderline of the osteotomy but larger areas with connective tissue or even fibrous cartilage in the center of the gap.

CONCLUSION

This metaphyseal bone-healing model provides defined and adjustable biomechanical conditions. The histological images demonstrated intramembranous and endochondral bone healing in the osteotomy gap without callus formation. The model therefore seems appropriate to study metaphyseal bone healing under differing mechanical conditions.

Histological investigation of osseous changes of mandibular condyles with backscattered electron images

OBJECTIVES

This study aims to confirm the radiographic morphology of mandibular condyles with microscopic observation and to investigate the mechanism of osseous changes of adult human mandibular condyles.

METHODS

Following the radiographic examinations of ten temporomandibular joints (TMJs) from five cadavers using conventional tomography, helical CT and limited cone beam CT for dentistry (limited CBCT), ten mandibular condyles were removed. Micro CT images were made of the undecalcified dissected joints in the sagittal plane, perpendicular to the long axis of the condyles, to produce images similar in orientation to the radiographic images. Tissue morphology was observed through backscattered electron images and elemental analysis using electron probe microanalysis (EPMA). The microscopic findings were compared with the radiographic findings.

RESULTS

Limited CBCT images most accurately depicted erosive change of the bone cortex of the mandibular condyle. EPMA indicated bone remodelling in all mandibular condyles analysed, which showed cartilage calcification different from typical enchondral ossification. No finding of dissolution or disruption of the superficial soft tissue was observed in all specimens, including those presenting radiographic findings of osteoarthrosis. Active bone remodelling with hypertrophic changes of superficial soft tissue was observed in mandibular condyles, which implied radiographic findings of osteoarthrosis.

CONCLUSIONS

The high detectability of limited CBCT images on bony morphology of mandibular condyles was confirmed. It is speculated that bone remodelling of adult human mandibular condyles is brought about by cartilage calcification. It was suggested that the osseous changes classified as osteoarthrosis with radiographic images are normal bone remodelling.

A study of the role of nell-1 gene modified goat bone marrow stromal cells in promoting new bone formation

Nell-1 is a recently discovered secreted protein with the capacity to promote osteoblastic calvarial cell differentiation and mineralization and induce calvarial bone overgrowth and regeneration in various rodent models. However, the extent of Nell-1 osteoinductivity in large animal cells remains unknown. The objective of the study was to evaluate the feasibility of adenoviral encoding Nell-1 (AdNell-1) gene transfer into primary adult goat bone marrow stromal cells (BMSCs) in vitro and in vivo and to compare the osteoinductive effects with those produced by bone morphogenetic protein-2 (BMP-2), a well established osteoinductive molecule currently utilized for regional gene therapy. AdNell-1-transduced BMSCs expressed Nell-1 protein and underwent osteoblastic differentiation within 2 weeks in vitro, which is comparable to AdBMP-2. After intramuscular injection of nude mice, the AdNell-1- and AdBMP-2-transduced BMSCs revealed new bone formation, while untransduced or AdLacZ-transduced BMSCs showed mainly fibrotic tissue proliferation. At 4 weeks, BMP-2 induced significantly larger bone mass with a mature bone margin and central cavity filled with primarily fatty marrow tissue. Nell-1 samples had significantly less bone mass but were histologically similar to newly formed trabecular bone mixed with chondroid bone-like areas verified by type X collagen (ColX) immunohistochemistry. This distinct difference in histomorphology from the bone mass induced by BMP-2 suggests that there is a potential clinical role/advantage for Nell-1 in skeletal tissue engineering and regeneration.

Chondrocytes are released as viable cells during cartilage resorption associated with the formation of intrachondral canals in the rat tibial epiphysis

The development of cartilage canals is the first event of the ossification of the epiphyses in mammals. Canal formation differs from vascular invasion during primary ossification, since the former involves resorption of resting cartilage and is uncoupled from bone deposition. To learn more about the fate of resorbed chondrocytes during this process, we have carried out structural, cell proliferation, and in situ hybridization studies during the first stages of ossification of the rat tibial proximal epiphysis. Results concerning the formation of the cartilage canals implied the release of resting chondrocytes from the cartilage matrix to the canal cavity. Released chondrocytes had a well-preserved structure, expressed type-II collagen, and maintained the capacity to divide. All these data suggested that chondrocytes released into the canals remained viable for a specific time. Analysis of the proliferative activity at different regions of the cartilage canals showed that the percentage of proliferative chondrocytes at areas of active cartilage resorption was significantly higher than that in zones of low resorption. These results are consistent with the hypothesis that resting chondrocytes surrounding canals have a role in supplying cells for the development of the secondary ossification center. Since released chondrocytes are at an early stage of differentiation greatly preceding their entry into the apoptotic pathway and are exposed to a specific matrix, cellular, and humoral microenvironment, they might differentiate to other cell types and contribute to the ossification of the epiphysis.

