Osteoprogenitor cells of mature human skeletal muscle tissue: an in vitro study

Bone regeneration in critical-size defects of the mandible using biomechanically adapted CAD/CAM hybrid scaffolds: An in vivo study in miniature pigs

The study aimed to analyze bone regeneration in critical-size defects using hybrid scaffolds biomechanically adapted to the specific defect and adding the growth factor rhBMP-2. For this animal study, ten minipigs underwent bilateral defects in the corpus mandibulae and were subsequently treated with novel cylindrical hybrid scaffolds. These scaffolds were designed digitally to suit the biomechanical requirements of the mandibular defect, utilizing finite element analysis. The scaffolds comprised zirconium dioxide-tricalcium phosphate (ZrO2-TCP) support struts and TCP foam ceramics. One scaffold in each animal was loaded with rhBMP-2 (100 μg/cm³), while the other served as an unloaded negative control. Fluorescent dyes were administered every 2 weeks, and computed tomography (CT) scans were conducted every 4 weeks. Euthanasia was performed after 3 months, and samples were collected for examination using micro-CT and histological evaluation of both hard and soft tissue. Intravital CT examinations revealed minor changes in radiographic density from 4 to 12 weeks postoperatively. In the group treated with rhBMP-2, radiographic density shifted from 2513 ± 128 (mean ± SD) to 2606 ± 115 Hounsfield units (HU), while the group without rhBMP-2 showed a change from 2430 ± 131 to 2601 ± 67 HU. Prior to implantation, the radiological density of samples measured 1508 ± 30 mg HA/cm³, whereas post-mortem densities were 1346 ± 71 mg HA/cm³ in the rhBMP-2 group and 1282 ± 91 mg HA/cm³ in the control group (p = 0.045), as indicated by micro-CT measurements. The histological assessment demonstrated successful ossification in all specimens. The newly formed bone area proportion was significantly greater in the rhBMP-2 group (48 ± 10%) compared with the control group without rhBMP-2 (42 ± 9%, p = 0.03). The mean area proportion of remaining TCP foam was 23 ± 8% with rhBMP-2 and 24 ± 10% without rhBMP-2. Successful bone regeneration was accomplished by implanting hybrid scaffolds into critical-size mandibular defects. Loading these scaffolds with rhBMP-2 led to enhanced bone regeneration and a uniform distribution of new bone formation within the hybrid scaffolds. Further studies are required to determine the adaptability of hybrid scaffolds for larger and potentially segmental defects in the maxillofacial region.

Preliminary study on the electromagnetic field treatment of osteoporosis in rats

OBJECTIVE:

In our study, the influence of PEMF on skeleton morphology and bone metabolism on rats with disuse osteoporosis was investigated, and the possibility of using it for the treatment of disuse osteoporosis was explored.

METHODS:

The rats in the ALN group were treated with alendronate, and the rats in the PEMF group were exposed to pulsed electromagnetic fields (3.82 mT, 10 Hz) for 40 mind-1. Rats were sacrificed by the end of 2, 4, 8 and 12 weeks, and serum and right leg bones were collected. Serum BMP-2, BGP concentrations and bone metabolism and biomechanical parameters were measured.

RESULTS:

The bone structural mechanical indices and material mechanical indices of the right femur in all groups of mice during weeks 2 and 4 were decreased. At week 8 the bone structural mechanical index and maximum stress of the right femur in the ALN group were markedly raised compared with the CON group (P< 0.01). Only maximum stress and strain were improved in the ALN group and had a significant difference (P< 0.05) at week 12. The serum BGP and BMP-2 concentration in the PEMF and ALN groups was increased (P< 0.05) at week 2, but this increase was not synchronized. After 8 weeks, BGP and BMP-2 level in the PEMF group was observably elevated (P<0.01) in contrast to the ALN group.

CONCLUSION:

From the experimental time interval analysis, PEMF can improve the mechanical stability of bone structure more gently and permanently than alendronate.

Severe Heterotopic Ossification in the Skeletal Muscle and Endothelial Cells Recruitment to Chondrogenesis Are Enhanced by Monocyte/Macrophage Depletion

Altered macrophage infiltration upon tissue damage results in inadequate healing due to inappropriate remodeling and stem cell recruitment and differentiation. We investigated in vivo whether cells of endothelial origin phenotypically change upon heterotopic ossification induction and whether infiltration of innate immunity cells influences their commitment and alters the ectopic bone formation. Liposome-encapsulated clodronate was used to assess macrophage impact on endothelial cells in the skeletal muscle upon acute damage in the ECs specific lineage-tracing Cdh5CreER^T2:R26REYFP/dtTomato transgenic mice. Macrophage depletion in the injured skeletal muscle partially shifts the fate of ECs toward endochondral differentiation. Upon ectopic stimulation of BMP signaling, monocyte depletion leads to an enhanced contribution of ECs chondrogenesis and to ectopic bone formation, with increased bone volume and density, that is reversed by ACVR1/SMAD pathway inhibitor dipyridamole. This suggests that macrophages contribute to preserve endothelial fate and to limit the bone lesion in a BMP/injury-induced mouse model of heterotopic ossification. Therefore, alterations of the macrophage-endothelial axis may represent a novel target for molecular intervention in heterotopic ossification.

Scaffold implantation in the omentum majus of rabbits for new bone formation

Restoration of the mandible after defects caused by ablative surgery remains challenging. Microvascular free flaps from the scapula, fibula or iliac crest remain the 'gold standard'. A drawback of these methods is donor-side morbidity, availability and the shape of the bone. Former cases have shown that prefabrication of a customized bone flap in the latissimus dorsi muscle may be successful; however, this method is still associated with high donor-side morbidity. Osteogenesis in the omentum majus of rabbits by wrapping the periosteum into it was confirmed recently and is particularly interesting for bone endocultivation. Twelve adult male New Zealand white rabbits were used. In each, two hydroxyapatite blocks were implanted in the greater omentum with autologous bone or autologous bone + rhBMP-2. Bone density measurements were performed by CT scans. Fluorochrome labelling was used for new bone formation detection. The animals were sacrificed at week 10, and the specimens were harvested for histological and histomorphometric analysis. In histological and fluorescence microscopic analysis, new bone formation could be found, as well as new blood vessels and connective tissue. No significant differences were found regarding the histological analysis and bone density measurements between the groups. It could be demonstrated that the omentum majus is a practical way to use one's own body as a bioreactor for prefabrication of tissue-engineered bony constructs. Regarding the influence and exact dose of rhBMP-2, further research is necessary. To establish and improve this method, further large-animal experimental studies are also necessary.

