Angiogenesis and intramembranous osteogenesis

Three-dimensional-printed polycaprolactone scaffolds with interconnected hollow-pipe structures for enhanced bone regeneration

Three-dimensional (3D)-printed scaffolds are widely used in tissue engineering to help regenerate critical-sized bone defects. However, conventional scaffolds possess relatively simple porous structures that limit the delivery of oxygen and nutrients to cells, leading to insufficient bone regeneration. Accordingly, in the present study, perfusable and permeable polycaprolactone scaffolds with highly interconnected hollow-pipe structures that mimic natural micro-vascular networks are prepared by an indirect one-pot 3D-printing method. In vitro experiments demonstrate that hollow-pipe-structured (HPS) scaffolds promote cell attachment, proliferation, osteogenesis and angiogenesis compared to the normal non-hollow-pipe-structured scaffolds. Furthermore, in vivo studies reveal that HPS scaffolds enhance bone regeneration and vascularization in rabbit bone defects, as observed at 8 and 12 weeks, respectively. Thus, the fabricated HPS scaffolds are promising candidates for the repair of critical-sized bone defects.

Application of BMP in Bone Tissue Engineering

At present, bone nonunion and delayed union are still difficult problems in orthopaedics. Since the discovery of bone morphogenetic protein (BMP), it has been widely used in various studies due to its powerful role in promoting osteogenesis and chondrogenesis. Current results show that BMPs can promote healing of bone defects and reduce the occurrence of complications. However, the mechanism of BMP in vivo still needs to be explored, and application of BMP alone to a bone defect site cannot achieve good therapeutic effects. It is particularly important to modify implants to carry BMP to achieve slow and sustained release effects by taking advantage of the nature of the implant. This review aims to explain the mechanism of BMP action in vivo, its biological function, and how BMP can be applied to orthopaedic implants to effectively stimulate bone healing in the long term. Notably, implantation of a system that allows sustained release of BMP can provide an effective method to treat bone nonunion and delayed bone healing in the clinic.

The vascularization paradox of non-union formation

Despite major research efforts to elucidate mechanisms of non-union formation, failed fracture healing remains a common complication in orthopedic surgery. Adequate vascularization has been recognized as a crucial factor for successful bone regeneration, as newly formed microvessels guarantee the supply of the callus tissue with vital oxygen, nutrients, and growth factors. Accordingly, a vast number of preclinical studies have focused on the development of vascularization strategies to stimulate fracture repair. However, recent evidence suggests that stimulation of blood vessel formation is an oversimplified approach to support bone regeneration. This review discusses the role of vascularization during bone regeneration and delineates a phenomenon, for which we coin the term “the vascularization paradox of non-union-formation”. This view is based on the results of a variety of experimental studies that suggest that the callus tissue of non-unions is indeed densely vascularized and that pro-angiogenic mediators, such as vascular endothelial growth factor, are sufficiently expressed at the facture site. By gaining further insights into the molecular and cellular basis of non-union vascularization, it may be possible to develop more optimized treatment approaches or even prevent the non-union formation in the future.

Understanding Reactive Oxygen Species in Bone Regeneration: A Glance at Potential Therapeutics and Bioengineering Applications

Although the complex mechanism by which skeletal tissue heals has been well described, the role of reactive oxygen species (ROS) in skeletal tissue regeneration is less understood. It has been widely recognized that a high level of ROS is cytotoxic and inhibits normal cellular processes. However, with more recent discoveries, it is evident that ROS also play an important, positive role in skeletal tissue repair, specifically fracture healing. Thus, dampening ROS levels can potentially inhibit normal healing. On the same note, pathologically high levels of ROS cause a sharp decline in osteogenesis and promote nonunion in fracture repair. This delicate balance complicates the efforts of therapeutic and engineering approaches that aim to modulate ROS for improved tissue healing. The physiologic role of ROS is dependent on a multitude of factors, and it is important for future efforts to consider these complexities. This review first discusses how ROS influences vital signaling pathways involved in the fracture healing response, including how they affect angiogenesis and osteogenic differentiation. The latter half glances at the current approaches to control ROS for improved skeletal tissue healing, including medicinal approaches, cellular engineering, and enhanced tissue scaffolds. This review aims to provide a nuanced view of the effects of ROS on bone fracture healing which will inspire novel techniques to optimize the redox environment for skeletal tissue regeneration.

Osteogenic and angiogenic profiles of the palatal process of the maxilla and the palatal process of the palatine bone

Hard palate consists anteriorly of the palatal process of the maxilla (ppmx) and posteriorly of the palatal process of the palatine (ppp). Currently, palatal osteogenesis is receiving increasing attention. This is the first study to provide an overview of the osteogenesis process of the mouse hard palate. We found that the period in which avascular mesenchymal condensation becomes a vascularized bone structure corresponds to embryonic day (E) 14.5 to E16.5 in the hard palate. The ppmx and ppp differ remarkably in morphology and molecular respects during osteogenesis. Osteoclasts in the ppmx and ppp are heterogeneous. There was a multinucleated giant osteoclast on the bone surface at the lateral-nasal side of the ppmx, while osteoclasts in the ppp were more abundant and adjacent to blood vessels but were smaller and had fewer nuclei. In addition, bone remodeling in the hard palate was asymmetric and exclusively occurred on the nasal side of the hard palate at E18.5. During angiogenesis, CD31-positive endothelial cells were initially localized in the surrounding of palatal mesenchymal condensation and then invaded the condensation in a sprouting fashion. At the transcriptome level, we found 78 differentially expressed genes related to osteogenesis and angiogenesis between the ppmx and ppp. Fifty-five related genes were up/downregulated from E14.5 to E16.5. Here, we described the morphogenesis and the heterogeneity in the osteogenic and angiogenic genes profiles of the ppmx and ppp, which are significant for subsequent studies of normal and abnormal subjects.

Immunomodulatory effects and mechanisms of distraction osteogenesis

Distraction osteogenesis (DO) is widely used for bone tissue engineering technology. Immune regulations play important roles in the process of DO like other bone regeneration mechanisms. Compared with others, the immune regulation processes of DO have their distinct features. In this review, we summarized the immune-related events including changes in and effects of immune cells, immune-related cytokines, and signaling pathways at different periods in the process of DO. We aim to elucidated our understanding and unknowns about the immunomodulatory role of DO. The goal of this is to use the known knowledge to further modify existing methods of DO, and to develop novel DO strategies in our unknown areas through more detailed studies of the work we have done.

Extracorporeal shockwave relieves endothelial injury and dysfunction in steroid-induced osteonecrosis of the femoral head via miR-135b targeting FOXO1: in vitro and in vivo studies

Injury and dysfunction of endothelial cells (ECs) are closely related to the pathogenesis of steroid-induced osteonecrosis of the femoral head (ONFH), while MicroRNAs (miRNAs) play an essential role in the processes. Extracorporeal shockwave treatment (ESWT) has been used in the non-invasive treatment of various diseases including musculoskeletal and vascular disorders. In particular, ESWT with low energy levels showed a beneficial effect in ischemic tissues. However, there has been no comprehensive assessment of the effect of ESWT and miRNAs on steroid-induced ONFH. In the present study, we investigated the role and mechanism of ESWT and miRNAs both in vitro and in vivo. Using a steroid-induced ONFH rat model, we found that ESWT significantly enhances proliferation and angiogenesis as well as alleviates apoptosis. In two types of ECs, ESWT can promote cell proliferation and migration, enhance angiogenesis, and inhibit apoptosis. Notably, our study demonstrates that miR-135b is downregulated and modulated forkhead box protein O1 (FOXO1) in ECs treated with dexamethasone. Remarkably, both miR-135b knockdown and FOXO1 overexpression reversed the beneficial effect of ESWT on ECs. Additionally, our data suggest that ESWT activates the FOXO1-related pathway to impact proliferation, apoptosis, and angiogenesis. Taken together, this study indicates that ESWT relieves endothelial injury and dysfunction in steroid-induced ONFH via miR-135b targeting FOXO1.