Delivery of Growth Factors Using Gene Therapy to Enhance Bone Healing

OBJECTIVE

To review the delivery of growth factors using gene therapy for enhancing long-bone fracture healing.

STUDY DESIGN

Literature review.

METHODS

MEDLINE and CAB Abstracts literature search (1980-2004).

RESULTS

Non-union and infected non-union are relatively common complications of long-bone fractures in human and veterinary patients. Growth factors are cytokines that regulate many cell functions important in fracture healing. Exogenous growth factors can be delivered to the fracture site as recombinant proteins or using gene therapy. Recombinant human bone morphogenetic protein-2 and -7 (rhBMP-2 and -7), in particular, enhance fracture healing in numerous experimental and clinical studies. Some limitations with use of recombinant proteins may be overcome by use of gene therapy. Gene therapy involves delivery of the growth factor gene to cells at the fracture site using a viral or non-viral vector. The gene is then expressed (protein synthesis) by cells at the fracture site. Delivery of the BMP gene to the fracture site using gene therapy has been evaluated in laboratory animal models of non-union, with favorable results and without complications.

CONCLUSION

Delivery of growth factors, particularly members of BMP family, to the fracture site using gene therapy may be a method to enhance fracture healing. Use of this technology may improve the prognosis for patients with long-bone fractures.

CLINICAL RELEVANCE

Clinical application of gene therapy could improve the prognosis for human and veterinary patients with long-bone fractures, but has not been evaluated clinically.

Endochondral ossification in fracture callus during long bone repair: the localisation of 'cavity-lining cells' within the cartilage

Successful fracture healing typically involves the production of a cartilaginous callus, which is eventually remodelled into new bone. The blood vessels in the advancing front of endochondral ossification are likely to play an important role in the replacement of cartilage with bone within the callus. This was investigated by histology and immunohistochemistry techniques carried out on rabbit tibial osteotomy tissue. Cavities within the cartilage were identified by histology and in many cases, there appeared to be vascular structures within them, identified by the immunolocalisation of the transmembrane proteins CD31 and CD34. Osteocalcin localisation and Alizarin red histology was carried out to identify 'osteoblastic' cells and mineral localisation within the cartilaginous callus respectively. However, it was the identification of a population of cells lining the cavities within the cartilage that became the main focus of this study. These cells were 'osteoblastic' in nature, (positive localisation of osteocalcin), and were also positive for the adhesion proteins CD31 and CD34. It is thought that these cells play a role in the conversion of cartilage to bone during the fracture healing process.

Characterization of the development of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells during osteoinduction by rhBMP-2: a histochemical and ultrastructural study

OBJECTIVE

To investigate the characteristics of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells induced by recombinant human bone morphogenetic protein-2 (rhBMP-2).

MATERIALS AND METHODS

rhBMP-2 (5 microg) combined with atelocollagen was implanted into calf muscles of rats and removed on days 7, 10, 14, 21, or 28. Tissue sections were examined using: (i) hematoxylin/Alcian blue/Sirius red stain, (ii) enzyme histochemistry for alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase activity, (iii) immunohistochemistry for types I, II, and X collagen, and (iv) electron microscopy.

RESULTS

On day 7, numerous fibroblast-like cells with ALP activity were present on the pellet rim. On day 10, chondroid matrix (CM) had formed, contained both type I collagen and proteoglycans, and often continued into the BMP pellet. On day 14, bone-like matrix formed around hypertrophic chondrocytes simultaneously with endochondral ossification. Coexpression of types I and II collagen within chondrocytes and osteocytes was observed throughout the time course of the experiment.

CONCLUSION

These results suggest that fibroblast-like cells invading the pellet differentiate into chondrocytes and form CM under the scaffold of the carrier component. It appears that some chondrocytes change their phenotype to produce the bone-like matrix and remain within the endochondral bone. This process enables rapid osteogenesis to occur.