Identification and Characterization of Skeletal Muscle Stem Cells from Human Orbicularis Oculi Muscle

Skeletal muscle stem cell (SMSC) transplantation has shown great therapeutical potential in repairing muscle loss and dysfunction, but the muscle acquisition is usually a traumatic procedure causing pain and morbidity to the donor. In this study, we investigated the feasibility of isolating SMSCs from human orbicularis oculi muscle (OOM), which is routinely removed and discarded during ophthalmic cosmetic surgeries. OOM fragments were harvested from 18 female healthy donors undergoing upper eyelid plasties. Plastic-adherent cells were isolated from the muscles using a two-step plating method combined with collagenase digestion. A total of 15 cell cultures were successfully established from the muscle samples. These adherent cells were positive for the specific markers of SMSCs and could be directed toward the osteogenic, adipogenic, chondrogenic, and myogenic phenotypes in the presence of lineage-specific inductive media. Moreover, after cultured in the myogenic inductive medium for 3 weeks, the muscle cells were injected into the tibialis anterior muscles of nude mice and the cell fate was detected using a DiI-labeling technique. In vivo myogenesis was evidenced by the expression of DiI fluorescence after cell transplantation. The donor cells could be found in the satellite cell position and incorporated into the host myofibers. Our results demonstrated that human OOM represents a novel source of myogenic precursors with stem cell-like properties, which may provide a foundation for the SMSC-based therapeutics of skeletal muscle diseases.

In Vitro and In Vivo Osteogenesis of Human Orbicularis Oculi Muscle-Derived Stem Cells

BACKGROUND

Cell-based therapies for treating bone defects require a source of stem cells with osteogenic potential. There is evidence from pathologic ossification within muscles that human skeletal muscles contain osteogenic progenitor cells. However, muscle samples are usually acquired through a traumatic biopsy procedure which causes pain and morbidity to the donor. Herein, we identified a new alternative source of skeletal muscle stem cells (SMSCs) without conferring morbidity to donors.

METHODS

Adherent cells isolated from human orbicularis oculi muscle (OOM) fragments, which are currently discarded during ophthalmic cosmetic surgeries, were obtained using a two-step plating method. The cell growth kinetics, immunophenotype and capabilities of in vitro multilineage differentiation were evaluated respectively. Moreover, the osteogenically-induced cells were transduced with GFP gene, loaded onto the porous β-tricalcium phosphate (β-TCP) bioceramics, and transplanted into the subcutaneous site of athymic mice. Ectopic bone formation was assessed and the cell fate in vivo was detected.

RESULTS

OOM-derived cells were fibroblastic in shape, clonogenic in growth, and displayed phenotypic and behavioral characteristics similar to SMSCs. In particular, these cells could be induced into osteoblasts in vitro evidenced by the extracellular matrix calcification and enhanced alkaline phosphatase (ALP) activity and osteocalcin (OCN) production. New bone formation was found in the cell-loaded bioceramics 6 weeks after implantation. By using the GFP-labeling technique, these muscle cells were detected to participate in the process of ectopic osteogenesis in vivo.

CONCLUSION

Our data suggest that human OOM tissue is a valuable and noninvasive resource for osteoprogenitor cells to be used in bone repair and regeneration.

Pericytes in Muscular Dystrophies

The muscular dystrophies are an heterogeneous group of inherited myopathies characterised by the progressive wasting of skeletal muscle tissue. Pericytes have been shown to make muscle in vitro and to contribute to skeletal muscle regeneration in several animal models, although recent data has shown this to be controversial. In fact, some pericyte subpopulations have been shown to contribute to fibrosis and adipose deposition in muscle. In this chapter, we explore the identity and the multifaceted role of pericytes in dystrophic muscle, potential therapeutic applications and the current need to overcome the hurdles of characterisation (both to identify pericyte subpopulations and track cell fate), to prevent deleterious differentiation towards myogenic-inhibiting subpopulations, and to improve cell proliferation and engraftment efficacy.

Pericytes in Skeletal Muscle

Skeletal muscle regeneration is a highly orchestrated process and involves the activation of many cellular and molecular pathways. Although satellite cells (SCs) are the major cell type responsible for muscle regeneration, pericytes show remarkable myogenic potential and various advantages as cell therapy in muscular disorders. This chapter first introduces the structure, marker expression, origin, and category of pericytes. Next, we discuss their functions in muscular dystrophy and/or muscle injuries, focusing on their myogenic, adipogenic, fibrogenic, chondrogenic, and osteogenic activities. Understanding this knowledge will promote the development of innovative cell therapies for muscle disorders, including muscular dystrophy.

The superior regenerative potential of muscle-derived stem cells for articular cartilage repair is attributed to high cell survival and chondrogenic potential

Three populations of muscle-derived cells (PP1, PP3, and PP6) were isolated from mouse skeletal muscle using modified preplate technique and retrovirally transduced with BMP4/GFP.  In vitro, the PP1 cells (fibroblasts) proliferated significantly slower than the PP3 (myoblasts) and PP6 cells (muscle-derived stem cells); the PP1 and PP6 cells showed a superior rate of survival compared with PP3 cells under oxidative stress; and the PP6 cells showed significantly superior chondrogenic capabilities than PP1 and PP3 cells. In vivo, the PP6 cells promoted superior cartilage regeneration compared with the other muscle-derived cell populations. The cartilage defects in the PP6 group had significantly higher histological scores than those of the other muscle-derived cell groups, and GFP detection revealed that the transplanted PP6 cells showed superior in vivo cell survival and chondrogenic capabilities compared with the PP1 and PP3 cells. PP6 cells (muscle-derived stem cells) are superior to other primary muscle-derived cells for use as a cellular vehicle for BMP4-based ex vivo gene therapy to heal full-thickness osteo-chondral defects. The superiority of the PP6/muscle-derived stem cells appears to be attributable to a combination of increased rate of in vivo survival and superior chondrogenic differentiation capacity.