Effect of Hypoxia-Inducible Factor-1α on Osteogenesis of Titanium Dioxide Nanotube Bone Marrow Mesenchymal Stem Cells with Different Diameters Under Periodic Tensile Stress

Bone marrow mesenchymal stem cells (BMSC) have the ability to multi polarize with multiple tropisms and participate in tissue remodeling. This study assessed the effect of titanium dioxide nanotubes with different diameters on ossification of BMSC cells and HIF-1α expression in BMSC ossification. Titanium dioxide nanotubes with different diameters were prepared and then the following groups were set according to the size of pressure; Ti group, NT10 group, NT30 group, and NT60 group. Analysis of cell morphology was done by fluorescence microscope, while adhesion and proliferation were assessed by MTT assay. Moreover, ALP activity, collagen secretion and outer matrix mineralization and expression of HIF-1α, VEGF, and TWIST were assessed by RT-PCR and Western blot. The P3 generation of BMSC cells was successfully obtained. Three types of nanotubes were arranged regularly and contact angle showed NT60<NT30<NT10<Ti, protein adsorption presented NT10>Ti>NT30>NT60 (P < 0.05). Cells from NT30 and N60 groups showed obvious expansion with pseudopodia and pseudo plates of cells. Cell adhesion showed changes in sizes of NT10>Ti>NT30>NT66. NT60 group showed lower cell proliferation and higher ALP activity and collagen secretion than the other groups. NT30 and NT60 group presented higher mineralization level, larger diameter, and higher degree of promotion. The NT30 group presented lowest content of HIF-1α (0.12 ± 0.03), VEGF (0.013 ± 0.004), and TWIST (0.014 ± 0.003). Inoculation of BMSCs on titanium dioxide nanotubes of different diameters under cyclical tensile stress environment can promote growth of BMSC cells in a diameter-dependent manner.

Diversity of Vascular Niches in Bones and Joints During Homeostasis, Ageing, and Diseases

The bones and joints in the skeletal system are composed of diverse cell types, including vascular niches, bone cells, connective tissue cells and mineral deposits and regulate whole-body homeostasis. The capacity of maintaining strength and generation of blood lineages lies within the skeletal system. Bone harbours blood and immune cells and their progenitors, and vascular cells provide several immune cell type niches. Blood vessels in bone are phenotypically and functionally diverse, with distinct capillary subtypes exhibiting striking changes with age. The bone vasculature has a special impact on osteogenesis and haematopoiesis, and dysregulation of the vasculature is associated with diverse blood and bone diseases. Ageing is associated with perturbed haematopoiesis, loss of osteogenesis, increased adipogenesis and diminished immune response and immune cell production. Endothelial and perivascular cells impact immune cell production and play a crucial role during inflammation. Here, we discuss normal and maladapted vascular niches in bone during development, homeostasis, ageing and bone diseases such as rheumatoid arthritis and osteoarthritis. Further, we discuss the role of vascular niches during bone malignancy.

Calcification and resorption of mouse Meckel's cartilage analyzed by von Kossa and tartrate-resistant acid phosphatase histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry

Meckel's cartilage is essential for the normal development of the mandible. The fate of the intermediate portion of Meckel's cartilage is unique as most of it disappears soon after birth except for the part that forms the sphenomandibular ligament. The mechanism of the disappearance of Meckel's cartilage is unknown; therefore, this study was designed to investigate the process of Meckel's cartilage degradation, focusing on cartilage matrix calcification and the appearance of chondroclasts. Developing mouse mandibles at embryonic days 15, 16, 17, and 18, and postnatal day 2 were processed for whole-mount staining with alcian blue and alizarin red. The mandibles on embryonic days 15, 16, 17, and 18 were fixed and embedded in paraffin. Adjacent sections were processed for von Kossa and tartrate-resistant acid phosphatase (TRAP) histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS). Calcification and the element concentrations of calcium, phosphorus, and carbon were examined with von Kossa histochemistry and SEM/EDS. The involvement of chondroclasts was investigated using TRAP histochemistry. The results demonstrated that the intermediate portion of Meckel's cartilage is resorbed by chondroclasts after chondrocyte hypertrophy and cartilage matrix calcification and that the mineral concentration of calcified Meckel's cartilage is comparable to that of the surrounding bone. This study contributes to the understanding of the mechanism of Meckel's cartilage resorption and provides useful insights into the development of the mandible.

A timeseries analysis of the fracture callus extracellular matrix proteome during bone fracture healing

While most bones fully self-heal, certain diseases require bone allograft to assist with fracture healing. Bone allografts offer promise as treatments for such fractures due to their osteogenic properties. However, current bone allografts made of decellularized bone extracellular matrix (ECM) have high failure rates, and thus grafts which improve fracture healing outcomes are needed. Understanding specific changes to the ECM proteome during normal fracture healing would enable the identification of key proteins that could be used enhance osteogenicity of bone allograft. Here, we performed a timeseries analysis of the fracture callus in mice to investigate proteomic and mineralization changes to the ECM at key stages of fracture healing. We found that changes to the ECM proteome largely coincide with the distinct phases of fracture healing. Basement membrane proteins (AGRN, COL4, LAMA), cartilage proteins (COL2A1, ACAN), and collagen crosslinking enzymes (LOXL, PLOD, ITIH) were initially upregulated, followed by bone specific proteoglycans and collagens (IBSP, COL1A1). Various tissue proteases (MMP2, 9, 13, 14; CTSK, CTSG, ELANE) were expressed at different levels throughout fracture healing. These changes coordinated with mineralization of the fracture callus, which increased steeply during the initial stages of healing. Interestingly the later timepoint was characterized by a response to wound healing and high expression of clotting factors (F2, 7, 9, 10). We identified ELANE and ITIH2 as tissue remodeling enzymes having no prior known involvement with fracture healing. This data can be further mined to identify regenerative proteins for enhanced bone graft design.

Relating multivariate shapes to genescapes using phenotype-biological process associations for craniofacial shape

Realistic mappings of genes to morphology are inherently multivariate on both sides of the equation. The importance of coordinated gene effects on morphological phenotypes is clear from the intertwining of gene actions in signaling pathways, gene regulatory networks, and developmental processes underlying the development of shape and size. Yet, current approaches tend to focus on identifying and localizing the effects of individual genes and rarely leverage the information content of high-dimensional phenotypes. Here, we explicitly model the joint effects of biologically coherent collections of genes on a multivariate trait – craniofacial shape – in a sample of n = 1145 mice from the Diversity Outbred (DO) experimental line. We use biological process Gene Ontology (GO) annotations to select skeletal and facial development gene sets and solve for the axis of shape variation that maximally covaries with gene set marker variation. We use our process-centered, multivariate genotype-phenotype (process MGP) approach to determine the overall contributions to craniofacial variation of genes involved in relevant processes and how variation in different processes corresponds to multivariate axes of shape variation. Further, we compare the directions of effect in phenotype space of mutations to the primary axis of shape variation associated with broader pathways within which they are thought to function. Finally, we leverage the relationship between mutational and pathway-level effects to predict phenotypic effects beyond craniofacial shape in specific mutants. We also introduce an online application that provides users the means to customize their own process-centered craniofacial shape analyses in the DO. The process-centered approach is generally applicable to any continuously varying phenotype and thus has wide-reaching implications for complex trait genetics.

Electrospun Fibers Improving Cellular Respiration via Mitochondrial Protection

Cellular respiration is the prerequisite for cell survival and functions, and mitochondrial function and microcirculation oxygen supply are essential for cellular respiration. However, in diabetic fracture, cellular respiration of bone marrow stem cells (BMSCs) is disrupted because of the dysfunction of mitochondria and microcirculation disorders. Here, the electrospun fibers of GelMA loaded with Hif-1 pathway activator (DFO) are constructed to improve the cellular respiration of BMSCs via protecting mitochondrial function and reconstructing microcirculation. The sequential process of electrospinning and UV crosslinking endowed the electrospun fibers with breathability and the biomechanical properties like the periosteum. In vitro biomolecular experiments showed that by crosslinking grafted polyethylene glycol acrylate liposomes loaded with DFO, the functional electrospun fibers can release DFO locally to activate Hif-1 in BMSCs, which can regulate the balance of Bcl-2/Bax to protect mitochondria and upregulate the expression of VEGF to reconstruct microcirculation. Animal experiments confirmed that the functional electrospun fibers can promote the recovery of diabetic fracture in vivo. These suggested that the functional electrospun fibers can improve cellular respiration for cell survival and functions of BMSCs. This study provides a new treatment strategy for diabetic fracture and other tissue regeneration on basis of cellular respiration improvement.