Biocompatibility of three-dimensional chondrocyte grafts in large tibial defects of rabbits

OBJECTIVE

To evaluate biocompatibility and effects of implantation of 3-dimensional chondrocyte-agarose autografts in tibial defects in rabbits and to compare in vitro and in vivo chondrocyte-agarose constructs with respect to cell viability, differentiation, and matrix production.

ANIMALS

24 adult New Zealand White rabbits.

PROCEDURE

Three-dimensional constructs with (grafted group) or without (control group) autogenous chondrocytes were implanted into tibial defects of rabbits and cultured in vitro. During an 8-week period, defects were evaluated radiographically, grossly, histologically, biochemically, and immunohistochemically. In vitro constructs were evaluated histologically, biochemically, and immunohistochemically.

RESULTS

Tibial defects had significantly higher radiographic densitometry values at 4 and 6 weeks after implantation in grafted group rabbits, compared with control group rabbits. Number of observed centers of endochondral ossification was significantly greater in defects of grafted group rabbits, compared with control group rabbits. On day 14, glycosaminoglycan concentration was significantly higher in tibial defects of grafted group rabbits, compared to defects of control group rabbits or in vitro constructs. At weeks 2, 4, and 8, glycosaminoglycan concentrations were significantly lower in the in vitro control constructs, compared with other groups. Collagen type I was present in bone and bony callous in defects of grafted and control group rabbits. Collagen type II was identified in cartilaginous tissues of grafted and control group rabbits. Collagen type X was associated with hypertrophic chondrocytes. Only type II collagen was found in the in vitro chondrocyte constructs.

CONCLUSIONS AND CLINICAL RELEVANCE

Chondrocyte-agarose grafts are biocompatible in large tibial defects and appear to provide a cell source for augmenting endochondral ossification.

The fate of soft callus chondrocytes during long bone fracture repair

Following fracture, the cartilaginous tissue of the soft callus is eventually replaced by bone. Removal of the cartilage is a critical part of the bone healing process but information concerning the changes in chondrocytes during this process is sparse. The aim of the study was to investigate the fate of chondrocytes in the soft callus during the bone repair process using a rabbit tibial fracture model. Fracture tissue was processed for collagen I-III and keratan sulphate immunohistochemistry to study changes in matrix composition and the TUNEL technique (terminal deoxynucleotidyl transferase medicated dUTP nick-end labelling) to identify death of soft callus chondrocytes. Transmission electron microscopy (TEM) was also carried out to investigate the ultrastructure of chondrocytes within the soft callus. Results showed that the size of the cartilage area decreased over time and that cartilage matrix was replaced with new matrix rich in collagen I and III. Chondrocytes became engulfed in the new matrix and appeared to stop producing cartilage matrix. Chondrocyte cell death was seen at the border of the soft callus, just within the newly produced matrix. TEM revealed that these dying/dead cells were not typically apoptotic in appearance. In conclusion, results indicate that chondrocytes of the soft callus die as a result of the progressive production of bone matrix which eventually engulfs them and leads to the remodelling of the area and eventual bone repair.

Pleiotrophin/Osteoblast-stimulating factor 1: dissecting its diverse functions in bone formation

OSF-1, more commonly known as pleiotrophin (PTN) or heparin-binding growth-associated molecule (HB-GAM), belongs to a new family of secreted HB proteins, which are structurally unrelated to any other growth factor family. The aims of this study were to dissect the diverse functions of PTN in bone formation. The study showed that PTN was synthesized by osteoblasts at an early stage of osteogenic differentiation and was present at sites of new bone formation, where PTN was stored in the new bone matrix. Low concentrations (10 pg/ml) of PTN stimulated osteogenic differentiation of mouse bone marrow cells and had a modest effect on their proliferation, whereas higher concentrations (ng/ml) had no effect. However, PTN did not have the osteoinductive potential of bone morphogenetic proteins (BMPs) because it failed to convert C2C12 cells, a premyoblastic cell line, to the osteogenic phenotype, whereas recombinant human BMP-2 (rhBMP-2) was able to do so. When PTN was present together with rhBMP-2 during the osteoinductive phase, PTN inhibited the BMP-mediated osteoinduction in C2C12 cells at concentrations between 0.05 pg/ml and 100 ng/ml. However, when added after osteoinduction had been achieved, PTN enhanced further osteogenic differentiation. An unusual effect of PTN (50 ng/ml) was the induction of type I collagen synthesis by chondrocytes in organ cultures of chick nasal cartilage and rat growth plates. Thus, PTN had multiple effects on bone formation and the effects were dependent on the concentration of PTN and the timing of its presence. To explain these multiple effects, we propose that PTN is an accessory signaling molecule, which is involved in a variety of processes in bone formation. PTN enhances or inhibits primary responses depending on the prevailing concentrations, the primary stimulus, and the availability of appropriate receptors.