Bone Tissue Engineering: Past-Present-Future

Bone is one of the few tissues to display a true potential for regeneration. Fracture healing is an obvious example where regeneration occurs through tightly regulated sequences of molecular and cellular events which recapitulate tissue formation seen during embryogenesis. Still in some instances, bone regeneration does not occur properly (i.e. critical size lesions) and an appropriate therapeutic intervention is necessary. Successful replacement of bone by tissue engineering will likely depend on the recapitulation of this flow of events. In fact, bone regeneration requires cross-talk between microenvironmental factors and cells; for example, resident mesenchymal progenitors are recruited and properly guided by soluble and insoluble signaling molecules. Tissue engineering attempts to reproduce and to mimic this natural milieu by delivering cells capable of differentiating into osteoblasts, inducing growth factors and biomaterials to support cellular attachment, proliferation, migration, and matrix deposition. In the last two decades, a significant effort has been made by the scientific community in the development of methods and protocols to repair and regenerate tissues such as bone, cartilage, tendons, and ligaments. In this same period, great advancements have been achieved in the biology of stem cells and on the mechanisms governing "stemness". Unfortunately, after two decades, effective clinical translation does not exist, besides a few limited examples. Many years have passed since cell-based regenerative therapies were first described as "promising approaches", but this definition still engulfs the present literature. Failure to envisage translational cell therapy applications in routine medical practice evidences the existence of unresolved scientific and technical struggles, some of which still puzzle researchers in the field and are presented in this chapter.

Contributions of muscle-resident progenitor cells to homeostasis and disease

Adult skeletal muscle maintains a homeostatic state with modest levels of cellular turnover, unlike the skin or blood. However, the muscle is highly sensitive to tissue injury, which unleashes a cascade of regenerative and inflammatory processes. Muscle regeneration involves cross-talk between numerous cytokine signaling axes, and the coordinated activity of multiple muscle-resident and circulating progenitor populations. Satellite cells, closely associated with myofibers, are established as the canonical muscle stem cell, with self-renewal and myofiber-regenerating capacity. However, a heterogeneous group of mesenchymal progenitor cells residing within the muscle interstitium are also highly responsive to muscle injury and exhibit varying degrees of regenerative potential. These cells interact with satellite cells via direct and indirect mechanisms to regulate regeneration or repair. We describe the known phylogenetic and functional relationships of the multiple progenitor populations residing within skeletal muscle, their putative roles in the coordination of injury repair, and their possible contributions to health and disease.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

The inhibition effects of insulin on BMP2-induced muscle heterotopic ossification

Bone morphogenetic proteins (BMPs) play an important role in regulating osteoblastic differentiation and bone formation. But the diffuse of BMPs into muscle tissues around bone injury sites often leads to heterotopic ossification, which has been regarded as one of major side-effects of BMP implementation in bone defect patients. It raises great demands for exploring effective methods that preventing BMP-induced heterotopic ossification while not interrupting the osteoinductive activity of BMPs for in situ bone defect repair. Here we found insulin, a positive regulator for bone regeneration, inhibited BMP2-induced muscle heterotopic ossification by suppressing the expression of bone transcription factor Osterix. By analyzing downstream molecules of insulin pathway, we found AKT/mTOR/GSK3 signaling was responsible for the inhibition of insulin on BMP2-induced ossification, and GSK3 inhibitor SB216763 attenuated BMP2-induced muscle heterotopic ossification. The data might shed light on developing effective clinical therapy for inhibiting muscle heterotopic ossification when BMPs were used bone defect repair.

Sustained release of bone morphogenetic protein 2 via coacervate improves the osteogenic potential of muscle-derived stem cells

Muscle-derived stem cells (MDSCs) isolated from mouse skeletal muscle by a modified preplate technique exhibit long-term proliferation, high self-renewal, and multipotent differentiation capabilities in vitro. MDSCs retrovirally transduced to express bone morphogenetic proteins (BMPs) can differentiate into osteocytes and chondrocytes and enhance bone and articular cartilage repair in vivo, a feature that is not observed with nontransduced MDSCs. These results emphasize that MDSCs require prolonged exposure to BMPs to undergo osteogenic and chondrogenic differentiation. A sustained BMP protein delivery approach provides a viable and potentially more clinically translatable alternative to genetic manipulation of the cells. A unique growth factor delivery platform comprised of native heparin and a synthetic polycation, poly(ethylene argininylaspartate diglyceride) (PEAD), was used to bind, protect, and sustain the release of bone morphogenetic protein-2 (BMP2) in a temporally and spatially controlled manner. Prolonged exposure to BMP2 released by the PEAD:heparin delivery system promoted the differentiation of MDSCs to an osteogenic lineage in vitro and induced the formation of viable bone at an ectopic site in vivo. This new strategy represents an alternative approach for bone repair mediated by MDSCs while bypassing the need for gene therapy.

Platelet-rich plasma promotes the proliferation of human muscle derived progenitor cells and maintains their stemness

Human muscle-derived progenitor cells (hMDPCs) offer great promise for muscle cell-based regenerative medicine; however, prolonged ex-vivo expansion using animal sera is necessary to acquire sufficient cells for transplantation. Due to the risks associated with the use of animal sera, the development of a strategy for the ex vivo expansion of hMDPCs is required. The purpose of this study was to investigate the efficacy of using platelet-rich plasma (PRP) for the ex-vivo expansion of hMDPCs. Pre-plated MDPCs, myoendothelial cells, and pericytes are three populations of hMDPCs that we isolated by the modified pre-plate technique and Fluorescence Activated Cell Sorting (FACS), respectively. Pooled allogeneic human PRP was obtained from a local blood bank, and the effect that thrombin-activated PRP-releasate supplemented media had on the ex-vivo expansion of the hMDPCs was tested against FBS supplemented media, both in vitro and in vivo. PRP significantly enhanced short and long-term cell proliferation, with or without FBS supplementation. Antibody-neutralization of PDGF significantly blocked the mitogenic/proliferative effects that PRP had on the hMDPCs. A more stable and sustained expression of markers associated with stemness, and a decreased expression of lineage specific markers was observed in the PRP-expanded cells when compared with the FBS-expanded cells. The in vitro osteogenic, chondrogenic, and myogenic differentiation capacities of the hMDPCs were not altered when expanded in media supplemented with PRP. All populations of hMDPCs that were expanded in PRP supplemented media retained their ability to regenerate myofibers in vivo. Our data demonstrated that PRP promoted the proliferation and maintained the multi-differentiation capacities of the hMDPCs during ex-vivo expansion by maintaining the cells in an undifferentiated state. Moreover, PDGF appears to be a key contributing factor to the beneficial effect that PRP has on the proliferation of hMDPCs.

