Bone regeneration properties of granulocyte colony-stimulating factor via neovascularization and osteogenesis

From inflammation to bone formation: the intricate role of neutrophils in skeletal muscle injury and traumatic heterotopic ossification

Neutrophils are emerging as an important player in skeletal muscle injury and repair. Neutrophils accumulate in injured tissue, thus releasing inflammatory factors, proteases and neutrophil extracellular traps (NETs) to clear muscle debris and pathogens when skeletal muscle is damaged. During the process of muscle repair, neutrophils can promote self-renewal and angiogenesis in satellite cells. When neutrophils are abnormally overactivated, neutrophils cause collagen deposition, functional impairment of satellite cells, and damage to the skeletal muscle vascular endothelium. Heterotopic ossification (HO) refers to abnormal bone formation in soft tissue. Skeletal muscle injury is one of the main causes of traumatic HO (tHO). Neutrophils play a pivotal role in activating BMPs and TGF-β signals, thus promoting the differentiation of mesenchymal stem cells and progenitor cells into osteoblasts or osteoclasts to facilitate HO. Furthermore, NETs are specifically localized at the site of HO, thereby accelerating the formation of HO. Additionally, the overactivation of neutrophils contributes to the disruption of immune homeostasis to trigger HO. An understanding of the diverse roles of neutrophils will not only provide more information on the pathogenesis of skeletal muscle injury for repair and HO but also provides a foundation for the development of more efficacious treatment modalities for HO.

The fusion of keratinized epithelium, an indication of early implant placement in the aesthetic area: an animal study

BACKGROUND

In the period of the early implant placement, the socket is mainly occupied by provisional matrix (PM). Keratinized epithelium (KE) is critical for primary wound closure. Although both KE and PM are important, the detailed relationship among migrating KE, PM formation and indication of the early implant placement is still unclear.

OBJECTIVE

This research aimed to locate a healing stage of KE with highest osteogenic PM formation after tooth extraction, which could be treated as the optimal time point for early implant placement.

MATERIAL AND METHODS

Mice were sacrificed on days 1, 2, 3, 4 and 6 after incisor extraction. Clinical, histological, and immunohistochemical evaluations of the extraction sockets were performed, and statistical analyses were conducted. We then inserted implants into the PM with the greatest bioactivity and observed its osseointegration pattern for 3, 10, 17 and 30 days.

RESULT

When KE fusion was reached, sockets were dominated by PM with the greatest expression of osteocalcin (OC, P < 0.05) and high levels of CD34 and Runx2. OC and Runx2 expression were positively correlated with KE coverage (P < 0.05). When the implant was inserted at 4 days' healing, the PM maintained its osteogenic ability, and osseointegration proceeded perfectly.

CONCLUSION

The migration of KE was correlated with the formation of highly osteogenic and angiogenic PM. And the fusion of KE could be treated as an indication for early implant placement.

Phase III clinical trial of autologous CD34 + cell transplantation to accelerate fracture nonunion repair

BACKGROUND

We previously demonstrated that CD34 + cell transplantation in animals healed intractable fractures via osteogenesis and vasculogenesis; we also demonstrated the safety and efficacy of this cell therapy in an earlier phase I/II clinical trial conducted on seven patients with fracture nonunion. Herein, we present the results of a phase III clinical trial conducted to confirm the results of the previous phase studies using a larger cohort of patients.

METHODS

CD34 + cells were mobilized via administration of granulocyte colony-stimulating factor, harvested using leukapheresis, and isolated using magnetic cell sorting. Autologous CD34 + cells were transplanted in 15 patients with tibia nonunion and 10 patients with femur nonunion, who were followed up for 52 weeks post transplantation. The main outcome was a reduction in time to heal the tibia in nonunion patients compared with that in historical control patients. We calculated the required number of patients as 15 based on the results of the phase I/II study. An independent data monitoring committee performed the radiographic assessments. Adverse events and medical device failures were recorded.

RESULTS

All fractures healed during the study period. The time to radiological fracture healing was 2.8 times shorter in patients with CD34 + cell transplantation than in the historical control group (hazard ratio: 2.81 and 95% confidence interval 1.16-6.85); moreover, no safety concerns were observed.

CONCLUSIONS

Our findings strongly suggest that autologous CD34 + cell transplantation is a novel treatment option for fracture nonunion.

TRIAL REGISTRATION

UMIN-CTR, UMIN000022814. Registered on 22 June 2016.

Immunohistochemical localization of CD31, CD34, and periostin in vocal fold polyps

BACKGROUND

Long-term voice-abuse or sudden vocal fold microvascular disruption can lead to injury and subsequent repair/remodeling of the vocal fold mucosa. Periostin is known to be involved in airway remodeling and in various otolaryngological diseases. In ischemic heart disease, increased CD31 expression has been observed around cardiomyocytes during remodeling, and endothelial proliferation has been reported to occur at these sites.

OBJECTIVES

We investigated the expression and the roles of CD31, CD34, and periostin in the formation of vocal fold polyps.

MATERIALS AND METHODS

Fifty-seven samples of vocal fold polyps were investigate histopathologically and immunohistochemically.

RESULT

Expression of CD31 and CD34 was detected in 41 (71.9%) and 53 (93.0%) samples, respectively, obtained from patients with vocal fold polyp. Expression of periostin was detected in 41 (71.9%) samples obtained from patients with vocal polyps. The vocal polyp samples could be classified into three histological subtypes. Three patterns of CD31 and CD34 expression were observed in the vocal polyp. Four patterns of periostin expression were observed in vocal polyps. An association was observed between the CD31 expression pattern and the histological subtype of vocal fold polyps.

CONCLUSION AND SIGNIFICANCE

In vocal fold polyps, evaluation of vascular endothelial markers may be useful for staging.