Mechanical tension in distraction osteogenesis regulates chondrocytic differentiation

Differentiation of chondrocytes to cells of osteoblastic phenotype occurs during an interim period of bone development, fracture repair and distraction osteogenesis. To study the relationship between tension-stress and chondrogenesis, uniaxial strains (0 microstrains, 2000 microstrains, 20000 microstrains, 200000 microstrains, 300000 microstrains) were applied in a rabbit model of mandibular distraction osteogenesis. The results demonstrated that cell differentiation, apoptosis and tissue development in the newly formed gap tissue showed a correlation to the applied strain magnitudes. Only strains of 20000 microstrains resulted in a statistically significant (P<0.05) formation of cartilage struts with embedded chondrocyte-like cells. However, chondrocyte-like cells were rarely detected in samples distracted at lower or higher strain magnitudes. Osteoblasts appeared to replace cartilaginous matrix by mineralized bone matrix. The phenotypic change from chondrocytes to osteoblasts was accompanied by a decreased proteoglycan synthesis. a change in the expression from type II collagen towards type I and involved asymmetric cell divisions and apoptotic cell death. Therefore, we suggest that mechanical strain is an external stimulus responsible for phenotypic cell alterations.

Expression of type I, type II, and type X collagen genes during altered endochondral ossification in the femoral epiphysis of osteosclerotic (oc/oc) mice

The osteosclerotic (oc/oc) mouse, a genetically distinct murine mutation that has a functional defect in its osteoclasts, also has rickets and shows an altered endochondral ossification in the epiphyseal growth plate. The disorder is morphologically characterized by an abnormal extension of hypertrophic cartilage at 10 days after birth, which is later (21 days after birth) incorporated into the metaphyseal woven bone without breakdown of the cartilage matrix following vascular invasion of chondrocyte lacunae. In situ hybridization revealed that the extending hypertrophic chondrocytes expressed type I and type II collagen mRNA, as well as that of type X collagen and that the osteoblasts in the metaphysis expressed type II and type X collagen mRNA, in addition to type I collagen mRNA. The topographic distribution of the signals suggests a possible co-expression of each collagen gene in the individual cells. Immunohistochemically, an overlapping deposition of type I, type II, and type X collagen was observed in both the extending cartilage and metaphyseal bony trabeculae. Such aberrant gene expression and synthesis of collagen indicate that pathologic ossification takes place in the epiphyseal/metaphyseal junction of oc/oc mouse femur in different way than in normal endochondral ossification. This abnormality is probably not due to a developmental disorder in the epiphyseal plate but to the failure in conversion of cartilage into bone, since the epiphyseal plate otherwise appeared normal, showing orderly stratified zones with a proper expression of cartilage-specific genes.

Phenotypic stability of articular chondrocytes in vitro: the effects of culture models, bone morphogenetic protein 2, and serum supplementation

Numerous in vitro culture models have been developed for the investigation of chondrocyte and cartilage biology. In this study, we investigated the stability of the chondrocytic phenotype in monolayer, aggregate, pellet, and explant culture models and assessed the effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) and serum supplementation on the phenotype in each model. Phenotypic effects were assessed by analyses of procollagen type II, aggrecan, (V + C)- fibronectin, and procollagen type I messenger RNA expression. In monolayer cultures, we noted a characteristic loss of procollagen type II and induction of procollagen type I expression. The aggregate and pellet culture models supported matrix protein gene expression profiles more reflective of in vivo levels. In explant cultures, expression of matrix protein genes was consistently depressed. Treatment with rhBMP-2 significantly increased the expression of procollagen type II and aggrecan in monolayer cultures; however, other models showed comparatively little response. Similarly, serum supplementation significantly down-regulated procollagen type II and aggrecan expression in monolayer cultures but had less effect on gene expression in the other models. Serum supplementation increased procollagen type I expression in monolayer and aggregate cultures. These results suggest that the influence of exogenous BMP-2 and serum on expression of chondrocyte-specific matrix protein genes is influenced by aspects of substrate attachments, cellular morphology, and/or cytoskeletal organization. Finally, the analyses of fibronectin expression suggest that V and C region alternative splicing in chondrocytes is linked to the establishment of a three-dimensional multicellular complex.