Quantitative 3D imaging of the cranial microvascular environment at single-cell resolution

Vascularization is critical for skull development, maintenance, and healing. Yet, there remains a significant knowledge gap in the relationship of blood vessels to cranial skeletal progenitors during these processes. Here, we introduce a quantitative 3D imaging platform to enable the visualization and analysis of high-resolution data sets (>100 GB) throughout the entire murine calvarium. Using this technique, we provide single-cell resolution 3D maps of vessel phenotypes and skeletal progenitors in the frontoparietal cranial bones. Through these high-resolution data sets, we demonstrate that CD31hiEmcnhi vessels are spatially correlated with both Osterix+ and Gli1+ skeletal progenitors during postnatal growth, healing, and stimulated remodeling, and are concentrated at transcortical canals and osteogenic fronts. Interestingly, we find that this relationship is weakened in mice with a conditional knockout of PDGF-BB in TRAP+ osteoclasts, suggesting a potential role for osteoclasts in maintaining the native cranial microvascular environment. Our findings provide a foundational framework for understanding how blood vessels and skeletal progenitors spatially interact in cranial bone, and will enable more targeted studies into the mechanisms of skull disease pathologies and treatments. Additionally, our technique can be readily adapted to study numerous cell types and investigate other elusive phenomena in cranial bone biology.

Betaine Attenuates Osteoarthritis by Inhibiting Osteoclastogenesis and Angiogenesis in Subchondral Bone

Osteoarthritis (OA) is the most common type of arthritis with no effective therapy. Subchondral bone and overlying articular cartilage are closely associated and function as “osteo-chondral unit” in the joint. Abnormal mechanical load leads to activated osteoclast activity and increased bone resorption in the subchondral bone, which is implicated in the onset of OA pathogenesis. Thus, inhibiting subchondral bone osteoclast activation could prevent OA onset. Betaine, isolated from the Lycii Radicis Cortex (LRC), has been demonstrated to exert anti-inflammatory, antifibrotic and antiangiogenic properties. Here, we evaluated the effects of betaine on anterior cruciate ligament transection (ACLT)-induced OA mice. We observed that betaine decreased the number of matrix metalloproteinase 13 (MMP-13)-positive and collagen X (Col X)-positive cells, prevented articular cartilage proteoglycan loss and lowered the OARSI score. Betaine decreased the thickness of calcified cartilage and increased the expression level of lubricin. Moreover, betaine normalized uncoupled subchondral bone remodeling as defined by lowered trabecular pattern factor (Tb.pf) and increased subchondral bone plate thickness (SBP). Additionally, aberrant angiogenesis in subchondral bone was blunted by betaine treatment. Mechanistically, we demonstrated that betaine suppressed osteoclastogenesis in vitro by inhibiting reactive oxygen species (ROS) production and subsequent mitogen-activated protein kinase (MAPK) signaling. These data demonstrated that betaine attenuated OA progression by inhibiting hyperactivated osteoclastogenesis and maintaining microarchitecture in subchondral bone.

Periosteal medial femoral condyle free flap for metacarpal nonunion

Background

Metacarpal nonunion is a rare condition. The osteogenic capacity of periosteal free flap was investigated in five patients with metacarpal nonunion and impaired bone vascularization.

Patients and methods

Surgery was performed between 64 and 499 days after the initial bone osteosynthesis. The average age was 27.6 (range 16–32) years. Nonunion was caused by excessive periosteal removal in two patients, extensive open trauma in three. Four nonunions were diaphyseal, one metaphyseal. A periosteal medial femoral condyle free flap was raised on the descending genicular artery for four patients, the superomedial genicular artery for one. After osteosynthesis with a plate, the flap was wrapped around the metacarpal, overlapping the bone proximally and distally. The recipient vessel were the dorsal branch of the radial artery and a vena comitans in the anatomical snuffbox in four patients, at the base of the first webspace in one.

Results

The flap size ranged from 5 × 3.5 cm to 8 × 4 cm. No postoperative complication occurred. Radiological bone union was achieved 3 to 8 months after surgery. One patient had a full range of motion, one a slight extension lag of the proximal interphalangeal joint, two moderate joint stiffness of the proximal interphalangeal or metacarpophalangeal joint (one requiring plate removal and extensor tenolysis), one severe stiffness that allowed using a hook grip which was the aim of the surgery.

Conclusion

In case of metacarpal nonunion with impaired bone vascularization, the periosteal medial femoral condyle free flap provides an effective and biomimetic approach to bone healing.

Myeloproliferative Disorders and its Effect on Bone Homeostasis: The Role of Megakaryocytes

Myeloproliferative Neoplasms (MPNs) are a heterogeneous group of chronic hematological diseases that arise from the clonal expansion of abnormal hematopoietic stem cells, of which Polycythemia Vera (PV), Essential Thrombocythemia (ET), and Primary Myelofibrosis (PMF) have been extensively reviewed in context of clonal expansion, fibrosis and other phenotypes. Here, we review current knowledge on the influence of different forms of MPN on bone health. Studies implicated various degrees of effect of different forms of MPN on bone density, and on osteoblast proliferation and differentiation, using murine models and human data. The majority of studies show that bone volume is generally increased in PMF patients, whereas it is slightly decreased or not altered in ET and PV patients, although possible differences between male and female phenotypes were not fully explored in most MPN forms. Osteosclerosis seen in PMF patients is a serious complication that can lead to bone marrow failure, and the loss of bone reported in some ET and PV patients can lead to osteoporotic fractures. Some MPN forms are associated with increased number of megakaryocytes (MKs), and several of the MK-associated factors in MPN are known to affect bone development. Here, we review known mechanisms involved in these processes, with focus on the role of MKs and secreted factors. Understanding MPN-associated changes in bone health could improve early intervention and treatment of this side effect of the pathology.

Low level laser therapy promotes bone regeneration by coupling angiogenesis and osteogenesis

BACKGROUND

Bone tissue engineering is a new concept bringing hope for the repair of large bone defects, which remains a major clinical challenge. The formation of vascularized bone is key for bone tissue engineering. Growth of specialized blood vessels termed type H is associated with bone formation. In vivo and in vitro studies have shown that low level laser therapy (LLLT) promotes angiogenesis, fracture healing, and osteogenic differentiation of stem cells by increasing reactive oxygen species (ROS). However, whether LLLT can couple angiogenesis and osteogenesis, and the underlying mechanisms during bone formation, remains largely unknown.

METHODS

Mouse bone marrow mesenchymal stem cells (BMSCs) combined with biphasic calcium phosphate (BCP) grafts were implanted into C57BL/6 mice to evaluate the effects of LLLT on the specialized vessel subtypes and bone regeneration in vivo. Furthermore, human BMSCs and human umbilical vein endothelial cells (HUVECs) were co-cultured in vitro. The effects of LLLT on cell proliferation, angiogenesis, and osteogenesis were assessed.

RESULTS

LLLT promoted the formation of blood vessels, collagen fibers, and bone tissue and also increased CD31^hiEMCN^hi-expressing type H vessels in mBMSC/BCP grafts implanted in mice. LLLT significantly increased both osteogenesis and angiogenesis, as well as related gene expression (HIF-1α, VEGF, TGF-β) of grafts in vivo and of co-cultured BMSCs/HUVECs in vitro. An increase or decrease of ROS induced by H₂O₂ or Vitamin C, respectively, resulted in an increase or decrease of HIF-1α, and a subsequent increase and decrease of VEGF and TGF-β in the co-culture system. The ROS accumulation induced by LLLT in the co-culture system was significantly decreased when HIF-1α was inhibited with DMBPA and was followed by decreased expression of VEGF and TGF-β.

CONCLUSIONS

LLLT enhanced vascularized bone regeneration by coupling angiogenesis and osteogenesis. ROS/HIF-1α was necessary for these effects of LLLT. LLLT triggered a ROS-dependent increase of HIF-1α, VEGF, and TGF-β and resulted in subsequent formation of type H vessels and osteogenic differentiation of mesenchymal stem cells. As ROS also was a target of HIF-1α, there may be a positive feedback loop between ROS and HIF-1α, which further amplified HIF-1α induction via the LLLT-mediated ROS increase. This study provided new insight into the effects of LLLT on vascularization and bone regeneration in bone tissue engineering.