Biological Functions of Let-7e-5p in Promoting the Differentiation of MC3T3-E1 Cells

MicroRNAs let-7c and let-7f, two members of the let-7 family, were involved in regulating osteoblast differentiation and have an important role in bone formation. Let-7e-5p, which also belonged to the let-7 family, presented in the differentiation of adipose-derived stem cells and mouse embryonic stem cells. However, the role of let-7e-5p in osteoblast differentiation was unclear. Thus, this study aimed to elucidate the function of let-7e-5p in osteoblast differentiation and its mechanism. Firstly, we found that the let-7e-5p mimic promoted osteoblast differentiation but not the proliferation of MC3T3-E1 cells by positively regulating the expression levels of osteogenic-associated genes (RUNX2, OCN, OPN, and OSX), the activity of ALP, and formation of mineralized nodules. Moreover, we ascertained that the let-7e-5p mimic downregulated the post-transcriptional expression of SOCS1 by specifically binding to the 3′ untranslated region of SOCS1 mRNA. Also, let-7e-5p-induced SOCS1 downregulation increased the protein levels of p-STAT5 and IGF-1, which were both modulated by SOCS1 molecules. Furthermore, let-7e-5p abrogated the inhibition of osteogenic differentiation mediated by SOCS1 overexpression. Therefore, these results suggested that let-7e-5p regulated the differentiation of MC3T3-E1 cells through the JAK2/STAT5 pathway to upregulate IGF-1 gene expression by inhibiting SOCS1. These findings may provide a new insight into the regulatory role of let-7e-5p in osteogenic differentiation and imply the existence of a novel mechanism underlying let-7e-5p-mediated osteogenic differentiation.

The Therapeutic Potential of Hematopoietic Stem Cells in Bone Regeneration

The repair process of bone fractures is a complex biological mechanism requiring the recruitment and in situ functionality of stem/stromal cells from the bone marrow (BM). BM mesenchymal stem/stromal cells have been widely explored in multiple bone tissue engineering applications, whereas the use of hematopoietic stem cells (HSCs) has been poorly investigated in this context. A reasonable explanation is the fact that the role of HSCs and their combined effect with other elements of the hematopoietic niches in the bone-healing process is still elusive. Therefore, in this review we intend to highlight the influence of HSCs in the bone repair process, mainly through the promotion of osteogenesis and angiogenesis at the bone injury site. For that, we briefly describe the main biological characteristics of HSCs, as well as their hematopoietic niches, while reviewing the biomimetic engineered BM niche models. Moreover, we also highlighted the role of HSCs in translational in vivo transplantation or implantation as promoters of bone tissue repair.

Systemic Administration of G-CSF Accelerates Bone Regeneration and Modulates Mobilization of Progenitor Cells in a Rat Model of Distraction Osteogenesis

Granulocyte colony-stimulating factor (G-CSF) was shown to promote bone regeneration and mobilization of vascular and osteogenic progenitor cells. In this study, we investigated the effects of a systemic low dose of G-CSF on both bone consolidation and mobilization of hematopoietic stem/progenitor cells (HSPCs), endothelial progenitor cells (EPCs) and mesenchymal stromal cells (MSCs) in a rat model of distraction osteogenesis (DO). Neovascularization and mineralization were longitudinally monitored using positron emission tomography and planar scintigraphy. Histological analysis was performed and the number of circulating HSPCs, EPCs and MSCs was studied by flow cytometry. Contrary to control group, in the early phase of consolidation, a bony bridge with lower osteoclast activity and a trend of an increase in osteoblast activity were observed in the distracted callus in the G-CSF group, whereas, at the late phase of consolidation, a significantly lower neovascularization was observed. While no difference was observed in the number of circulating EPCs between control and G-CSF groups, the number of MSCs was significantly lower at the end of the latency phase and that of HSPCs was significantly higher 4 days after the bone lengthening. Our results indicate that G-CSF accelerates bone regeneration and modulates mobilization of progenitor cells during DO.

TiO2 bioactive implant surfaces doped with specific amount of Sr modulate mineralization

One of the main problems that remain in the implant industry is poor osseointegration due to bioinertness of implants. In order to promote bioactivity, calcium (Ca), phosphorus (P) and strontium (Sr) were incorporated into a TiO₂ porous layer produced by micro-arc oxidation. Ca and P as bioactive elements are already well reported in the literature, however, the knowledge of the effect of Sr is still limited. In the present work, the effect of various amounts of Sr was evaluated and the morphology, chemical composition and crystal structure of the oxide layer were investigated. Furthermore, in vitro studies were carried out using human osteoblast-like cells. The oxide layer formed showed a triplex structure, where higher incorporation of Sr increased Ca/P ratio, amount of rutile and promoted the formation of SrTiO₃ compound. Biological tests revealed that lower concentrations of Sr did not compromise initial cell adhesion neither viability and interestingly improved mineralization. However, higher concentration of Sr (and consequent higher amount of rutile) showed to induce collagen secretion but with compromised mineralization, possibly due to a delayed mineralization process or induced precipitation of deficient hydroxyapatite. Ca-P-TiO₂ porous layer with less concentration of Sr seems to be an ideal candidate for bone implants.

Hydrogel Encapsulation of Mesenchymal Stem Cells and Their Derived Exosomes for Tissue Engineering

Tissue engineering has been an inveterate area in the field of regenerative medicine for several decades. However, there remains limitations to engineer and regenerate tissues. Targeted therapies using cell-encapsulated hydrogels, such as mesenchymal stem cells (MSCs), are capable of reducing inflammation and increasing the regenerative potential in several tissues. In addition, the use of MSC-derived nano-scale secretions (i.e., exosomes) has been promising. Exosomes originate from the multivesicular division of cells and have high therapeutic potential, yet neither self-replicate nor cause auto-immune reactions to the host. To maintain their biological activity and allow a controlled release, these paracrine factors can be encapsulated in biomaterials. Among the different types of biomaterials in which exosome infusion is exploited, hydrogels have proven to be the most user-friendly, economical, and accessible material. In this paper, we highlight the importance of MSCs and MSC-derived exosomes in tissue engineering and the different biomaterial strategies used in fabricating exosome-based biomaterials, to facilitate hard and soft tissue engineering.

Neurogenic Heterotopic Ossifications Develop Independently of Granulocyte Colony-Stimulating Factor and Neutrophils

Neurogenic heterotopic ossifications (NHOs) are incapacitating heterotopic bones in periarticular muscles that frequently develop following traumatic brain or spinal cord injuries (SCI). Using our unique model of SCI-induced NHO, we have previously established that mononucleated phagocytes infiltrating injured muscles are required to trigger NHO via the persistent release of the pro-inflammatory cytokine oncostatin M (OSM). Because neutrophils are also a major source of OSM, we investigated whether neutrophils also play a role in NHO development after SCI. We now show that surgery transiently increased granulocyte colony-stimulating factor (G-CSF) levels in blood of operated mice, and that G-CSF receptor mRNA is expressed in the hamstrings of mice developing NHO. However, mice defective for the G-CSF receptor gene Csf3r, which are neutropenic, have unaltered NHO development after SCI compared to C57BL/6 control mice. Because the administration of recombinant human G-CSF (rhG-CSF) has been trialed after SCI to increase neuroprotection and neuronal regeneration and has been shown to suppress osteoblast function at the endosteum of skeletal bones in human and mice, we investigated the impact of a 7-day rhG-CSF treatment on NHO development. rhG-CSF treatment significantly increased neutrophils in the blood, bone marrow, and injured muscles. However, there was no change in NHO development compared to saline-treated controls. Overall, our results establish that unlike monocytes/macrophages, neutrophils are dispensable for NHO development following SCI, and rhG-CSF treatment post-SCI does not impact NHO development. Therefore, G-CSF treatment to promote neuroregeneration is unlikely to adversely promote or affect NHO development in SCI patients. © 2020 American Society for Bone and Mineral Research.