The expression of the fibrillar collagen genes during fracture healing: heterogeneity of the matrices and differentiation of the osteoprogenitor cells

The cells that express the genes for the fibrillar collagens, types I, II, III and V, during callus development in rabbit tibial fractures healing under stable and unstable mechanical conditions were localized. The fibroblast-like cells in the initial fibrous matrix express types I, III and V collagen mRNAs. Osteoblasts, and osteocytes in the newly formed membranous bone under the periosteum, express the mRNAs for types I, III and V collagens, but osteocytes in the mature trabeculae express none of these mRNAs. Cartilage formation starts at 7 days in calluses forming under unstable mechanical conditions. The differentiating chondrocytes express both types I and II collagen mRNAs, but later they cease expression of type I collagen mRNA. Both types I and II collagens were located in the cartilaginous areas. The hypertrophic chondrocytes express neither type I, nor type II, collagen mRNA. Osteocalcin protein was located in the bone and in some cartilaginous regions. At 21 days, irrespective of the mechanical conditions, the callus consists of a layer of bone; only a few osteoblasts lining the cavities now express type I collagen mRNA. We suggest that osteoprogenitor cells in the periosteal tissue can differentiate into either osteoblasts or chondrocytes and that some cells may exhibit an intermediate phenotype between osteoblasts and chondrocytes for a short period. The finding that hypertrophic chondrocytes do not express type I collagen mRNA suggests that they do not transdifferentiate into osteoblasts during endochondral ossification in fracture callus.

Association of matrix acid and alkaline phosphatases with mineralization of cartilage and endochondral bone

The activities of acid and alkaline phosphatases were localized by enzyme histochemistry in the chondroepiphyses of 5 week old rabbits. Using paraformaldehyde-lysine-periodate as fixative, the activity of acid phosphatase was particularly well preserved and could be demonstrated not only in osteoclasts, but also in chondrocytes as well as in the cartilage and early endochondral matrices. The acid phosphatase in the chondrocytes and the matrix was tartrate-resistant, but inhibited by 2 mM sodium fluoride, whereas for osteoclasts 50-100 mM sodium fluoride were required for inhibition. Simultaneous localisation of both acid and alkaline phosphatase activities was possible in tissue that had been fixed in 85% ethanol and processed immediately. In the growth plates of the secondary ossification centre and the physis, there was a sequential localisation of the two phosphatases associated with chondrocyte maturation. The matrix surrounding immature epiphyseal chondrocytes or resting/proliferating growth plate chondrocytes contained weak acid phosphatase activity. Maturing chondrocytes were positive for alkaline phosphatase which spread to the matrix in the pre-mineralizing zone, in a pattern that was consistent with the known location of matrix vesicles. The region of strong alkaline phosphatase activity was the precise region where acid phosphatase activity was reduced. With the onset of cartilage calcification, alkaline phosphatase activity disappeared, but strong acid phosphatase activity was found in close association with the early mineral deposition. Acid phosphatase activity was also present in the matrix of the endochondral bone, but was only found in early spicules which had recently mineralised. The results suggest that alkaline phosphatase activity is required in preparation of mineralization, whereas acid phosphatase activity might have a contributory role during the early progression of mineral formation.