Effects of nanocrystalline hydroxyapatite concentration and skeletal site on bone and cartilage formation in rats

Most fractures heal by a combination of endochondral and intramembranous ossification dependent upon strain and vascularity at the fracture site. Many biomaterials-based bone regeneration strategies rely on the use of calcium phosphates such as nano-crystalline hydroxyapatite (nHA) to create bone-like scaffolds. In this study, nHA was dispersed in reactive polymers to form composite scaffolds that were evaluated both in vitro and in vivo. Matrix assays, immunofluorescent staining, and Western blots demonstrated that nHA influenced mineralization and subsequent osteogenesis in a dose-dependent manner in vitro. Furthermore, nHA dispersed in polymeric composites promoted osteogenesis by a similar mechanism as particulated nHA. Scaffolds were implanted into a 2-mm defect in the femoral diaphysis or metaphysis of Sprague-Dawley rats to evaluate new bone formation at 4 and 8 weeks. Two formulations were tested: a poly(thioketal urethane) scaffold without nHA (PTKUR) and a PTKUR scaffold augmented with 22 wt% nHA (22nHA). The scaffolds supported new bone formation in both anatomic sites. In the metaphysis, augmentation of scaffolds with nHA promoted an intramembranous healing response. Within the diaphysis, nHA inhibited endochondral ossification. Immunohistochemistry was performed on cryo-sections of the bone/scaffold interface in which CD146, CD31, Endomucin, CD68, and Myeloperoxidase were evaluated. No significant differences in the infiltrating cell populations were observed. These findings suggest that nHA dispersed in polymeric composites induces osteogenic differentiation of adherent endogenous cells, which has skeletal site-specific effects on fracture healing. STATEMENT OF SIGNIFICANCE: Understanding the mechanism by which synthetic scaffolds promote new bone formation in preclinical models is crucial for bone regeneration applications in the clinic where complex fracture cases are seen. In this study, we found that dispersion of nHA in polymeric scaffolds promoted in vitro osteogenesis in a dose-dependent manner through activation of the PiT1 receptor and subsequent downstream Erk1/2 signaling. While augmentation of polymeric scaffolds with nHA enhanced intramembranous ossification in metaphyseal defects, it inhibited endochondral ossification in diaphyseal defects. Thus, our findings provide new insights into designing synthetic bone grafts that complement the skeletal site-specific fracture healing response.

Anti-VEGF antibody delivered locally reduces bony bar formation following physeal injury in rats

Physeal injuries can result in the formation of a "bony bar" which can lead to bone growth arrest and deformities in children. Vascular endothelial growth factor (VEGF) has been shown to play a role in bony bar formation, making it a potential target to inhibit bony repair tissue after physeal injury. The goal of this study was to investigate whether the local delivery of anti-VEGF antibody (α-VEGF; 7.5 μg) from alginate:chitosan hydrogels to the tibial physeal injury site in rats prevents bony bar formation. We tested the effects of quick or delayed delivery of α-VEGF using both 90:10 and 50:50 ratio alginate:chitosan hydrogels, respectively. Male and female 6-week-old Sprague-Dawley rats received a tibial physeal injury and the injured site injected with alginate-chitosan hydrogels: (1) 90:10 (Quick Release); (2) 90:10 + α-VEGF (Quick Release + α-VEGF); (3) 50:50 (Slow Release); (4) 50:50 +  α-VEGF (Slow Release +  α-VEGF); or (5) Untreated. At 2, 4, and 24 weeks postinjury, animals were euthanized and tibiae assessed for bony bar and vessel formation, repair tissue type, and limb lengthening. Our results indicate that Quick Release + α-VEGF reduced bony bar and vessel formation, while also increasing cartilage repair tissue. Further, the quick release of α-VEGF neither affected limb lengthening nor caused deleterious side-effects in the adjacent, uninjured physis. This α-VEGF treatment, which inhibits bony bar formation without interfering with normal bone elongation, could have positive implications for children suffering from physeal injuries.

Ridge preservation applying a novel hydrogel for early angiogenesis and osteogenesis evaluation: an experimental study in canine

Ridge preservation is universally acknowledged as the conventional method for the post-extraction healing yet there are no standard materials for the ideal healing outcome. Herein, a composite gel comprising gelatin nanoparticles (GNPs) and injectable platelet-rich-fibrin (i-PRF) as the potential candidate for extracted socket healing is introduced. The combination of GNPs and i-PRF not only possesses favorable mechanical properties to withstand external force but also accelerate the blood clotting time significantly. In addition, six beagle dogs were adopted to assess the angiogenic and osteogenic capacity of GNPs+i-PRF gel in vivo. The GNPs+i-PRF gel significantly produced the most blood vessels area, woven bone and low osteoclast activity in extracted sockets at 2 weeks postoperation and remarkably generated corticalization on the alveolar ridge crest at 8 weeks postoperation according to histological results. Therefore, GNPs+i-PRF gel can be recommended as the candidate grafting material regarding ridge preservation for its cost effectiveness, excellent biocompatibility, facilitation of blood clotting and favorable capacity of promoting angiogenesis and osteogenesis.

Vascularization Strategies in Bone Tissue Engineering

Bone is a highly vascularized tissue, and its development, maturation, remodeling, and regeneration are dependent on a tight regulation of blood vessel supply. This condition also has to be taken into consideration in the context of the development of artificial tissue substitutes. In classic tissue engineering, bone-forming cells such as primary osteoblasts or mesenchymal stem cells are introduced into suitable scaffolds and implanted in order to treat critical-size bone defects. However, such tissue substitutes are initially avascular. Because of the occurrence of hypoxic conditions, especially in larger tissue substitutes, this leads to the death of the implanted cells. Therefore, it is necessary to devise vascularization strategies aiming at fast and efficient vascularization of implanted artificial tissues. In this review article, we present and discuss the current vascularization strategies in bone tissue engineering. These are based on the use of angiogenic growth factors, the co-implantation of blood vessel forming cells, the ex vivo microfabrication of blood vessels by means of bioprinting, and surgical methods for creating surgically transferable composite tissues.

Spatio-temporal immunolocalization of VEGF-A, Runx2, and osterix during the early steps of intramembranous ossification of the alveolar process in rat embryos

Vascular endothelial growth factor A (VEGF-A) is expressed by several cell types and is a crucial factor for angiogenic-osteogenic coupling. However, the immunolocalization of VEGF-A during the early stages of the alveolar process formation remains underexplored. Thus, we analyzed the spatio-temporal immunolocalization of VEGF-A and its relationship with Runt-related transcription factor 2 (Runx2) and osterix (Osx) during the early steps of intramembranous ossification of the alveolar process in rat embryos. Embryo heads (E) of 16, 18 and 20-day-old rats were processed for paraffin embedding. Histomorphometry and immunohistochemistry to detect VEGF-A, Runx2, and Osx (osteoblast differentiation markers) were performed. The volume density of bone tissue including bone cells and blood vessels increased significantly in E18 and E20. Cells showing high VEGF-A immunoreactivity were initially observed within a perivascular niche in the ectomesenchyme; afterwards, these cells were diffusely located near bone formation sites. Runx2-and Osx-immunopositive cells were observed in corresponded regions of cells showing strong VEGF-A immunoreactivity. Although these immunostained cells were observed in all specimens, this immunolocalization pattern was more evident in E16 specimens and gradually decreased in E18 and E20 specimens. Double immunofluorescence labelling showed intracellular co-localization of Osx and VEGF-A in cells surrounding the developing alveolar process, indicating a crucial role of VEGF-A in osteoblast differentiation. Our results showed VEGF-A immunoexpression in osteoblasts and its precursors during the maxillary alveolar process formation of rat embryos. Moreover, the VEGF-A-positive cells located within a perivascular niche at the early stages of the alveolar process development suggest a crosstalk between endothelium and ectomesenchymal cells, reinforcing the angiogenic-osteogenic coupling in this process.

Systemic therapy of MSCs in bone regeneration: a systematic review and meta-analysis

Objectives

Over the past decades, many studies focused on mesenchymal stem cells (MSCs) therapy for bone regeneration. Due to the efficiency of topical application has been widely dicussed and systemic application was also a feasible way for new bone formation, the aim of this study was to systematically review systemic therapy of MSCs for bone regeneration in pre-clinical studies.