Costimulatory Effect of Rough Calcium Phosphate Coating and Blood Mononuclear Cells on Adipose-Derived Mesenchymal Stem Cells In Vitro as a Model of In Vivo Tissue Repair

Calcium phosphate (CaP) materials do not always induce ectopic vascularization and bone formation; the reasons remain unclear, and there are active discussions of potential roles for post-implantation hematoma, circulating immune and stem cells, and pericytes, but studies on adipose-derived stem cells (AMSCs) in this context are lacking. The rough (average surface roughness R_a = 2-5 µm) scaffold-like CaP coating deposited on pure titanium plates by the microarc oxidation method was used to investigate its subcutaneous vascularization in CBA/CaLac mice and in vitro effect on cellular and molecular crosstalk between human blood mononuclear cells (hBMNCs) and AMSCs (hAMSCs). Postoperative hematoma development on the CaP surface lasting 1-3 weeks may play a key role in the microvessel elongation and invasion into the CaP relief at the end of the 3rd week of injury and BMNC migration required for enhanced wound healing in mice. Satisfactory osteogenic and chondrogenic differentiation but poor adipogenic differentiation of hAMSCs on the rough CaP surface were detected in vitro by differential cell staining. The fractions of CD73⁺ (62%), CD90⁺ (0.24%), and CD105⁺ (0.41%) BMNCs may be a source of autologous circulating stem/progenitor cells for the subcutis reparation, but allogenic hBMNC participation is mainly related to the effects of CD4⁺ T cells co-stimulated with CaP coating on the in vitro recruitment of hAMSCs, their secretion of angiogenic and osteomodulatory molecules, and the increase in osteogenic features within the period of in vivo vascularization. Cellular and molecular crosstalk between BMNCs and AMSCs is a model of effective subcutis repair. Rough CaP surface enhanced angio- and osteogenic signaling between cells. We believe that preconditioning and/or co-transplantation of hAMSCs with hBMNCs may broaden their potential in applications related to post-implantation tissue repair and bone bioengineering caused by microarc CaP coating.

Local delivery of insulin/IGF-1 for bone regeneration: carriers, strategies, and effects

Bone defects caused by trauma, tumor resection, congenital malformation and infection are still a major challenge for clinicians. Biomimetic bone materials have attracted more and more attention in science and industry. Insulin and insulin-like growth factor-1 (IGF-1) have been increasingly recognized as an inducible factor for osteogenesis and angiogenesis. Spatiotemporal release of insulin may serve as the promising strategy. Considering the successful application of nanoparticles in drug loading, various insulin delivery systems have been developed, including (poly (lactic-co-glycolic acid), PLGA), hydroxyapatite (HA), gelatin, chitosan, alginate, and (γ-glutamic acid)/β-tricalcium phosphate, γ-PGA/β-TCP). Here, we have reviewed the progress on nanoparticles carrying insulin/IGF for bone regeneration. In addition, the key regulatory mechanism of insulin in bone regeneration is also summarized. The future application strategies and the challenges in bone regeneration are also discussed.

Macrophage subtype and cytokine expression characterization during the acute inflammatory phase of mouse bone fracture repair

Fracture repair is a complex process requiring heterotypic interactions between osteogenic cells and immune cells. Recent evidence indicates that macrophages are critically involved in fracture repair. Polarized macrophage populations differentially promote and regulate inflammation in other tissues, but little is known about the various macrophage subtypes and their signaling activities following a bone fracture. The authors hypothesized that classically activated (M1 subtype) and alternatively activated (M2 subtype) macrophages are active during the early repair process to initiate and regulate the inflammatory response. To test our hypothesis, bone marrow was collected from intact femurs (naïve group), contralateral and fractured femurs of mice on days 0, 1, 2, 4, and 7 postfracture. Macrophages were isolated from the bone marrow and macrophage subtypes were identified using flow cytometry with antibodies to F4/80, MHC II, CD86, CD11c, and CD40. Bone marrow cytokine levels were measured using xMAP. Flow cytometry revealed dynamic changes in M1 subtype (F4/80+/MHC II+/CD86+), M2 subtype (F4/80+/MHC II-/CD86-), and dendritic cell (DCs; MHCII+/CD11c+/CD40+) populations following fracture as compared to naïve controls. M1 subtype levels were correlated with IL-1α, IL-1ß, IL-2, IL-17, Eotaxin, and MCP-1, while DCs were correlated with IL-6, G-CSF, LIF, KC, and VEGF-A. The results indicate that M1 and M2 subtypes and DCs are recruited to the fracture site early during the repair process and consequently may work in tandem to regulate the inflammatory response required to recruit osteogenic cells needed for later stages of repair.

Vascularization Strategies in the Prevention of Nonunion Formation

Delayed healing and nonunion formation are major challenges in orthopedic surgery, which require the development of novel treatment strategies. Vascularization is considered one of the major prerequisites for successful bone healing, providing an adequate nutrient supply and allowing the infiltration of progenitor cells to the fracture site. Hence, during the last decade, a considerable number of studies have focused on the evaluation of vascularization strategies to prevent or to treat nonunion formation. These involve (1) biophysical applications, (2) systemic pharmacological interventions, and (3) tissue engineering, including sophisticated scaffold materials, local growth factor delivery systems, cell-based techniques, and surgical vascularization approaches. Accumulating evidence indicates that in nonunions, these strategies are indeed capable of improving the process of bone healing. The major challenge for the future will now be the translation of these strategies into clinical practice to make them accessible for the majority of patients. If this succeeds, these vascularization strategies may markedly reduce the incidence of nonunion formation. Impact statement Delayed healing and nonunion formation are a major clinical problem in orthopedic surgery. This review provides an overview of vascularization strategies for the prevention and treatment of nonunions. The successful translation of these strategies in clinical practice is of major importance to achieve adequate bone healing.