Osteoblast recruitment and bone formation enhanced by cell matrix-associated heparin-binding growth-associated molecule (HB-GAM)

Bone has an enormous capacity for growth, regeneration, and remodeling. This capacity is largely due to induction of osteoblasts that are recruited to the site of bone formation. The recruitment of osteoblasts has not been fully elucidated, though the immediate environment of the cells is likely to play a role via cell- matrix interactions. We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation. Intriguingly, N-syndecan, which acts as a receptor for HB-GAM, is expressed by osteoblasts/osteoblast precursors, whose ultrastructural phenotypes suggest active cell motility. The hypothesis that HB-GAM/N-syndecan interaction mediates osteoblast recruitment, as inferred from developmental studies, was tested using osteoblast-type cells that express N-syndecan abundantly. These cells migrate rapidly to HB-GAM in a haptotactic transfilter assay and in a migration assay where HB-GAM patterns were created on culture wells. The mechanism of migration is similar to that previously described for the HB-GAM-induced migratory response of neurons. Our hypothesis that HB-GAM/N-syndecan interaction participates in regulation of osteoblast recruitment was tested using two different in vivo models: an adjuvant-induced arthritic model and a transgenic model. In the adjuvant-induced injury model, the expression of HB-GAM and of N-syndecan is strongly upregulated in the periosteum accompanying the regenerative response of bone. In the transgenic model, the HB-GAM expression is maintained in mesenchymal tissues with the highest expression in the periosteum. The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness. HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

Expression of bone matrix proteins mRNA during distraction osteogenesis

Distraction osteogenesis is a recently advanced principle of bone lengthening in which a bone separated by osteotomy is subjected to slow progressive distraction using an external fixation device. Appropriate mechanical tension-stress is believed not to break the callus but rather to stimulate osteogenesis. To study the molecular features of this process, the expression and localization of the mRNAs encoding osteopontin (OPN), osteocalcin (OC), matrix Gla protein (MGP), osteonectin (ON), and collagen type I and I during distraction osteogenesis were examined by in situ hybridization and Northern blot analysis. The process can be divided into three distinct phases: the lag phase for 7 days between osteotomy and the beginning of distraction, the distraction phase for 21 days, and the consolidation phase for several weeks. The histologic and molecular events taking place during the lag phase were similar to those observed in fracture healing. The osteotomy site was surrounded by external callus consisting of hyaline cartilage. As distraction started at the rate of 0.25 mm/12 h, the cartilaginous callus was elongated, deformed, and eventually separated into proximal and distal segments. The chondrocytes were stretched along the tension vector and became fibroblast-like in shape. Although morphologically these cells were distinguishable from osteogenic cells, they expressed OPN, OC, and alkaline phosphatase mRNAs. As distraction advanced, the cartilaginous callus was progressively replaced by bony callus by endochondral ossification and thereafter new bone was formed directly by intramembranous ossification. OPN mRNA was detected in preosteoblasts and osteoblasts at the boundary between fibrous tissue and new bone. ON, MGP, and OC mRNAs appeared early in the differentiation stage. The variety of cell types expressing mRNA encoding bone matrix proteins in distraction osteogenesis was much greater than that detected in the embryonic bone formation and fracture healing process. Moreover, the levels of OPN, ON, MGP, and OC mRNA expression markedly increased during the distraction phase. These results suggested that mechanical tension-stress modulates cell shape and phenotype, and stimulates the expression of the mRNA for bone matrix proteins.

Is acid phosphatase activity present in bone matrix at sites of endochondral ossification in rabbit fracture callus?

It has been suggested that acid phosphatase activity is present in newly formed bone matrix at sites of endochondral ossification in rabbit fracture calluses. Because acid phosphatases are usually found intracellularly, it was decided to test this possibility more rigorously. Tissue from 10- and 14-day healing rabbit fractures was subjected to a series of critical tests for acid phosphatases with a pH optimum of 5.0. Fluoride, tartrate and molybdate were used as potential inhibitors of acid phosphatase activity. The effects of several counterstaining protocols were also investigated. A fluoride- and tartrate-resistant acid phosphatase is located in osteoclasts and mononuclear phagocytes. Diffuse staining of the bone matrix is seen, but it is dependent upon the length of incubation in the substrate medium and the distance from the acid phosphatase-reacting cells. It is concluded that the coloration of the bone matrix is probably caused by diffusion of the dye and reaction product and is, therefore, artifactual.