Methods

The article search was conducted in PubMed and Embase databases. Original research articles that assessed potential effect of systemic application of MSCs for bone regeneration in vivo were selected and evaluated in this review, according to eligibility criteria. The efficacy of MSC systemic treatment was analyzed by random effects meta-analysis, and the outcomes were expressed in standard mean difference (SMD) and its 95% confidence interval. Subgroup analyses were conducted on animal species and gender, MSCs types, frequency and time of injection, and bone diseases.

Results

Twenty-three articles were selected in this review, of which 21 were included in meta-analysis. The results showed that systemic therapy increased bone mineral density (SMD 3.02 [1.84, 4.20]), bone volume to tissue volume ratio (2.10 [1.16, 3.03]), and the percentage of new bone area (7.03 [2.10, 11.96]). Bone loss caused by systemic disease tended to produce a better response to systemic treatment (p=0.05 in BMD, p=0.03 in BV/TV).

Conclusion

This study concluded that systemic therapy of MSCs promotes bone regeneration in preclinical experiments. These results provided important information for the systemic application of MSCs as a potential application of bone formation in further animal experiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02456-w.

Induced Pluripotent Stem Cell-Derived Endothelial Networks Accelerate Vascularization But Not Bone Regeneration

Vascularization is critical for engineering mineralized tissues. It has been previously shown that biomaterials containing preformed endothelial networks anastomose to host vasculature following implantation. However, the networks alone may not increase regeneration. In addition, a clinically applicable source of cells for vascularization is needed. In this study, vascular networks were generated from endothelial cells (ECs) derived from human induced pluripotent stem cells (iPSCs). Network formation by iPSC-ECs within fibrin gels was investigated in a mesenchymal stem cells (MSCs) coculture spheroid model. Statistical design of experiments technique was evaluated for its predicting capability during the optimization of experimental parameters. The prevascularized units were combined with hydroxyapatite nanoparticles to develop a vascularized composite hydrogel that was implanted in a rodent critical-sized cranial defect model. Immunohistological staining for human-specific CD31 at week 1 indicated the presence and maintenance of the implanted vessels. At 8 weeks, the prevascularized systems resulted in higher vessel density over MSC-only scaffolds. The implanted vessels appeared to establish flow with host vasculature. While there was a slight increase in bone volume in the prevascularized bone construct compared to MSC-only bone constructs, there was not a profound increase in bone regeneration. These results show that scaffolds with network structures can be generated from ECs derived from iPSC and that the networks survive and inosculate with the host postimplantation in a bone model. Impact statement Vascularization is critical for engineering bone. Prevascularized scaffolds have been shown to improve postimplantation vascularization. Herein, vascularized networks were generated from induced pluripotent cells derived from endothelial cells. These vascularized units were combined with a fibrin/hydroxyapatite scaffold to develop a prevascularized construct for bone regeneration. Implantation of these scaffolds in a small animal cranial defect model resulted in network inosculation and increased vascularization, but exhibited only a limited effect on bone formation. This study provides insight into the challenges of generating vascularized bone.

CSF1R-dependent macrophages control postnatal somatic growth and organ maturation

Homozygous mutation of the Csf1r locus (Csf1rko) in mice, rats and humans leads to multiple postnatal developmental abnormalities. To enable analysis of the mechanisms underlying the phenotypic impacts of Csf1r mutation, we bred a rat Csf1rko allele to the inbred dark agouti (DA) genetic background and to a Csf1r-mApple reporter transgene. The Csf1rko led to almost complete loss of embryonic macrophages and ablation of most adult tissue macrophage populations. We extended previous analysis of the Csf1rko phenotype to early postnatal development to reveal impacts on musculoskeletal development and proliferation and morphogenesis in multiple organs. Expression profiling of 3-week old wild-type (WT) and Csf1rko livers identified 2760 differentially expressed genes associated with the loss of macrophages, severe hypoplasia, delayed hepatocyte maturation, disrupted lipid metabolism and the IGF1/IGF binding protein system. Older Csf1rko rats developed severe hepatic steatosis. Consistent with the developmental delay in the liver Csf1rko rats had greatly-reduced circulating IGF1. Transfer of WT bone marrow (BM) cells at weaning without conditioning repopulated resident macrophages in all organs, including microglia in the brain, and reversed the mutant phenotypes enabling long term survival and fertility. WT BM transfer restored osteoclasts, eliminated osteopetrosis, restored bone marrow cellularity and architecture and reversed granulocytosis and B cell deficiency. Csf1rko rats had an elevated circulating CSF1 concentration which was rapidly reduced to WT levels following BM transfer. However, CD43hi non-classical monocytes, absent in the Csf1rko, were not rescued and bone marrow progenitors remained unresponsive to CSF1. The results demonstrate that the Csf1rko phenotype is autonomous to BM-derived cells and indicate that BM contains a progenitor of tissue macrophages distinct from hematopoietic stem cells. The model provides a unique system in which to define the pathways of development of resident tissue macrophages and their local and systemic roles in growth and organ maturation.

DESARROLLO Y CRECIMIENTO FEMORAL DE CONEJAS DE LA LÍNEA NEW ZEALAND

En este proyecto se investigan los cambios que acontecen en el desarrollo y crecimiento de conejos hembras de la línea New Zealand (CoNZ) en sus huesos femorales. Los animales fueron mantenidas en jaulas individuales desde las 2 semanas de edad, con comida  y agua ad libitum y se sacrificaron en tiempos mensualmente consecutivos: 1, 2, 3, 4, 5, y 6 meses. Tras la obtención de las piezas femorales, y a partir de estudios imagenológicos se  determinaron los ángulos del cuello femoral (Af), la longitud total (L), la densidad mineral ósea total, del centro óseo y de la metáfisis femoral (DMOt, DMOco y DMOmf respectivamente), analizándose las variaciones intergrupales por el test Wilcoxon, y corrección de Bonferroni. Se realizaron estudios histológicos de los cortes descalcificados de las piezas femorales. Los análisis sobre los Af mostraron un incremento significativo durante el primer mes mientras que L se estabilizó a partir del 4to mes. Los valores de DMOt mostraron un plateau a partir del cuarto mes, si bien las DMOco y DMOmf ya a partir del tercer mes no mostraron incrementos significativos. Histológicamente se observó para el cuarto mes ausencia de las diferentes zonas características del cartílago de crecimiento metafisiario, con presencia únicamente de un pequeño remanente de células condrales. Desde el quinto mes se observa ausencia total de cartílago, con presencia únicamente de tejido osteoide (TO). La interpretación integrada de los resultados nos permite afirmar, que a partir del cuarto mes de desarrollo, el fémur de CoNZ adquiere características compatibles con un periodo de adultez.

Functional identification of a rare vascular endothelial growth factor a (VEGFA) variant associating with the nonsyndromic cleft lip with/without cleft palate

Vascular endothelial growth factor A (VEGFA) is a crucial growth factor, which participates in multiple processes of human growth and development, such as angiogenesis and osteogenesis and is also necessary for development of palate. The purpose of this study was to investigate the effect of a rare VEGFA mutation (NM_001025366.2 773 T > C p.Val258Ala) on the cell functions and osteogenesis. Here, we found that the VEGFA mutation has adverse effects on the function of human embryonic palatal plate mesenchymal (HEPM) cells, and may affect the development of palate. The VEGFA mutation has adverse effects on promoting cell proliferation and migration and inhibiting apoptosis in HEPM and HEK–293 cells. In addition, the mutant VEGFA allele has a negative influence on osteogenesis. Taken together, the rare variant of the VEGFA gene had an adverse effect on cell functions and osteogenesis, which may impact the development of the palate. And these findings suggested that VEGFA mutation (c.773 T > C) may lead to nonsyndromic cleft lip with or without cleft palate and also provide a new insight into the mechanism of VEGFA gene in osteogenesis and palatogenesis.