Chemokines and Bone

Chemokines are a family of small proteins, subdivided by their conserved cysteine residues and common structural features. Chemokines interact with their cognate G-protein-coupled receptors to elicit downstream signals that result in cell migration, proliferation, and survival. This review presents evidence for how the various CXC and CC subfamily chemokines influence bone hemostasis by acting on osteoclasts, osteoblasts, and progenitor cells. Also discussed are the ways in which chemokines contribute to bone loss as a result of inflammatory diseases such as rheumatoid arthritis, HIV infection, and periodontal infection. Both positive and negative effects of chemokines on bone formation and bone loss are presented. In addition, the role of chemokines in altering the bone microenvironment through effects on angiogenesis and tumor invasion is discussed. Very few therapeutic agents that influence bone formation by targeting chemokines or chemokine receptors are available, although a few are currently being evaluated.

Clinical and radiographic outcomes of allogeneic block grafts for maxillary lateral ridge augmentation: A randomized clinical trial

BACKGROUND

A main drawback of bone block graft surgery is the resorption occurring in early stages of healing. To our knowledge, there are no studies comparing outcomes of freeze-dried bone allograft (FDBA) blocks with different architecture.

PURPOSE

The aim of this work was to investigate different factors that can affect graft resorption and to compare the resorption rates of two different types of allogeneic blocks, corticocancellous and cancellous.

MATERIALS AND METHODS

A randomized clinical trial was designed. Twenty-eight patients referred for onlay bone augmentation prior to implant placement were included in the study. Preoperative computerized tomography (CT) was taken for all patients. Patients received FDBA blocks of either cancellous or corticocancellous bone obtained from the iliac crest. After a 4-month follow-up, postoperative CT was taken. Then, another surgery was performed, with the purpose to place dental implants. The aforementioned groups were compared for bone resorption and implant outcome using analysis of covariance (ANCOVA) and repeated ANOVA measures, respectively. Demographic data, trabecular bone density, and graft sites were also analyzed.

RESULTS

A total of 93 implants were placed in the augmented bone sites over 28 patients. A 100% survival rate was achieved during a mean follow-up period of 24 months in both groups. Higher bone resorption rate was found with cancellous bone grafts (29.2% ± 2.6) compared with corticocancellous grafts (19.3% ± 2.3). Moreover, higher resorption rates in patients with lower bone density (<185 Hounsfield Units) (31.7% ± 3.1) and smokers (26.39% ± 2.3) were observed when compared with patients with higher bone density (>185 Hounsfield Units) (16.8% ± 2.1) and nonsmokers (22.1% ± 2.3), respectively.

CONCLUSION

Within the limitations of this study, these findings indicate that both corticocancellous and cancellous FDBA grafts constitute a clinical acceptable alternative for bone reconstruction, although cancellous grafts present higher resorption rates. Moreover, host factors such as patient's low bone density and smoking habits may also increase graft resorption rates.

More friend than foe: the emerging role of neutrophils in tissue repair

Neutrophils are the most abundant immune cells in humans and serve as first responders to a myriad of host perturbations. Equipped with a plethora of antimicrobial molecules, neutrophils invade sites of inflammation to eradicate pathogens and clear debris. Traditionally, neutrophils were thought to cause collateral tissue damage before dying at the site. However, the presence of neutrophil infiltration into sterile injuries (in the absence of infections) suggests additional roles for these cells. Now, the view of neutrophils as indiscriminate killers seems to be changing as evolving evidence suggests that neutrophils actively orchestrate resolution of inflammation and contribute to tissue repair. Novel concepts include the idea that neutrophils are key to revascularization and subsequently reverse-transmigrate back to the vasculature, actively leaving sites of tissue damage to re-home to functional niches in the lung and bone marrow. This Review scrutinizes the role of neutrophils in tissue damage and repair, discussing recent findings and raising unresolved questions around this intriguing immune cell.

Osteogenesis and angiogenesis are simultaneously enhanced in BMP2-/VEGF-transfected adipose stem cells through activation of the YAP/TAZ signaling pathway

While bone has the capability to heal itself, there is a great difficulty in reconstituting large bone defects created by heavy trauma or the resection of malignant tumors.

Granulocyte-colony stimulating factor enhances bone fracture healing

BACKGROUND

Circulating mesenchymal stem cells contribute to bone repair. Their incorporation in fracture callus is correlated to their bioavailability. In addition, Granulocyte-colony stimulating factor induces the release of vascular and mesenchymal progenitors. We hypothesized that this glycoprotein stimulates fracture healing, and analyzed the effects of its administration at low doses on bone healing.

METHODS

27 adult male Sprague-Dawley rats underwent mid-femur osteotomy stabilized by centromedullar pinning. In a post (pre) operative group, rats were subcutaneously injected with 5 μg/kg per day of Granulocyte-colony stimulating factor for 5 days after (before) surgery. In a control group, rats were injected with saline solution for 5 days immediately after surgery. A radiographic consolidation score was calculated. At day 35, femurs were studied histologically and underwent biomechanical tests.

FINDINGS

5 weeks after surgery, mean radiographic scores were significantly higher in the Preop group 7.75 (SD 0.42) and in the Postop group 7.67 (SD 0.52) than in the control group 6.75 (SD 0.69). Biomechanical tests showed femur stiffness to be more than three times higher in both the Preop 109.24 N/mm (SD 51.86) and Postop groups 100.05 N/mm (SD 60.24) than in control 32.01 N/mm (SD 15.78). Mean maximal failure force was twice as high in the Preop group 68.66 N (SD 27.78) as in the control group 34.21 N (SD 11.79). Histological results indicated a later consolidation process in control than in treated groups.

INTERPRETATION

Granulocyte-colony stimulating factor injections strongly stimulated early femur fracture healing, indicating its potential utility in human clinical situations such as programmed osteotomy and fracture.

Neutrophils in Tissue Trauma of the Skin, Bone, and Lung: Two Sides of the Same Coin

Following severe tissue injury, patients are exposed to various danger- and microbe-associated molecular patterns, which provoke a strong activation of the neutrophil defense system. Neutrophils trigger and modulate the initial posttraumatic inflammatory response and contribute critically to subsequent repair processes. However, severe trauma can affect central neutrophil functions, including circulation half-life, chemokinesis, phagocytosis, cytokine release, and respiratory burst. Alterations in neutrophil biology may contribute to trauma-associated complications, including immune suppression, sepsis, multiorgan dysfunction, and disturbed tissue regeneration. Furthermore, there is evidence that neutrophil actions depend on the quality of the initial stimulus, including trauma localization and severity, the micromilieu in the affected tissue, and the patient's overall inflammatory status. In the present review, we describe the effects of severe trauma on the neutrophil phenotype and dysfunction and the consequences for tissue repair. We particularly concentrate on the role of neutrophils in wound healing, lung injury, and bone fractures, because these are the most frequently affected tissues in severely injured patients.