Osteogenic differentiation capacity of human skeletal muscle-derived progenitor cells

Heterotopic ossification (HO) is defined as the formation of ectopic bone in soft tissue outside the skeletal tissue. HO is thought to result from aberrant differentiation of osteogenic progenitors within skeletal muscle. However, the precise origin of HO is still unclear. Skeletal muscle contains two kinds of progenitor cells, myogenic progenitors and mesenchymal progenitors. Myogenic and mesenchymal progenitors in human skeletal muscle can be identified as CD56⁺ and PDGFRα⁺ cells, respectively. The purpose of this study was to investigate the osteogenic differentiation potential of human skeletal muscle-derived progenitors. Both CD56⁺ cells and PDGFRα⁺ cells showed comparable osteogenic differentiation potential in vitro. However, in an in vivo ectopic bone formation model, PDGFRα⁺ cells formed bone-like tissue and showed successful engraftment, while CD56⁺ cells did not form bone-like tissue and did not adapt to an osteogenic environment. Immunohistological analysis of human HO sample revealed that many PDGFRα⁺ cells were localized in proximity to ectopic bone formed in skeletal muscle. MicroRNAs (miRNAs) are known to regulate many biological processes including osteogenic differentiation. We investigated the participation of miRNAs in the osteogenic differentiation of PDGFRα⁺ cells by using microarray. We identified miRNAs that had not been known to be involved in osteogenesis but showed dramatic changes during osteogenic differentiation of PDGFRα⁺ cells. Upregulation of miR-146b-5p and -424 and downregulation of miR-7 during osteogenic differentiation of PDGFRα⁺ cells were confirmed by quantitative real-time RT-PCR. Inhibition of upregulated miRNAs, miR-146b-5p and -424, resulted in the suppression of osteocyte maturation, suggesting that these two miRNAs have the positive role in the osteogenesis of PDGFRα⁺ cells. Our results suggest that PDGFRα⁺ cells may be the major source of HO and that the newly identified miRNAs may regulate osteogenic differentiation process of PDGFRα⁺ cells.

Therapeutic potential of perivascular cells from human pluripotent stem cells

Vascularization of injured tissues or artificial grafts is a major challenge in tissue engineering, stimulating a continued search for alternative sources for vasculogenic cells and the development of therapeutic strategies. Human pluripotent stem cells (hPSCs), either embryonic or induced, offer a plentiful platform for the derivation of large numbers of vasculogenic cells, as required for clinical transplantations. Various protocols for generation of vasculogenic smooth muscle cells (SMCs) from hPSCs have been described with considerably different SMC derivatives. In addition, we recently identified hPSC-derived pericytes, which are similar to their physiological counterparts, exhibiting unique features of blood vessel-residing perivascular cells, as well as multipotent mesenchymal precursors with therapeutic angiogenic potential. In this review we refer to methodologies for the development of a variety of perivascular cells from hPSCs with respect to developmental induction, differentiation capabilities, potency and their dual function as mesenchymal precursors. The therapeutic effect of hPSC-derived perivascular cells in experimental models of tissue engineering and regenerative medicine are described and compared to those of their native physiological counterparts.

BMP2 is superior to BMP4 for promoting human muscle-derived stem cell-mediated bone regeneration in a critical-sized calvarial defect model

Muscle-derived cells have been successfully isolated using a variety of different methods and have been shown to possess multilineage differentiation capacities, including an ability to differentiate into articular cartilage and bone in vivo; however, the characterization of human muscle-derived stem cells (hMDSCs) and their bone regenerative capacities have not been fully investigated. Genetic modification of these cells may enhance their osteogenic capacity, which could potentially be applied to bone regenerative therapies. We found that hMDSCs, isolated by the preplate technique, consistently expressed the myogenic marker CD56, the pericyte/endothelial cell marker CD146, and the mesenchymal stem cell markers CD73, CD90, CD105, and CD44 but did not express the hematopoietic stem cell marker CD45, and they could undergo osteogenic, chondrogenic, adipogenic, and myogenic differentiation in vitro. In order to investigate the osteoinductive potential of hMDSCs, we constructed a retroviral vector expressing BMP4 and GFP and a lentiviral vector expressing BMP2. The BMP4-expressing hMDSCs were able to undergo osteogenic differentiation in vitro and exhibited enhanced mineralization compared to nontransduced cells; however, when transplanted into a calvarial defect, they failed to regenerate bone. Local administration of BMP4 protein and cell pretreatment with N-acetylcysteine (NAC), which improves cell survival, did not enhance the osteogenic capacity of the retro-BMP4-transduced cells. In contrast, lenti-BMP2-transduced hMDSCs not only exhibited enhanced in vitro osteogenic differentiation but also induced robust bone formation and nearly completely healed a critical-sized calvarial defect in CD-1 nude mice 6 weeks following transplantation. Herovici's staining of the regenerated bone demonstrated that the bone matrix contained a large amount of type I collagen. Our findings indicated that the hMDSCs are likely mesenchymal stem cells of muscle origin and that BMP2 is more efficient than BMP4 in promoting the bone regenerative capacity of the hMDSCs in vivo.

The role of endothelial-mesenchymal transition in heterotopic ossification

Heterotopic ossification (HO) is a process by which bone forms in soft tissues, in response to injury, inflammation, or genetic disease. This usually occurs by initial cartilage formation, followed by endochondral ossification. A rare disease called fibrodysplasia ossificans progressiva (FOP) allows this mechanism to be induced by a combination of genetic mutation and acute inflammatory responses. FOP patients experience progressive HO throughout their lifetime and form an ectopic skeleton. Recent studies on FOP have suggested that heterotopic cartilage and bone is of endothelial origin. Vascular endothelial cells differentiate into skeletal cells through a mesenchymal stem cell intermediate that is generated by endothelial-mesenchymal transition (EndMT). Local inflammatory signals and/or other changes in the tissue microenvironment mediate the differentiation of endothelial-derived mesenchymal stem cells into chondrocytes and osteoblasts to induce HO. We discuss the current evidence for the endothelial contribution to heterotopic bone formation.