Light rare earth elements hinder bone development via inhibiting type H vessels formation in mice

Light rare earth elements (LREEs) are widely used in medical, industrial, and agricultural fields. Wide application of light rare earth and exposure to these elements in human society leads to increasing accumulation of LREE in human skeletal system. However, the effects of LREEs on human bone health is not clear. In this study, we found that LREE reduced CD31highEmcnhigh endothelial cell mediated type H vessels formation at the metaphyseal sites, resulting in reduced bone mass and low bone quality in mouse bone development. To explore the underlying mechanism, we induced bone marrow macrophages (BMMs) to preosteoclasts (pOCs) with exposure of LREE (Pr3+, Nd3+, Sm3+). The cytotoxicity of LREE was evaluated by CCK-8. Platelet-derived growth factor (PDGF-BB) is the cytokine secreted by pOCs that most responsible for inducing Type H vessel formation. We used ELISA kit to determine the PDGF-BB level in pOC supernatant, and mouse serum finding that the PDGF-BB level was reduced by LREEs treatment. Then we tested the ability of migration and tube formation of HUVECs using condition medium from pOCs. The migration and tube formation ability of HUVECs were both suppressed with LREEs pretreatment. We concluded that LREEs hinder mouse bone development by suppressing type H vessels associated bone formation. DATA AND MATERIALS AVAILABILITY: All data generated or analyzed during this study are included in this article. Please contact the corresponding author for unique material requests. Some material used in the reported research may require requests to collaborators and agreements with both commercial and non-profit institutions, as specified in the paper. Requests are reviewed by Third Military Medical University to verify whether the request is subject to any intellectual property or confidentiality obligations. Any material that can be shared will be released via a Material Transfer Agreement.

Bone physiological microenvironment and healing mechanism: Basis for future bone-tissue engineering scaffolds

Bone-tissue defects affect millions of people worldwide. Despite being common treatment approaches, autologous and allogeneic bone grafting have not achieved the ideal therapeutic effect. This has prompted researchers to explore novel bone-regeneration methods. In recent decades, the development of bone tissue engineering (BTE) scaffolds has been leading the forefront of this field. As researchers have provided deep insights into bone physiology and the bone-healing mechanism, various biomimicking and bioinspired BTE scaffolds have been reported. Now it is necessary to review the progress of natural bone physiology and bone healing mechanism, which will provide more valuable enlightenments for researchers in this field. This work details the physiological microenvironment of the natural bone tissue, bone-healing process, and various biomolecules involved therein. Next, according to the bone physiological microenvironment and the delivery of bioactive factors based on the bone-healing mechanism, it elaborates the biomimetic design of a scaffold, highlighting the designing of BTE scaffolds according to bone biology and providing the rationale for designing next-generation BTE scaffolds that conform to natural bone healing and regeneration.

The Potential of FGF-2 in Craniofacial Bone Tissue Engineering: A Review

Bone is a hard-vascularized tissue, which renews itself continuously to adapt to the mechanical and metabolic demands of the body. The craniofacial area is prone to trauma and pathologies that often result in large bone damage, these leading to both aesthetic and functional complications for patients. The "gold standard" for treating these large defects is autologous bone grafting, which has some drawbacks including the requirement for a second surgical site with quantity of bone limitations, pain and other surgical complications. Indeed, tissue engineering combining a biomaterial with the appropriate cells and molecules of interest would allow a new therapeutic approach to treat large bone defects while avoiding complications associated with a second surgical site. This review first outlines the current knowledge of bone remodeling and the different signaling pathways involved seeking to improve our understanding of the roles of each to be able to stimulate or inhibit them. Secondly, it highlights the interesting characteristics of one growth factor in particular, FGF-2, and its role in bone homeostasis, before then analyzing its potential usefulness in craniofacial bone tissue engineering because of its proliferative, pro-angiogenic and pro-osteogenic effects depending on its spatial-temporal use, dose and mode of administration.

Investigation of Vasculogenesis Inducing Biphasic Scaffolds for Bone Tissue Engineering

Vascularization is the main obstacle for the bone tissue engineering strategies since the defect size is generally large. Incorporation of angiogenic factors is one of the strategies employed in order to accelerate vascularization and improve bone healing. In this study, a biphasic scaffold consisting of fibrous poly(lactide-co-glycolide) (PLGA) and poly(lactide-co-glycolide)-block-poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PLGA-PEG-PLGA) hydrogel loaded with vascular endothelial growth factor-A (VEGF) inducer, GS4012, was constructed. Mesenchymal stem cells isolated from rat bone marrow (rBMSCs) were used for differentiation into osteogenic cells, and endothelial cells isolated from rat peripheral blood (rPBECs) were used to test the in vitro endothelial cell recruitment. The biphasic scaffold was tested for cell proliferation, ALP expression, VEGF induction, expression of osteogenic genes by rBMSCs, and recruitment of rPBECs in vitro and for improved bone healing and vascularization in vivo on critical size rat cranial defects. Endothelial migration through porous insert and VEGF induction were obtained in vitro in response to GS4012 as well as the upregulation of ALP, Runx2, Col I, and OC gene expressions. The biphasic scaffold was also shown to be effective in improving endothelial cell recruitment, vascularization, and bone healing in vivo. Thus, the proposed design has a great potential for the healing of critical size bone defect in tissue engineering studies according to both in vitro and in vivo investigations.

Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis

Angiogenesis and osteogenesis are coupled. However, the cellular and molecular regulation of these processes remains to be further investigated. Both tissues have recently been recognized as endocrine organs, which has stimulated research interest in the screening and functional identification of novel paracrine factors from both tissues. This review aims to elaborate on the novelty and significance of endocrine regulatory loops between bone and the vasculature. In addition, research progress related to the bone vasculature, vessel-related skeletal diseases, pathological conditions, and angiogenesis-targeted therapeutic strategies are also summarized. With respect to future perspectives, new techniques such as single-cell sequencing, which can be used to show the cellular diversity and plasticity of both tissues, are facilitating progress in this field. Moreover, extracellular vesicle-mediated nuclear acid communication deserves further investigation. In conclusion, a deeper understanding of the cellular and molecular regulation of angiogenesis and osteogenesis coupling may offer an opportunity to identify new therapeutic targets.

The Glycoprotein/Cytokine Erythropoietin Promotes Rapid Alveolar Ridge Regeneration In Vivo by Promoting New Bone Extracellular Matrix Deposition in Conjunction with Coupled Angiogenesis/Osteogenesis

The loss of bone following tooth extraction poses a significant clinical problem for maxillofacial esthetics, function, and future implant placement. In the present study, the efficacy of an erythropoietin-impregnated collagen scaffold as an alveolar ridge augmentation material versus a conventional collagen scaffold and a BioOss inorganic bovine bone xenograft was examined. The collagen/Erythropoietin (EPO) scaffold exhibited significantly more rapid and complete osseous regeneration of the alveolar defect when compared to bone xenograft and the collagen membrane alone. The new EPO induced extracellular matrix was rich in Collagen I, Collagen III, Fibronectin (Fn) and E-cadherin, and featured significantly increased levels of the osteogenic transcription factors Runt-related transcription factor 2 (Runx2) and Osterix (Osx). Histomorphometric evaluation revealed a significant two-fold increase in the number of capillaries between the EPO and the BioOss group. Moreover, there was a highly significant 3.5-fold higher level of vascular endothelial growth factor (VEGF) in the collagen/EPO-treated group compared to controls. The significant effect of EPO on VEGF, FN, and RUNX2 upregulation was confirmed in vitro, and VEGF pathway analysis using VEGF inhibitors confirmed that EPO modulated extracellular matrix protein expression through VEGF even in the absence of blood vessels. Together, these data demonstrate the effectiveness of an EPO-impregnated collagen scaffold for bone regeneration as it induces rapid matrix production and osseoinduction adjacent to new capillaries via VEGF.

Insights into the mechanism of vascular endothelial cells on bone biology

In the skeletal system, blood vessels not only function as a conduit system for transporting gases, nutrients, metabolic waste, or cells but also provide multifunctional signal molecules regulating bone development, regeneration, and remodeling. Endothelial cells (ECs) in bone tissues, unlike in other organ tissues, are in direct contact with the pericytes of blood vessels, resulting in a closer connection with peripheral connective tissues. Close-contact ECs contribute to osteogenesis and osteoclastogenesis by secreting various cytokines in the paracrine or juxtacrine pathways. An increasing number of studies have revealed that extracellular vesicles (EVs) derived from ECs can directly regulate maturation process of osteoblasts and osteoclasts. The different pathways focus on targets at different distances, forming the basis of the intimate spatial and temporal link between bone tissue and blood vessels. Here, we provide a systematic review to elaborate on the function of ECs in bone biology and its underlying mechanisms based on three aspects: paracrine, EVs, and juxtacrine. This review proposes the possibility of a therapeutic strategy targeting blood vessels, as an adjuvant treatment for bone disorders.