Five Days Granulocyte Colony-Stimulating Factor Treatment Increases Bone Formation and Reduces Gap Size of a Rat Segmental Bone Defect: A Pilot Study

Bone is an organ with high natural regenerative capacity and most fractures heal spontaneously when appropriate fracture fixation is provided. However, additional treatment is required for patients with large segmental defects exceeding the endogenous healing potential and for patients suffering from fracture non-unions. These cases are often associated with insufficient vascularization. Transplantation of CD34+ endothelial progenitor cells (EPCs) has been successfully applied to promote neovascularization of bone defects, however including extensive ex vivo manipulation of cells. Here, we hypothesized, that treatment with granulocyte colony-stimulating factor (G-CSF) may improve bone healing by mobilization of CD34+ progenitor cells into the circulation, which in turn may facilitate vascularization at the defect site. In this pilot study, we aimed to characterize the different cell populations mobilized by G-CSF and investigate the influence of cell mobilization on the healing of a critical size femoral defect in rats. Cell mobilization was investigated by flow cytometry at different time points after five consecutive daily G-CSF injections. In a pilot study, bone healing of a 4.5-mm critical femoral defect in F344 rats was compared between a saline-treated control group and a G-CSF treatment group. In vivo microcomputed tomography and histology were applied to compare bone formation in both treatment groups. Our data revealed that leukocyte counts show a peak increase at the first day after the last G-CSF injection. In addition, we found that CD34+ progenitor cells, including EPCs, were significantly enriched at day 1, and further increased at day 5 and day 11. Upregulation of monocytes, granulocytes and macrophages peaked at day 1. G-CSF treatment significantly increased bone volume and bone density in the defect, which was confirmed by histology. Our data show that different cell populations are mobilized by G-CSF treatment in cell specific patterns. Although in this pilot study no bridging of the critical defect was observed, significantly improved bone formation by G-CSF treatment was clearly shown.

Immunological characterization of the early human fracture hematoma

The initial inflammatory phase of fracture healing is of great importance for the clinical outcome. We aimed to develop a detailed time-dependent analysis of the initial fracture hematoma. We analyzed the composition of immune cell subpopulations by flow cytometry and the concentration of cytokines and chemokines by bioplex in 42 samples from human fractures of long bones <72 h post-trauma. The early human fracture hematoma is characterized by maturation of granulocytes and migration of monocytes/macrophages and hematopoietic stem cells. Both T helper cells and cytotoxic T cells proliferate within the fracture hematoma and/or migrate to the fracture site. Humoral immunity characteristics comprise high concentration of pro-inflammatory cytokines such as IL-6, IL-8, IFNγ and TNFα, but also elevated concentration of anti-inflammatory cytokines, e.g., IL-1 receptor antagonist and IL-10. Furthermore, we found that cells of the fracture hematoma represent a source for key chemokines. Even under the bioenergetically restricted conditions that exist in the initial fracture hematoma, immune cells are not only present, but also survive, mature, function and migrate. They secrete a cytokine/chemokine cocktail that contributes to the onset of regeneration. We hypothesize that this specific microenvironment of the initial fracture hematoma is among the crucial factors that determine fracture healing.

Variation in lipid mediator and cytokine levels during mouse femur fracture healing

Fracture healing is regulated by a variety of inflammatory mediators and growth factors which act over time to regenerate the injured tissue. This study used a mouse femur fracture model to quantify the temporal expression pattern of lipid mediators, cytokines, and related mRNAs during healing. Cyclooxygenase (COX-1 and COX-2) and 5-lipoxygenase (5-LO) derived lipid mediators, cytokines, and mRNA levels were quantified using mass spectrometry (LC-MS/MS), bead-based multiplex assays (xMAP), and quantitative PCR of cDNA (RTqPCR), respectively. Our analysis found that, the early inflammatory response (between 0 and 4 days after fracture) in the mouse femur fracture model coincided with elevated levels of COX-derived lipid mediators and inflammatory cytokines but with decreased levels of 5-LO-derived lipid mediators. Further, the COX-derived lipid mediators remained elevated for at least 7 days after fracture, suggesting that the COX-derived lipid mediators have additional functions during later phases of the fracture healing response. Differences were also found between mRNA levels and corresponding cytokines and lipid mediator levels, supporting a role for post-transcriptional regulation of gene expression. The temporal changes in fracture callus lipid mediator levels and inflammatory cytokines support a general positive role for inflammatory cytokines and COX-derived lipid mediators on fracture healing and a general negative role for 5-lipoxygenase derived lipid mediators during the initial stages of repair. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1883-1893, 2016.

The influence of locally applied granulocyte-colony stimulating factor on osteoporotic bone

OBJECTIVES

The compromised capacity of bone healing in osteoporotic population renders a serious concern of patients and clinicians. This study aimed to investigate the influence of G-CSF on bone reconstruction using an osteoporotic animal model.

MATERIALS AND METHODS

Sixty skeletal mature female Spraque-Dawley rats underwent bilateral ovariectomy (OVX) and were assigned into three groups (n = 20). Three months after OVX, defects of 5 mm in cranial and 2 mm in femur were surgically created on all the animals. The defects were left unfilled, filled with gelatin sponge (GS), or filled with granulocyte-colony stimulating factor (G-CSF) infused GS. Specimens were retrieved for histomorphometric and micro-CT analyses at weeks 1, 4, 8, and 12 after surgery.

RESULTS

At early stage of week 1 to week 8, the histomorphometric and micro-CT analysis demonstrated more advanced bone formation in femur in the control group; by week 12, all groups achieved cortical closure. In cranial bone, more advanced bone formation was exhibited in G-CSF-treated group at both early and late stages, although this observation was not statistically significant.

CONCLUSIONS

The results indicated that in osteoporotic bone, G-CSF may advance bone healing in cranial bone where spontaneous bone formation was insufficient.

Effect of Granulocyte-Colony Stimulating Factor on Endothelial Cells and Osteoblasts

OBJECTIVES

Some animal studies showed that granulocyte-colony stimulating factor (G-CSF) provides beneficial environment for bone healing. It has been well documented that endothelial cells and osteoblasts play critical roles in multiple phases of bone healing. However, the biological effects of G-CSF on these cells remain controversial. This study aimed to investigate the influence of G-CSF at various concentrations on endothelial cells and osteoblasts.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVECs) and human osteoblasts (hOBs) were treated with G-CSF at 1000, 100, 10, and 0 ng/mL, respectively. The capacity of cell proliferation, migration, and tube formation of HUVECs was evaluated at 72, 8, and 6 hours after treatment, respectively. The capacity of proliferation, differentiation, and mineralization of hOBs was evaluated at 24 hours, 72 hours, and 21 days after treatment, respectively.