Characterization of distinct mesenchymal-like cell populations from human skeletal muscle in situ and in vitro

Human skeletal muscle is an essential source of various cellular progenitors with potential therapeutic perspectives. We first used extracellular markers to identify in situ the main cell types located in a satellite position or in the endomysium of the skeletal muscle. Immunohistology revealed labeling of cells by markers of mesenchymal (CD13, CD29, CD44, CD47, CD49, CD62, CD73, CD90, CD105, CD146, and CD15 in this study), myogenic (CD56), angiogenic (CD31, CD34, CD106, CD146), hematopoietic (CD10, CD15, CD34) lineages. We then analysed cell phenotypes and fates in short- and long-term cultures of dissociated muscle biopsies in a proliferation medium favouring the expansion of myogenic cells. While CD56(+) cells grew rapidly, a population of CD15(+) cells emerged, partly from CD56(+) cells, and became individualized. Both populations expressed mesenchymal markers similar to that harboured by human bone marrow-derived mesenchymal stem cells. In differentiation media, both CD56(+) and CD15(+) cells shared osteogenic and chondrogenic abilities, while CD56(+) cells presented a myogenic capacity and CD15(+) cells presented an adipogenic capacity. An important proportion of cells expressed the CD34 antigen in situ and immediately after muscle dissociation. However, CD34 antigen did not persist in culture and this initial population gave rise to adipogenic cells. These results underline the diversity of human muscle cells, and the shared or restricted commitment abilities of the main lineages under defined conditions.

Serum after traumatic brain injury increases proliferation and supports expression of osteoblast markers in muscle cells

BACKGROUND

Traumatic brain injury is associated with an increased rate of heterotopic ossification within skeletal muscle, possibly as a result of humoral factors. In this study, we investigated whether cells from skeletal muscle adopt an osteoblastic phenotype in response to serum from patients with traumatic brain injury.

METHODS

Serum was collected from thirteen patients with severe traumatic brain injury, fourteen patients with a long-bone fracture, and ten control subjects. Primary cultures of skeletal muscle cells isolated from patients undergoing orthopaedic surgery were performed and characterized with use of immunofluorescence microscopy, reverse transcription-polymerase chain reaction, and Western blot analysis. Proliferation and osteoblastic differentiation were assessed with use of commercial cell assays, Western blot analysis (for Osterix protein), and the Villanueva bone stain.

RESULTS

All serum-treated cell populations expressed the osteoblast marker Osterix after one week in culture. Cells treated with serum from all study groups in mineralization medium had increased alkaline phosphatase activity and mineralized nodules within the mesenchymal cell subpopulation after three weeks in culture. Serum from patients with traumatic brain injury induced a significant increase (p = 0.02) in the rate of proliferation of primary skeletal muscle cells (1.87 [95% confidence interval, 1.66 to 2.09]) compared with the rate induced by serum from patients with a fracture (1.42 [95% confidence interval, 1.21 to 1.58]) or by serum from controls (1.35 [95% confidence interval, 1.15 to 1.54]).

CONCLUSIONS

Human serum supports the osteoblastic differentiation of cells derived from human skeletal muscle, and serum from patients with severe traumatic brain injury accelerates proliferation of these cells. These findings suggest the early presence of humoral factors following traumatic brain injury that stimulate the expansion of mesenchymal cells and osteoprogenitors within skeletal muscle.

New source of muscle-derived stem cells with potential for alveolar bone reconstruction in cleft lip and/or palate patients

Cleft lip and palate (CLP), one of the most frequent congenital malformations, affects the alveolar bone in the great majority of the cases, and the reconstruction of this defect still represents a challenge in the rehabilitation of these patients. One of the current most promising strategy to achieve this goal is the use of bone marrow stem cells (BMSC); however, isolation of BMSC or iliac bone, which is still the mostly used graft in the surgical repair of these patients, confers site morbidity to the donor. Therefore, in order to identify a new alternative source of stem cells with osteogenic potential without conferring morbidity to the donor, we have used orbicular oris muscle (OOM) fragments, which are regularly discarded during surgery repair (cheiloplasty) of CLP patients. We obtained cells from OOM fragments of four unrelated CLP patients (CLPMDSC) using previously described preplating technique. These cells, through flow cytometry analysis, were mainly positively marked for five mesenchymal stem cell antigens (CD29, CD90, CD105, SH3, and SH4), while negative for hematopoietic cell markers, CD14, CD34, CD45, and CD117, and for endothelial cell marker, CD31. After induction under appropriate cell culture conditions, these cells were capable to undergo chondrogenic, adipogenic, osteogenic, and skeletal muscle cell differentiation, as evidenced by immunohistochemistry. We also demonstrated that these cells together with a collagen membrane lead to bone tissue reconstruction in a critical-size cranial defects previously induced in nonimmunocompromised rats. The presence of human DNA in the new bone was confirmed by PCR with human-specific primers and immunohistochemistry with human nuclei antibodies. In conclusion, we showed that cells from OOM have phenotypic and behavior characteristics similar to other adult stem cells, both in vitro and in vivo. Our findings suggest that these cells represent a promising source of stem cells for alveolar bone grafting treatment, particularly in young CLP patients.

Inhibition of extracellular matrix assembly induces the expression of osteogenic markers in skeletal muscle cells by a BMP-2 independent mechanism

Background

The conversion of one cell type into another has been suggested to be, at the molecular level, the consequence of change(s) in the expression level of key developmental genes. Myoblasts have the ability to differentiate either to skeletal muscle or osteogenic lineage depending of external stimuli. Extracellular matrix (ECM) has been shown to be essential for skeletal muscle differentiation, through its direct interaction with myoblasts' cell receptors. We attempt to address if ECM also plays a role in the osteogenic differentiation of skeletal muscle cells.