A Review of Recent Developments in the Molecular Mechanisms of Bone Healing

Between 5 and 10 percent of fractures do not heal, a condition known as nonunion. In clinical practice, stable fracture fixation associated with autologous iliac crest bone graft placement is the gold standard for treatment. However, some recalcitrant nonunions do not resolve satisfactorily with this technique. For these cases, biological alternatives are sought based on the molecular mechanisms of bone healing, whose most recent findings are reviewed in this article. The pro-osteogenic efficacy of morin (a pale yellow crystalline flavonoid pigment found in old fustic and osage orange trees) has recently been reported, and the combined use of bone morphogenetic protein-9 (BMP9) and leptin might improve fracture healing. Inhibition with methyl-piperidino-pyrazole of estrogen receptor alpha signaling delays bone regeneration. Smoking causes a chondrogenic disorder, aberrant activity of the skeleton’s stem and progenitor cells, and an intense initial inflammatory response. Smoking cessation 4 weeks before surgery is therefore highly recommended. The delay in fracture consolidation in diabetic animals is related to BMP6 deficiency (35 kDa). The combination of bioceramics and expanded autologous human mesenchymal stem cells from bone marrow is a new and encouraging alternative for treating recalcitrant nonunions.

Osteogenesis, angiogenesis and immune response of Mg-Al layered double hydroxide coating on pure Mg

Layered double hydroxides (LDHs) are widely studied to enhance corrosion resistance and biocompatibility of Mg alloys, which are promising bone implants. However, the influence of LDH coating on the osteointegration of Mg implants lacks of a systematic study. In this work, Mg-Al LDH coating was prepared on pure Mg via hydrothermal treatment. The as-prepared Mg-Al LDH coated Mg exhibited better in vitro and in vivo corrosion resistance than bare Mg and Mg(OH)2 coated Mg. In vitro culture of mouse osteoblast cell line (MC3T3-E1) suggested that Mg-Al LDH coated Mg was more favorable for its osteogenic differentiation. In vitro culture of HUVECs revealed that cells cultured in the extract of Mg-Al LDH coated Mg showed superior angiogenic behaviors. More importantly, the immune response of Mg-Al LDH coated Mg was studied by in vitro culturing murine-derived macrophage cell line (RAW264.7). The results verified that Mg-Al LDH coated Mg could induce macrophage polarize to M2 phenotype (anti-inflammatory). Furthermore, the secreted factor in the macrophage-conditioned culture medium of Mg-Al LDH group was more suitable for the bone differentiation of rat bone marrow stem cells (rBMSCs) and the angiogenic behavior of human umbilical vein endothelial cells (HUVECs). Finally, the result of femoral implantation suggested that Mg-Al LDH coated Mg exhibited better osteointegration than bare Mg and Mg(OH)2 coated Mg. With favorable in vitro and in vivo performances, Mg-Al LDH is promising as protective coating on Mg for orthopedic applications.

Strontium-calcium phosphate hybrid cement with enhanced osteogenic and angiogenic properties for vascularised bone regeneration

Vascularized bone tissue engineering is regarded as one of the optimal treatment options for large bone defects. The lack of angiogenic properties and unsatisfactory physicochemical performance restricts calcium phosphate cement (CPC) from application in vascularized bone tissue engineering. Our previous studies have developed a starch and BaSO4 incorporated calcium phosphate hybrid cement (CPHC) with improved mechanical strength and handling properties. However, the bioactivity-especially the angiogenic ability-is still absent and requires further improvement. Herein, based on the reported CPHC and the osteogenic and angiogenic properties of strontium (Sr) ions, a strontium-enhanced calcium phosphate hybrid cement (Sr-CPHC) was developed to improve both biological and physicochemical properties of CPC. Compared to CPC, the initial setting time of Sr-CPHC was prolonged from 2.2 min to 20.7 min. The compressive strength of Sr-CPHC improved from 11.21 MPa to 45.52 MPa compared with CPC as well. Sr-CPHC was biocompatible and showed promotion of alkaline phosphatase (ALP) activity, calcium nodule formation and osteogenic relative gene expression, suggesting high osteogenic-inductivity. Sr-CPHC also facilitated the migration and tube formation of human umbilical vein endothelial cells (HUVECs) in vitro and up-regulated the expression of the vascular endothelial growth factor (VEGF) and Angiopoietin-1 (Ang-1). In vivo evaluation showed marked new bone formation in a rat calvarial defect model with Sr-CPHC implanted. Sr-CPHC also exhibited enhancement of neovascularization in subcutaneous connective tissue in a rat subcutaneous implantation model. Thus, the Sr-CPHC with the dual effects of osteogenesis and angiogenesis shows great potential for clinical applications such as the repair of ischemic osteonecrosis and critical-size bone defects.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Antenatal Corticosteroid Therapy Attenuates Angiogenesis Through Inhibiting Osteoclastogenesis in Young Mice

Antenatal corticosteroid therapy (ACT) has been shown to reduce morbidity and mortality rates in preterm delivery, but the fetus is more likely to face the risk of low bone mineralization and low fetal linear growth. However, the mechanism of ACT inducing low bone mineralization remains largely unknown. Pre-osteoclasts, which play an important role in angiogenesis and osteogenesis, are specifically regulating type H vessels (CD31^hiEmcn^hi) and vessel formation by secreting platelet-derived growth factor-BB (PDGF-BB). We find that the number of pre-osteoclasts and POC-secreted PDGF-BB is dramatically decreased in ACT mice, contributing to the reduction in type H vessels and bone mineralization during the mouse offspring. Quantitative analyses of micro-computed tomography show that the ACT mice have a significant reduction in the mass of trabecular bone relative to the control group. Mononuclear pre-osteoclasts in trabecular bone decreased in ACT mice, which leads to the amount of PDGF-BB reduced and attenuates type H vessel formation. After sorting the Rank+ osteoclast precursors using flow cytometry, we show that the enhancer of zeste homolog 2 (Ezh2) expression is decreased in Rank+ osteoclast precursors in ACT mice. Consistent with the flow data, by using small molecule Ezh2 inhibitor GSK126, we prove that Ezh2 is required for osteoclast differentiation. Downregulating the expression of Ezh2 in osteoclast precursors would reduce PDGF-BB production. Conditioned medium from osteoclast precursor cultures treated with GSK126 inhibited endothelial tube formation, whereas conditioned medium from vehicle group stimulated endothelial tube formation. These results indicate Ezh2 expression of osteoclast precursors is suppressed after ACT, which reduced the pre-osteoclast number and PDGF-BB secretion, thus inhibiting type H vessel formation and ACT-associated low bone mineralization.

Transcription Factors of the Alx Family: Evolutionarily Conserved Regulators of Deuterostome Skeletogenesis

Members of the alx gene family encode transcription factors that contain a highly conserved Paired-class, DNA-binding homeodomain, and a C-terminal OAR/Aristaless domain. Phylogenetic and comparative genomic studies have revealed complex patterns of alx gene duplications during deuterostome evolution. Remarkably, alx genes have been implicated in skeletogenesis in both echinoderms and vertebrates. In this review, we provide an overview of current knowledge concerning alx genes in deuterostomes. We highlight their evolutionarily conserved role in skeletogenesis and draw parallels and distinctions between the skeletogenic gene regulatory circuitries of diverse groups within the superphylum.