Programmed removal of chondrocytes during endochondral fracture healing

This investigation tested the hypothesis that the removal of chondrocytes during endochondral fracture healing involves an ordered process of programmed cell death. To accomplish this, unilateral closed fractures were created in the femora of 36 Sprague-Dawley rats. The rats were killed in groups of four on days 1, 3, 7, 14, 21, 28, 42, 49, and 56 after fracture. The femora were embedded in paraffin and tested for expression of specific markers of fragmented DNA with use of a terminal deoxyuridyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) technique. To determine the potential for transdifferentiation of chondrocytes to osteoblasts, calluses were also hybridized to detect expression of osteocalcin mRNA. Cell proliferation was assessed by an immunohistochemical detection method for proliferating cell nuclear antigen. A separate group of four rats was killed on day 28 to represent the later stage of the endochondral ossification, and the calluses were examined for cellular morphology with transmission electron microscopy. The results showed a coordination in both time and space of the activities of cellular proliferation and programmed cell death. Cell proliferation was most active in the earlier phases of fracture healing (days 1 through 14), although TUNEL expression was apparent in hypertrophic chondrocytes on day 14 after fracture and persisted until day 28. In the later stages of fracture healing (days 14 through 28), proliferating cell nuclear antigen was no longer synthesized in hard callus (intramembranous bone) and cell removal was the dominant activity in soft callus chondrocytes. Expression of osteocalcin mRNA was detected in osteoblasts but not in hypertrophic chondrocytes or in any other nonosteoblastic cell type. These findings support the hypothesis that the removal of chondrocytes during endochondral fracture healing is part of an ordered transition of tissue types in which the cellular mechanisms are genetically programmed to involve proliferation, maturation, and apoptotic cell death.

Cigarette smoking and bone healing: implications in foot and ankle surgery

Despite the known health risks associated with cigarettes, millions of Americans continue to smoke. Much has been reported on the adverse effects of cigarette smoke on wound healing. Recent experimental work and clinical observation have demonstrated the risk of impaired bone healing associated with cigarette smoking. The authors review the biological aspects of bone healing and analyze how the chemical components of cigarette smoke affect the bone healing process. Laboratory and clinical data are also reviewed. Cessation of cigarette smoking before foot and ankle surgery is recommended by the authors.

Regional and temporal changes in the acoustic properties of fracture callus in secondary bone healing

Controlled fractures were created in the right femora of 17 male Sprague-Dawley rats. The fractured limbs were harvested at 2, 4, 6, and 8 weeks after fracture, fixed, and embedded in polymethylmethacrylate. Midsagittal sections from each animal were evaluated with a scanning acoustic microscope, a device that generates an acoustic impedance map of the scanned material. The impedance of the fracture callus was measured in six regions on each specimen. These regions were chosen in an effort to distinguish between the impedance of the callus formed through intramembranous or endochondral ossification, and we found that the time course of increasing impedance differed for the fracture callus formed through the two pathways. Additionally, we found a significant difference in the mean impedance of the callus at each time period (p < or = 0.0013 for all comparisons), which resulted in an extremely linear relationship (r2 = 0.999) between mean callus impedance and healing time. This experimental model has become a popular choice for the investigation of fracture healing. As such, an accurate determination of the mechanical properties of the fracture callus is often sought. We propose that the implementation of scanning acoustic microscopy in the study of fracture healing may determine the changes in the material properties more accurately than conventional testing methods.

New aspects of endochondral ossification in the chick: chondrocyte apoptosis, bone formation by former chondrocytes, and acid phosphatase activity in the endochondral bone matrix

A detailed histological study of the growth plates from 9- to 20-day-old embryonic chick long bones was carried out with the aim of clarifying the long-debated question of the fate of the hypertrophic chondrocytes. Since resorption in chick bones does not occur synchronously across the plate as it does in mammals, specialized regions develop and the fate of the chondrocyte depends on its location within the growth plate. Where resorption took place, as at the sites of primary vascular invasion or at the main cartilage/marrow interface, chondrocytes underwent apoptosis before the lacunae were opened. In addition, spontaneous apoptosis of chondrocytes occurred at apparently random sites throughout all stages of chondrocyte differentiation. In older chick bones, a thick layer of endochondral bone matrix covered the cartilage edge. This consisted of type I collagen and the typical noncollagenous bone proteins but, in addition, contained tartrate-resistant acid phosphatase in the mineralized matrix. Where such matrix temporarily protected the subjacent cartilage from resorption, chondrocytes differentiated to bone-forming cells and deposited bone matrix inside their lacunae. At sites of first endochondral bone formation, some chondrocytes underwent an asymmetric cell division resulting in one daughter cell which underwent apoptosis, while the other cell remained viable and re-entered the cell cycle. This provided further support for the notion that chondrocytes as well as marrow stromal cells give rise to endochondral osteoblasts.