Results

Inhibition of proteoglycan sulfation by sodium chlorate in myoblast cultures strongly affects ECM synthesis and deposition and induces the expression of the osteogenic lineage markers alkaline phosphatase (ALP) and osteocalcin in mononuclear cells. Induction of ALP by sodium chlorate does not affect the expression of specific muscle determination transcription factors, such as MyoD and Myf-5, in the same cells. The osteogenic transcription factor Cbfa-1 expression is also unaffected. Induction of ALP is not inhibited by a soluble form of BMP receptor IA. This suggests that the deviation of the myogenic pathway of C2C12 myoblasts into the osteogenic lineage by inhibitors of proteoglycan sulfation is BMP-2 independent. The increase of osteogenic markers expression can be totally prevented by an exogenous ECM. Interestingly, a similar BMP-2-independent ALP activity induction can be observed in myoblasts cultured on an ECM previously synthesized by BMP-2 treated myoblasts. Under in vivo conditions of increased ECM turn-over and deposition, as in the mdx dystrophic muscle and during skeletal muscle regeneration, an induction and relocalization of ALP is observed in a subpopulation of skeletal muscle fibers, whereas in normal skeletal muscle, ALP expression is restricted to blood vessels and some endomysial mononuclear cells.

Conclusion

These results suggest that signals arising from the ECM induce the expression of osteogenic markers in muscle cells by a mechanism independent of BMP-2 and without affecting the expression of key muscle or osteogenic determination genes. An induction and relocalization of ALP is also observed in mdx and regenerating skeletal muscles, in vivo conditions of increased muscle ECM deposition or turnover.

Aldehyde dehydrogenase activity identifies a population of human skeletal muscle cells with high myogenic capacities

Aldehyde dehydrogenase 1A1 (ALDH) activity is one hallmark of human bone marrow (BM), umbilical cord blood (UCB), and peripheral blood (PB) primitive progenitors presenting high reconstitution capacities in vivo. In this study, we have identified ALDH(+) cells within human skeletal muscles, and have analyzed their phenotypical and functional characteristics. Immunohistofluorescence analysis of human muscle tissue sections revealed rare endomysial cells. Flow cytometry analysis using the fluorescent substrate of ALDH, Aldefluor, identified brightly stained (ALDH(br)) cells with low side scatter (SSC(lo)), in enzymatically dissociated muscle biopsies, thereafter abbreviated as SMALD(+) (for skeletal muscle ALDH(+)) cells. Phenotypical analysis discriminated two sub-populations according to CD34 expression: SMALD(+)/CD34(-) and SMALD(+)/CD34(+) cells. These sub-populations did not initially express endothelial (CD31), hematopoietic (CD45), and myogenic (CD56) markers. Upon sorting, however, whereas SMALD(+)/CD34(+) cells developed in vitro as a heterogeneous population of CD56(-) cells able to differentiate in adipoblasts, the SMALD(+)/CD34(-) fraction developed in vitro as a highly enriched population of CD56(+) myoblasts able to form myotubes. Moreover, only the SMALD(+)/CD34(-) population maintained a strong myogenic potential in vivo upon intramuscular transplantation. Our results suggest that ALDH activity is a novel marker for a population of new human skeletal muscle progenitors presenting a potential for cell biology and cell therapy.

Role of poly-L-lysine-coated plates and fetal calf serum concentration in sheep chondroprogenitor cell culturing

Conventional methods for differentiation of chondroprogenitor cells on plastic plates face several problems that hinder the application of this method for the treatment of chondrogenic injury. This work focused on the effect of poly-L-lysine (PLL)-coated plastic surfaces and fetal calf serum concentration on the chondroprogenitor cells. In the present study, cartilage was isolated from the articular cartilages of sheep and the cells were seeded on PLL-coated plates in various serum concentrations. Histochemical analysis was used to determine chondrogenic differentiation of the cells. According to our results, the cells formed three-dimensional masses and chondrogenic cells. In the present investigation, the best culture conditions for maximum proliferation of isolated cells were examined. Taken together, the results indicated that PLL may have some effect on the adhesive properties of chondroprogenitor cells and could be used for cartilage engineering.

Myoblast sensitivity and fibroblast insensitivity to osteogenic conversion by BMP-2 correlates with the expression of Bmpr-1a

Background

Osteoblasts are considered to primarily arise from osseous progenitors within the periosteum or bone marrow. We have speculated that cells from local soft tissues may also take on an osteogenic phenotype. Myoblasts are known to adopt a bone gene program upon treatment with the osteogenic bone morphogenetic proteins (BMP-2,-4,-6,-7,-9), but their osteogenic capacity relative to other progenitor types is unclear. We further hypothesized that the sensitivity of cells to BMP-2 would correlate with BMP receptor expression.

Methods

We directly compared the BMP-2 sensitivity of myoblastic murine cell lines and primary cells with osteoprogenitors from osseous tissues and fibroblasts. Fibroblasts forced to undergo myogenic conversion by transduction with a MyoD-expressing lentiviral vector (LV-MyoD) were also examined. Outcome measures included alkaline phosphatase expression, matrix mineralization, and expression of osteogenic genes (alkaline phosphatase, osteocalcin and bone morphogenetic protein receptor-1A) as measured by quantitative PCR.

Results

BMP-2 induced a rapid and robust osteogenic response in myoblasts and osteoprogenitors, but not in fibroblasts. Myoblasts and osteoprogenitors grown in osteogenic media rapidly upregulated Bmpr-1a expression. Chronic BMP-2 treatment resulted in peak Bmpr-1a expression at day 6 before declining, suggestive of a negative feedback mechanism. In contrast, fibroblasts expressed low levels of Bmpr-1a that was only weakly up-regulated by BMP-2 treatment. Bioinformatics analysis confirmed the presence of myogenic responsive elements in the proximal promoter region of human and murine BMPR-1A/Bmpr-1a. Forced myogenic gene expression in fibroblasts was associated with a significant increase in Bmpr-1a expression and a synergistic increase in the osteogenic response to BMP-2.

Conclusion

These data demonstrate the osteogenic sensitivity of muscle progenitors and provide a mechanistic insight into the variable response of different cell lineages to BMP-2.