RESULTS

HUVECs treated with 100 and 1000 ng/mL G-CSF showed a significantly higher value comparing with controls in migration assay (p < 0.001, p < 0.01, resp.); the group treated with 1000 ng/mL G-CSF showed a significantly lower value on tube formation. No significant difference was detected in groups of hOBs.

CONCLUSIONS

G-CSF showed favorable effects only on the migration of HUVECs, and no direct influence was found on hOBs.

Changing Paradigms in Cranio-Facial Regeneration: Current and New Strategies for the Activation of Endogenous Stem Cells

Craniofacial area represent a unique district of human body characterized by a very high complexity of tissues, innervation and vascularization, and being deputed to many fundamental function such as eating, speech, expression of emotions, delivery of sensations such as taste, sight, and earing. For this reasons, tissue loss in this area following trauma or for example oncologic resection, have a tremendous impact on patients' quality of life. In the last 20 years regenerative medicine has emerged as one of the most promising approach to solve problem related to trauma, tissue loss, organ failure etc. One of the most powerful tools to be used for tissue regeneration is represented by stem cells, which have been successfully implanted in different tissue/organs with exciting results. Nevertheless, both autologous and allogeneic stem cell transplantation raise many practical and ethical concerns that make this approach very difficult to apply in clinical practice. For this reason different cell free approaches have been developed aiming to the mobilization, recruitment, and activation of endogenous stem cells into the injury site avoiding exogenous cells implant but instead stimulating patients' own stem cells to repair the lesion. To this aim many strategies have been used including functionalized bioscaffold, controlled release of stem cell chemoattractants, growth factors, BMPs, Platelet-Rich-Plasma, and other new strategies such as ultrasound wave and laser are just being proposed. Here we review all the current and new strategies used for activation and mobilization of endogenous stem cells in the regeneration of craniofacial tissue.

Strategies to Stimulate Mobilization and Homing of Endogenous Stem and Progenitor Cells for Bone Tissue Repair

The gold standard for the treatment of critical-size bone defects is autologous or allogenic bone graft. This has several limitations including donor site morbidity and the restricted supply of graft material. Cell-based tissue engineering strategies represent an alternative approach. Mesenchymal stem cells (MSCs) have been considered as a source of osteoprogenitor cells. More recently, focus has been placed on the use of endothelial progenitor cells (EPCs), since vascularization is a critical step in bone healing. Although many of these approaches have demonstrated effectiveness for bone regeneration, cell-based therapies require time consuming and cost-expensive in vitro cell expansion procedures. Accordingly, research is becoming increasingly focused on the homing and stimulation of native cells. The stromal cell-derived factor-1 (SDF-1) - CXCR4 axis has been shown to be critical for the recruitment of MSCs and EPCs. Vascular endothelial growth factor (VEGF) is a key factor in angiogenesis and has been targeted in many studies. Here, we present an overview of the different approaches for delivering homing factors to the defect site by absorption or incorporation to biomaterials, gene therapy, or via genetically manipulated cells. We further review strategies focusing on the stimulation of endogenous cells to support bone repair. Finally, we discuss the major challenges in the treatment of critical-size bone defects and fracture non-unions.

Recent advances in bone regeneration using adult stem cells

Bone is a highly vascularized tissue reliant on the close spatial and temporal association between blood vessels and bone cells. Therefore, cells that participate in vasculogenesis and osteogenesis play a pivotal role in bone formation during prenatal and postnatal periods. Nevertheless, spontaneous healing of bone fracture is occasionally impaired due to insufficient blood and cellular supply to the site of injury. In these cases, bone regeneration process is interrupted, which might result in delayed union or even nonunion of the fracture. Nonunion fracture is difficult to treat and have a high financial impact. In the last decade, numerous technological advancements in bone tissue engineering and cell-therapy opened new horizon in the field of bone regeneration. This review starts with presentation of the biological processes involved in bone development, bone remodeling, fracture healing process and the microenvironment at bone healing sites. Then, we discuss the rationale for using adult stem cells and listed the characteristics of the available cells for bone regeneration. The mechanism of action and epigenetic regulations for osteogenic differentiation are also described. Finally, we review the literature for translational and clinical trials that investigated the use of adult stem cells (mesenchymal stem cells, endothelial progenitor cells and CD34⁺ blood progenitors) for bone regeneration.

Local drug delivery for enhancing fracture healing in osteoporotic bone

Fragility fractures can cause significant morbidity and mortality in patients with osteoporosis and inflict a considerable medical and socioeconomic burden. Moreover, treatment of an osteoporotic fracture is challenging due to the decreased strength of the surrounding bone and suboptimal healing capacity, predisposing both to fixation failure and non-union. Whereas a systemic osteoporosis treatment acts slowly, local release of osteogenic agents in osteoporotic fracture would act rapidly to increase bone strength and quality, as well as to reduce the bone healing period and prevent development of a problematic non-union. The identification of agents with potential to stimulate bone formation and improve implant fixation strength in osteoporotic bone has raised hope for the fast augmentation of osteoporotic fractures. Stimulation of bone formation by local delivery of growth factors is an approach already in clinical use for the treatment of non-unions, and could be utilized for osteoporotic fractures as well. Small molecules have also gained ground as stable and inexpensive compounds to enhance bone formation and tackle osteoporosis. The aim of this paper is to present the state of the art on local drug delivery in osteoporotic fractures. Advantages, disadvantages and underlying molecular mechanisms of different active species for local bone healing in osteoporotic bone are discussed. This review also identifies promising new candidate molecules and innovative approaches for the local drug delivery in osteoporotic bone.