Calcitonin Gene-Related Peptide Enhances Distraction Osteogenesis by Increasing Angiogenesis

Distraction osteogenesis (DO) is a well-established surgical technique for treating bone defect and limb lengthening. The major drawback of DO is the long treatment period as the external fixator has to be kept in place until consolidation is completed. Calcitonin gene-related peptide (CGRP) has been reported to promote angiogenesis by affecting endothelial progenitor cells (EPCs) in limb ischemia and wound healing. Thus, the goal of this study was to evaluate the angiogenic effect of exogenous CGRP on bone regeneration in a rat DO model. Exogenous CGRP was directly injected into the bone defect after each cycle of distraction in vivo. Microcomputed tomography, biomechanical test, and histological analysis were performed to assess the new bone formation. Angiography and immunofluorescence were performed to assess the formation of blood vessels. CD31⁺CD144⁺ EPCs in the bone defect were quantified with flow cytometry. In in vitro study, bone marrow stem cells (BMSCs) were used to investigate the effect of CGRP on EPCs production during endothelial differentiation. Our results showed that CGRP significantly promoted bone regeneration and vessel formation after consolidation. CGRP significantly increased the fraction of CD31⁺CD144⁺EPCs and the capillary density in the bone defect at the end of distraction phase. CGRP increased EPC population in the endothelial differentiation of BMSCs in vitro by activating PI3K/AKT signaling pathway. Furthermore, differentiated EPCs rapidly assembled into tube-like structures and promoted osteogenic differentiation of BMSCs. In conclusion, CGRP increased EPC population and promoted blood vessel formation and bone regeneration at the defect region in a DO model. Impact statement Distraction osteogenesis (DO) is a well-established surgical technique for limb lengthening and bone defect. The disadvantage of this technique is that external fixator is needed to be kept in place for about 12 months. This may result in increased risk of infection, financial burden, and negative psychological impacts. In this study, we have injected calcitonin gene-related peptide (CGRP) into the defect region after distraction and found that CGRP enhanced vessel formation and bone regeneration in a rat DO model. This suggests that a controlled delivery system for CGRP could be developed and applied clinically for accelerating bone regeneration in patients with DO.

Bone Microvasculature: Stimulus for Tissue Function and Regeneration

Bone is a highly vascularized organ, providing structural support to the body, and its development, regeneration, and remodeling depend on the microvascular homeostasis. Loss or impairment of vascular function can develop diseases, such as large bone defects, avascular necrosis, osteoporosis, osteoarthritis, and osteopetrosis. In this review, we summarize how vasculature controls bone development and homeostasis in normal and disease cases. A better understanding of this process will facilitate the development of novel disease treatments that promote bone regeneration and remodeling. Specifically, approaches based on tissue engineering components, such as stem cells and growth factors, have demonstrated the capacity to induce bone microvasculature regeneration and mineralization. This knowledge will have relevant clinical implications for the treatment of bone disorders by developing novel pharmaceutical approaches and bone grafts. Finally, the tissue engineering approaches incorporating vascular components may widely be applied to treat other organ diseases by enhancing their regeneration capacity. Impact statement Bone vasculature is imperative in the process of bone development, regeneration, and remodeling. Alterations or disruption of the bone vasculature leads to loss of bone homeostasis and the development of bone diseases. In this study, we review the role of vasculature on bone diseases and how vascular tissue engineering strategies, with a detailed emphasis on the role of stem cells and growth factors, will contribute to bone therapeutics.

Effects of VEGF on Prefabricated Vascularized Bone Allografts in Rats

BACKGROUND

 Massive bone defects after wide resection of malignant bone tumors or a serious injury require treatment using vascularized bone grafts. Although cadaveric bone allografts combined with vascularized bone autografts are currently thought to be ideal in terms of size and durability, this treatment requires the scarification of healthy bone tissue. In a previous study, we attempted to improve this situation by prefabricating a vascularized bone allograft in recipient rats. In this study, we added vascular endothelial growth factor (VEGF)-containing hydroxyapatite/collagen composite (HAp/Col) to a prefabricated vascularized bone allograft to stimulate angiogenesis, which is known to be important for bone formation.

METHODS

 Sprague Dawley rats (n = 50) were used as donors and Wistar rats (n = 50) as recipients. All rats were 9 weeks old. The recipient rats were divided into five groups according to the use of vascular bundles, HAp/Col, and an additive substance (VEGF). The bone allografts collected from the donors were transplanted into the thigh region of the recipients, and a saphenous vein and 10 μg HAp/Col with VEGF were inserted into the bone allografts through the slit. After 4 weeks, the transplanted bone allografts were harvested, and histologic and genetic evaluations were performed in relation to bone formation and resorption.

RESULTS

 The results showed that, compared with the control group, the implantation of the vascular bundles and VEGF-containing HAp/Col significantly stimulated angiogenesis and bone formation in the rats with the bone allografts. However, histological and genetic evaluations of bone resorption revealed that resorption was not observed in any group.

CONCLUSION

 These results suggest that VEGF-containing HAp/Col effectively stimulates angiogenesis and bone formation, but not bone resorption, in prefabricated vascularized bone allografts. This method could therefore become a useful tool for treating large bone defects.

Irregular Bone Defect Repair Using Tissue-Engineered Periosteum in a Rabbit Model

Background:

In previous studies, we succeeded in repairing a long bone defect with tissue-engineered periosteum (TEP), fabricated by incorporating rabbit mesenchymal stem cells with small intestinal submucosa. In this study, we investigated the feasibility of allogeneic irregular bone defect repair using TEP.

Methods:

We performed a subtotal resection of the scapula in 36 rabbits to establish a large irregular bone defect model. The rabbits were then randomly divided into three groups (n = 12 per group) and the defects were treated with TEP (Group 1), allogeneic deproteinized bone (DPB) (Group 2) or a hybrid of TEP and DPB (Group 3). At 4, 8, and 12 weeks after surgery, the rabbits were sacrificed, and the implants were harvested. X-ray radiographic and histological examinations were performed to detect bone healing. Ink-formaldehyde perfusion was introduced to qualitatively analyze vascularization in TEP engineered new bone.

Results:

The repair of scapular defects was diverse in all groups, shown by radiographic and histological tests. The radiographic scores in Group 1 and Group 3 were significantly higher than Group 2 at 8 and 12 weeks (p < 0.05). Histological scores further proved that Group 1 had significantly greater new bone formation compared to Group 3 (p < 0.05), while Group 2 had the lowest osteogenesis at all time-points (p < 0.001). Ink-formaldehyde perfusion revealed aboundant microvessels in TEP engineered new bone.

Conclusion:

We conclude that TEP is promising for the repair of large irregular bone defects. As a 3D scaffold, DPB could provide mechanical support and a shaping guide when combined with TEP. TEP engineered new bone has aboundant microvessels.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

Motivating role of type H vessels in bone regeneration

Coupling between angiogenesis and osteogenesis has an important role in both normal bone injury repair and successful application of tissue‐engineered bone for bone defect repair. Type H blood vessels are specialized microvascular components that are closely related to the speed of bone healing. Interactions between type H endothelial cells and osteoblasts, and high expression of CD31 and EMCN render the environment surrounding these blood vessels rich in factors conducive to osteogenesis and promote the coupling of angiogenesis and osteogenesis. Type H vessels are mainly distributed in the metaphysis of bone and densely surrounded by Runx2⁺ and Osterix⁺ osteoprogenitors. Several other factors, including hypoxia‐inducible factor‐1α, Notch, platelet‐derived growth factor type BB, and slit guidance ligand 3 are involved in the coupling of type H vessel formation and osteogenesis. In this review, we summarize the identification and distribution of type H vessels and describe the mechanism for type H vessel‐mediated modulation of osteogenesis. Type H vessels provide new insights for detection of the molecular and cellular mechanisms that underlie the crosstalk between angiogenesis and osteogenesis. As a result, more feasible therapeutic approaches for treatment of bone defects by targeting type H vessels may be applied in the future.

Integrated Transcriptome and Network Analysis Reveals Spatiotemporal Dynamics of Calvarial Suturogenesis

Craniofacial abnormalities often involve sutures, the growth centers of the skull. To characterize the organization and processes governing their development, we profile the murine frontal suture, a model for sutural growth and fusion, at the tissue- and single-cell level on embryonic days (E)16.5 and E18.5. For the wild-type suture, bulk RNA sequencing (RNA-seq) analysis identifies mesenchyme-, osteogenic front-, and stage-enriched genes and biological processes, as well as alternative splicing events modifying the extracellular matrix. Single-cell RNA-seq analysis distinguishes multiple subpopulations, of which five define a mesenchyme-osteoblast differentiation trajectory and show variation along the anteroposterior axis. Similar analyses of in vivo mouse models of impaired frontal suturogenesis in Saethre-Chotzen and Apert syndromes, Twist1+/- and Fgfr2+/S252W, demonstrate distinct transcriptional changes involving angiogenesis and ribogenesis, respectively. Co-expression network analysis reveals gene expression modules from which we validate key driver genes regulating osteoblast differentiation. Our study provides a global approach to gain insights into suturogenesis.