Differentiation potential of human muscle-derived cells towards chondrogenic phenotype in alginate beads culture

OBJECTIVE

The aim of this study was to evaluate the differentiation potential of two populations of muscle-derived cells (CD56- and CD56+) towards chondrogenic phenotype in alginate beads culture and to compare the effect of transforming growth factor beta 1 (TGFbeta1) on the differentiation process in these populations.

METHODS

Muscle CD56- and CD56+ cells were cultured in alginate beads, in a chondrogenic medium, containing or not TGFbeta1 (10 ng/ml). Cultures were maintained for 3, 7, 14 or 21 days in a humidified culture incubator. At harvest, one culture of each set was fixed for alcian blue staining and aggrecan detection. The steady-state level of matrix macromolecules mRNA was assessed by real-time polymerase chain reaction (PCR). Protein detection was performed by western-blot analysis. The binding activity of nuclear extracts to Cbfa1 DNA sequence was also evaluated by electrophoretic mobility shift assays (EMSA).

RESULTS

Chondrogenic differentiation of both CD56+ and CD56- muscle-derived cells was improved in alginate scaffold, even without growth factor, as suggested by increased chondrogenesis markers expression during the culture. Furthermore, TGFbeta1 enhanced the differentiation process and allowed to maintain a high expression of markers of mature chondrocytes. Of importance, the combination of alginate and TGFbeta1 treatment resulted in a further down-regulation of collagen type I and type X, as well as Cbfa1 both expression and binding activity.

CONCLUSIONS

Thus, alginate scaffold and chondrogenic medium are sufficient to lead both populations CD56+ and CD56- towards chondrogenic differentiation. Moreover, TGFbeta1 enhances this process and allows to maintain the chondrogenic phenotype by inhibiting terminal differentiation, particularly for CD56- cells.

The contribution of different cell lineages to bone repair: exploring a role for muscle stem cells

An anabolic response driven by osteoblasts is critical for the process of bone healing. Current evidence suggests that these osteoblasts may arise from multiple tissue types and cell lineages. Stem cells present in the bone marrow, periosteum, local soft tissues, vasculature, and/or circulation have been shown to have osteogenic potential. Transplanted cells from these sources have also been shown to incorporate into induced ectopic bone or repaired bone. While these experiments demonstrate the latent capacity of different lineages to assume an osteoblastic phenotype under pro-osteogenic conditions, the actual contribution of the different lineages to various repair situations in vivo remains unclear. This review explores the data arising from different bone formation and repair models. We propose a model suggesting that cells arising from the local tissues, particularly muscle cells, may play an important role in fracture repair under situations where the periosteal and/or bone marrow progenitor populations are depleted.

Osteogenic properties of human myogenic progenitor cells

Here, we identified human myogenic progenitor cells coexpressing Pax7, a marker of muscle satellite cells and bone-specific alkaline phosphatase, a marker of osteoblasts, in regenerating muscle. To determine whether human myogenic progenitor cells are able to act as osteoprogenitor cells, we cultured both primary and immortalized progenitor cells derived from the healthy muscle of a nondystrophic woman. The undifferentiated myogenic progenitors spontaneously expressed two osteoblast-specific proteins, bone-specific alkaline phosphatase and Runx2, and were able to undergo terminal osteogenic differentiation without exposure to an exogenous inductive agent such as bone morphogenetic proteins. They also expressed the muscle lineage-specific proteins Pax7 and MyoD, and lost their osteogenic characteristics in association with terminal muscle differentiation. Both myoblastic and osteoblastic properties are thus simultaneously expressed in the human myogenic cell lineage prior to commitment to muscle differentiation. In addition, C3 transferase, a specific inhibitor of Rho GTPase, blocked myogenic but not osteogenic differentiation of human myogenic progenitor cells. These data suggest that human myogenic progenitor cells retain the capacity to act as osteoprogenitor cells that form ectopic bone spontaneously, and that Rho signaling is involved in a critical switch between myogenesis and osteogenesis in the human myogenic cell lineage.

Fat tissue: views on reconstruction and exploitation

Transplantation of autologous fat as pedicle or transposition flaps has been a classical method in plastic surgery for tissue reconstruction. The injection of fat for soft tissue reconstruction is also an old innovation. This approach has some significant drawbacks such as resorption of the fat transplant. To regenerate additional and self-regenerating adipose tissue for reconstructive purposes, a thorough understanding of adipose tissue (mesodermal stem cells, adipoblasts, pre-adipocytes, mature, lipid-synthesizing, and lipid-storing white or brown adipocytes) on cellular and molecular levels is required. Several transcription factors that play a central role in the control of adipogenesis have been identified. Among these are the CCAAT/enhancer binding protein gene family and peroxisome proliferator-activated receptor-gamma. Hormones and growth factors, such as insulin and insulin-like growth factor (IGF), transfer external signals to differentiating adipocytes. In an attempt to improve the quality of tissue-engineered fat by culture-expanded adipocytes, various pre-adipocyte and stem cell culture conditions and expansion methods have been developed. In the presence of fetal calf serum, spontaneous differentiation of pre-adipocytes into fat cell clusters occurs to some degree. This in vitro differentiation can be enhanced by addition of inducing agents such as dexamethasone, isobutylmethylxantine, and insulin into the culture medium. Recent work has shown the multipotency of pre-adipocytes, which are fibroblast-like precursors of adipocytes. With use of specific culture conditions, human adipose tissue-derived stem cells can be induced to express markers of adipocyte, osteoblast, and myocyte cell lineages. The multipotent characteristics of adipose tissue-derived stem cells, as well as their abundance and accessibility in the human body, make them a potential cell source for tissue engineering applications.

alpha7 integrin expressing human fetal myogenic progenitors have stem cell-like properties and are capable of osteogenic differentiation

During muscle development, precursor cells fuse to form myofibers. Following injury in adult muscle, quiescent satellite cells become activated to regenerate muscle in a fashion similar to fetal development. Recent studies indicate that murine skeletal myoblasts can differentiate along multiple cell lineages including the osteoblastic pathway. However, little is known about the multipotency of human myogenic cells. Here, we isolate myogenic precursor cells from human fetal and adult muscle by sorting for the laminin-binding alpha7 integrin and demonstrate their differentiation potential and alteration in adhesive behavior. The alpha7-positive human fetal progenitors were efficient at forming myotubes and a majority expressed known muscle markers including M-cadherin and c-Met, but were heterogeneous for desmin and MyoD expression. To test their pluripotent differentiation potential, enriched populations of alpha7-positive fetal cells were subjected to inductive protocols. Although the myoblasts appeared committed to a muscle lineage, they could be converted to differentiate along the osteoblastic pathway in the presence of BMP-2. Interestingly, osteogenic cells showed altered adhesion and migratory activity that reflected growth factor-induced changes in integrin expression. These results indicate that alpha7-expressing fetal myoblasts are capable of differentiation to osteoblast lineage with a coordinated switch in integrin profiles and may represent a mechanism that promotes homing and recruitment of myogenic stem cells for tissue repair and remodeling.