Bone marrow derived stem cells in joint and bone diseases: a concise review

Stem cells have huge applications in the field of tissue engineering and regenerative medicine. Their use is currently not restricted to the life-threatening diseases but also extended to disorders involving the structural tissues, which may not jeopardize the patients' life, but certainly influence their quality of life. In fact, a particularly popular line of research is represented by the regeneration of bone and cartilage tissues to treat various orthopaedic disorders. Most of these pioneering research lines that aim to create new treatments for diseases that currently have limited therapies are still in the bench of the researchers. However, in recent years, several clinical trials have been started with satisfactory and encouraging results. This article aims to review the concept of stem cells and their characterization in terms of site of residence, differentiation potential and therapeutic prospective. In fact, while only the bone marrow was initially considered as a "reservoir" of this cell population, later, adipose tissue and muscle tissue have provided a considerable amount of cells available for multiple differentiation. In reality, recently, the so-called "stem cell niche" was identified as the perivascular space, recognizing these cells as almost ubiquitous. In the field of bone and joint diseases, their potential to differentiate into multiple cell lines makes their application ideally immediate through three main modalities: (1) cells selected by withdrawal from bone marrow, subsequent culture in the laboratory, and ultimately transplant at the site of injury; (2) bone marrow aspirate, concentrated and directly implanted into the injury site; (3) systemic mobilization of stem cells and other bone marrow precursors by the use of growth factors. The use of this cell population in joint and bone disease will be addressed and discussed, analysing both the clinical outcomes but also the basic research background, which has justified their use for the treatment of bone, cartilage and meniscus tissues.

Controlled release of granulocyte colony-stimulating factor enhances osteoconductive and biodegradable properties of Beta-tricalcium phosphate in a rat calvarial defect model

Autologous bone grafts remain the gold standard for the treatment of congenital craniofacial disorders; however, there are potential problems including donor site morbidity and limitations to the amount of bone that can be harvested. Recent studies suggest that granulocyte colony-stimulating factor (G-CSF) promotes fracture healing or osteogenesis. The purpose of the present study was to investigate whether topically applied G-CSF can stimulate the osteoconductive properties of beta-tricalcium phosphate (β-TCP) in a rat calvarial defect model. A total of 27 calvarial defects 5 mm in diameter were randomly divided into nine groups, which were treated with various combinations of a β-TCP disc and G-CSF in solution form or controlled release system using gelatin hydrogel. Histologic and histomorphometric analyses were performed at eight weeks postoperatively. The controlled release of low-dose (1 μg and 5 μg) G-CSF significantly enhanced new bone formation when combined with a β-TCP disc. Moreover, administration of 5 μg G-CSF using a controlled release system significantly promoted the biodegradable properties of β-TCP. In conclusion, the controlled release of 5 μg G-CSF significantly enhanced the osteoconductive and biodegradable properties of β-TCP. The combination of G-CSF slow-release and β-TCP is a novel and promising approach for treating pediatric craniofacial bone defects.

Clinical impact of circulating CD34-positive cells on bone regeneration and healing

Failures in fracture healing after conventional autologous and allogenic bone grafting are mainly due to poor vascularization. To meet the clinical demand, recent attentions in the regeneration and repair of bone have been focused on the use of stem cells such as bone marrow mesenchymal stem cells and circulating skeletal stem cells. Circulating stem cells are currently paid a lot of attention due to their ease of clinical setting and high potential for osteogenesis and angiogenesis. In this report, we focus on the first proof-of-principle experiments demonstrating the collaborative characteristics of circulating CD34(+) cells, known as endothelial and hematopoietic progenitor cell-rich population, which are capable to differentiate into both endothelial cells and osteoblasts. Transplantation of circulating CD34(+) cells provides a favorable environment for fracture healing via angiogenesis/vasculogenesis and osteogenesis, finally leading to functional recovery from fracture. Based on a series of basic studies, we performed a phase 1/2 clinical trial of autologous CD34(+) cell transplantation in patients with tibial or femoral nonunions and reported the safety and efficacy of this novel therapy. In this review, the current concepts and strategies in circulating CD34(+) cell-based therapy and its potential applications for bone repair will be highlighted.

Human cartilage fragments in a composite scaffold for single-stage cartilage repair: an in vitro study of the chondrocyte migration and the influence of TGF-β1 and G-CSF

PURPOSE

Minced chondral fragments are becoming popular as a source of cells for cartilage repair, as a growing interest is developing towards one-stage procedures to treat cartilage lesions. The purpose of this study is to (A) compare cell outgrowth from cartilage fragments of adult and young donors using two different types of scaffolds and (B) evaluate the influence of transforming-growth-factor-β1 (TGF-β1) and granulocyte colony-stimulating factor (G-CSF) on chondrocyte behaviour.

METHODS

In part (A) cartilage fragments from adult and young donors were either loaded onto an HA-derivative injectable paste scaffold or onto an HA-derivative membrane scaffold. Construct sections were then examined for cell counting after 1, 2 and 3 months. In part (B) only membrane scaffolds were prepared using cartilage fragments from young donors. Constructs were cultured either in standard growth medium or in the presence of specific growth factors, such as TGF-β1 or G-CSF or TGF-β1 + G-CSF. After 1 month, construct sections were examined for cell counting. Expression of chondrocyte markers (SOX9, CD151, CD49c) and proliferative markers (β-catenin, PCNA) was assessed using immunofluorescence techniques, both in unstimulated construct sections and in cells from unstimulated and stimulated construct cultures.

RESULTS

Part (A): histological analysis showed age-dependent and time-dependent chondrocyte migration. A significant difference (p < 0.05) was observed between young and older donors at the same time point. No difference was detected between the two types of scaffolds within the same group at the same time point. Part (B): after 1 month, the number of migrating cells/area significantly increased due to exposure to TGF-β1 and/or G-CSF (p < 0.05). Immunofluorescence revealed that outgrowing cells from unstimulated scaffold sections were positive for SOX9, CD151, CD49c and G-CSF receptor. Immunofluorescence of cells from construct cultures showed an increase in β-catenin in all stimulated groups and an increased PCNA expression in G-CSF-exposed cultures (p < 0.05).

CONCLUSION

Outgrowing cells may represent a subset of chondrocytes undergoing a phenotypic shift towards a proliferative state. TGF-β1, and to a greater extent G-CSF, may accelerate this outgrowth. The clinical relevance of this study may involve a potential future clinical application of scaffolds preloaded with growth factors as an additional coating for chondral fragments. Indeed, a controlled delivery of G-CSF, widely employed in various clinical settings, might improve the repair process driven by minced human cartilage fragments during one-stage cartilage repair.

Interactions between FGF21 and BMP-2 in osteogenesis

Lifestyle-related diseases are increasing and the challenge to create innovative drugs to treat such diseases is a main focus in medical science research. Fibroblast growth factor 21 (FGF21) is a powerful modulator of glucose and lipid metabolism, and is an innovative candidate drug already in clinical trials for type 2 diabetes mellitus and obesity. Bone fragility and impaired fracture healing induced by such lifestyle-related conditions are also a growing problem. Bone morphogenic proteins (BMPs) are well known osteogenic growth factors, and BMP-2 is used to augment bone formation in difficult clinical situations. There are many documented interactions between the FGF and BMP family proteins, although the interaction between FGF21 and BMP-2 remains unknown. The aim of this study was to reveal the effect of FGF21 toward BMP-2-dependent osteogenic activity, using C2C12 cells as a model system. We found that FGF21 enhanced BMP-2-dependent transcription and osteogenesis in the C2C12 cell line, which was confirmed by alkaline phosphatase activity, matrix mineralization, and gene expression. Mechanistically, FGF21 enhanced BMP-2-induced intracellular signaling through Smad proteins, but not through p44/42MAPK proteins. Furthermore, we identified a negative feedback loop in which BMP-2 decreased endogenous FGF21 mRNA expression. In summary, this study demonstrates interactions between BMP-2 and FGF21 pathways exist in vitro, and that FGF21 enhances the osteogenic activity of BMP-2 by up-regulating the BMP-2-dependent Smad signaling pathway.

Bone marrow-derived cell mobilization by G-CSF to enhance osseointegration of bone substitute in high tibial osteotomy

PURPOSE

To evaluate granulocyte colony-stimulating factor (G-CSF) efficacy in accelerating bone regeneration following opening-wedge high tibial valgus osteotomy for genu varum.

METHODS

A phase II trial was conducted for evaluating the preoperative administration of G-CSF given at 10 μg/kg/day for 3 consecutive days with an additional half-dose 4 h before the opening-wedge high tibial valgus osteotomy. Overall, 12 patients (Group A) received G-CSF treatment, and the subsequent 12 patients (Group B) underwent surgery without G-CSF. The osteotomy gap was filled by a bone graft substitute. Bone marrow cell (BMC) mobilization was monitored by CD34+ve cell and clonogenic progenitor cell analysis. All patients underwent a clinical (Lysholm Knee Scale and SF-36) and radiographic evaluation preoperatively, as well as at given intervals postsurgery.

RESULTS

All patients completed the treatment program without major side effects; G-CSF was well tolerated. BMC mobilization occurred in all Group A patients, with median peak values of circulating CD34+ve cells of 110/μL (range 29-256). Circulating clonogenic progenitors paralleled CD34+ve cell levels. A significant improvement in Lysholm Knee Scale was recorded at follow-up in Group A compared to Group B. At the radiographic evaluation, there was a significant increase in osseointegration at the bone-graft junction in Group A at 1, 2, 3 and 6 months postsurgery compared to Group B. The computerized tomography scan of the grafted area at 2 months postsurgery showed no significant difference in the quality of the newly formed bone between the two Groups.

CONCLUSIONS

Although the limited number of patients does not allow firm conclusions, the study suggests that G-CSF can be safely administered preoperatively in subjects undergoing opening-wedge high tibial valgus osteotomy; in addition, the clinical, radiographic and CT monitoring indicate that G-CSF and/or mobilized BMCs may hasten bone graft substitute osseointegration.

LEVEL OF EVIDENCE

I.

CD34-positive primitive vessels and other structures in human fetuses: an immunohistochemical study

CONCLUSION

CD34-positive mesenchymal tissues are likely to play a critical role in the pattern formation of laryngeal and pharyngeal walls. In the ear, nose, and throat regions, a future clinical relevance may be found in their use for muscle repair and regeneration.

OBJECTIVES

CD34 is a well-known marker of progenitor cells of blood vessels and stromal tissues. Thus, CD34-positive cells have recently been used clinically in the field of vascular and orthopedic biotechnology because of their capacity to assist regeneration of injured tissues. However, to our knowledge, the in situ distribution of CD34-positive cells has not yet been described in the human fetus, with the exception of a few organs. The purpose of this study was to describe the location of CD34-positive cells in the head and neck regions.

METHODS

Immunohistochemistry for CD34 was carried out using seven human fetuses (12 and 15 weeks of gestation).

RESULTS

CD34-positive structures showed a vessel-like appearance and were regularly arrayed along the nasal, oral, pharyngeal, and laryngeal mucosal epithelia, whereas in the laryngeal and pharyngeal striated muscles they were distributed diffusely as fibrous tissues such as the fascia and perimysium.

CD34-positive developing vessels and other structures in human fetuses: an immunohistochemical study

CD34 is a well-known marker of progenitor cells of blood vessels and stromal tissues. Thus, CD34-positive cells have recently been used clinically in the field of vascular and orthopedic biotechnology because of their capacity to assist regeneration of injured tissues. However, to our knowledge, the in situ distribution of CD34-positive cells has not yet been described in the human fetus, with the exception of a few organs. In the present study, we conducted immunohistochemistry for CD34 using 12 human fetuses (9-15 weeks of gestation). CD34-positive structures showed a vessel-like appearance and were regularly arrayed in the viscera, nerves and lymph nodes, whereas in the body wall and extremities, they were distributed diffusely as fibrous tissues, such as the fascia and perimysium. The myocardium was also divided and bundled by CD34-positive fibrous tissues. In striated muscles, limited examples of CD34 expression were found in the tongue and extraocular muscles in which only vessels were positive. Lymphatic vessels were negative for CD34. In addition to their contribution to vascular development in any part of the body, CD34-positive mesenchymal tissues seem to play a critical role in the pattern formation of skeletal muscle, synovial tissue and other muscle/tendon-associated tissues in human fetuses.

Osteal macrophages promote in vivo intramembranous bone healing in a mouse tibial injury model

Bone-lining tissues contain a population of resident macrophages termed osteomacs that interact with osteoblasts in vivo and control mineralization in vitro. The role of osteomacs in bone repair was investigated using a mouse tibial bone injury model that heals primarily through intramembranous ossification and progresses through all major phases of stabilized fracture repair. Immunohistochemical studies revealed that at least two macrophage populations, F4/80(+) Mac-2(-/low) TRACP(-) osteomacs and F4/80(+) Mac-2(hi) TRACP(-) inflammatory macrophages, were present within the bone injury site and persisted throughout the healing time course. In vivo depletion of osteomacs/macrophages (either using the Mafia transgenic mouse model or clodronate liposome delivery) or osteoclasts (recombinant osteoprotegerin treatment) established that osteomacs were required for deposition of collagen type 1(+) (CT1(+)) matrix and bone mineralization in the tibial injury model, as assessed by quantitative immunohistology and micro-computed tomography. Conversely, administration of the macrophage growth factor colony-stimulating factor 1 (CSF-1) increased the number of osteomacs/macrophages at the injury site significantly with a concurrent increase in new CT1(+) matrix deposition and enhanced mineralization. This study establishes osteomacs as participants in intramembranous bone healing and as targets for primary anabolic bone therapies.