Osteogenic induction of human periodontal ligament fibroblasts under two- and three-dimensional culture conditions

Human periodontal ligament fibroblasts (hPDLF) play a key role in the regeneration of periodontal compartment during guided tissue regeneration procedures. This property is attributed to the progenitor cell subsets residing in the area. The aim of this study was to investigate whether hPDLFs could undergo an osteogenic differentiation under two- and three-dimensional (2D and 3D) culture conditions upon osteogenic induction. hPDLFs were isolated from six healthy donors, cultured, and expanded according to standard protocols. Then, three osteogenic culture conditions (dexamethasone, ascorbic acid, and beta-glycerophosphate) were established: 1) 2D culture as single-cell monolayer, 2) 3D-static culture on mineralized poly(DL-lactic-co-glycolic acid) (PLGA) scaffold, and 3) 3D culture on mineralized PLGA scaffold inside the NASA-approved bioreactor stimulating microgravity conditions. After 21 days of osteogenic induction, the majority of monolayer cultures had undergone differentiation toward osteogenic lineage, as indicated by morphological changes, mineralization assay, and some phenotypical properties. However, immunohistochemistry revealed that the scaffold cultures expressed higher levels of osteogenic marker proteins compared with that of the monolayers. Secondly, hPDLF-PLGA constructs in bioreactor showed an increased expression of osteopontin and osteocalcin compared with that of static 3D culture after 21 days. Results indicate that human periodontal ligament contains a subpopulation of cells capable of undergoing osteogenic differentiation and presumably contributing to regeneration of bone defects in the adjacent area. Human PDLF-seeded mineralized PLGA scaffold in microgravity bioreactor may be used to support osteogenic differentiation in vitro. Thus, this system may offer new potential benefits as a tool for periodontal tissue engineering.

Functional heterogeneity of side population cells in skeletal muscle

Skeletal muscle regeneration has been exclusively attributed to myogenic precursors, satellite cells. A stem cell-rich fraction referred to as side population (SP) cells also resides in skeletal muscle, but its roles in muscle regeneration remain unclear. We found that muscle SP cells could be subdivided into three sub-fractions using CD31 and CD45 markers. The majority of SP cells in normal non-regenerating muscle expressed CD31 and had endothelial characteristics. However, CD31(-)CD45(-) SP cells, which are a minor subpopulation in normal muscle, actively proliferated upon muscle injury and expressed not only several regulatory genes for muscle regeneration but also some mesenchymal lineage markers. CD31(-)CD45(-) SP cells showed the greatest myogenic potential among three SP sub-fractions, but indeed revealed mesenchymal potentials in vitro. These SP cells preferentially differentiated into myofibers after intramuscular transplantation in vivo. Our results revealed the heterogeneity of muscle SP cells and suggest that CD31(-)CD45(-) SP cells participate in muscle regeneration.

Laminin, VEGF, and bone matrix protein expression in uroepithelial bone induction--a canine model

A biological and embryological bone induction from epithelial-mesenchymal cell interactions has been noticed in some developing tissues. However, the mechanism for bone formation induced by the epithelial-mesenchymal cell interactions is not clear. The aim of our study was to reveal the role of laminin, vascular endothelial growth factor (VEGF), and bone matrix proteins in mesenchymal cell differentiation during uroepithelial bone induction using a well-established canine model. In this model, a myoperitoneal muscle flap from the abdominal rectus sheath was transplanted into the bladder wall. After 6 weeks, the bladder samples were removed and assessed by histology and immunohistochemistry. This study demonstrated that bone formation occurred in two different directions with two distinct mechanisms. We noted that bone-forming cells in two types of bone formation derived from mesenchymal stem cell differentiation induced either from uroepithelium or bone autoinduction. Laminin was only expressed in peripheral regions of uroepithelium bone formation. Type II collagen was expressed both intracellularly and extracellularly around hypertrophic chondrocytes, whereas VEGF was mostly expressed in proliferating chondrocytes. This study indicates that components in basement membrane like laminin play a role in transitional epithelium-induced differentiation of mesenchymal cells to chondrocytes in muscle tissue. The sequential expression of bone matrix proteins by differentiated osteogenetic cells indicates a subsequent sequence of bone autoinduction.

Nonadherent cell population of human marrow culture is a complementary source of mesenchymal stem cells (MSCs)

To obtain enough quantity of osteogenic cells is a challenge for successful cell therapy in bone defect treatment, and cell numbers were usually achieved by culturing bone marrow cells in a relatively long duration. This study reports a simple and cost-effective method to enhance the number of mesenchymal stem cells (MSCs) by collecting and replating the nonadherent cell population of marrow MSCs culture. Bone marrow MSCs were isolated from 11 patients, cultured at a density of 1 x 10(5)/cm(2) to 1 x 10(6)/cm(2) in flasks. For the first three times of media change, the floating cells were centrifuged and replated in separate flasks. The total number of cells in both the primary and replating flasks were counted at day 21. Cell proliferation rate, potentials for osteogenic, chondrognenic, and adipogenic differentiation were examined in both cell types in vitro. In vivo osteogenic potentials of the cells were also tested in mice implantation model. The results showed that MSCs derived from nonadherent cell population of marrow cell cultures have similar cell proliferation and differentiation potentials as the originally attached MSCs in vitro. When implanted with hydroxyapatite/tricalcium phosphate (HA-TCP) materials subcutaneously in serve combined immune deficiency (SCID) mice, newly formed bony tissues were found in both cell type groups with osteocalcin expression. We have obtained 36.6% (20.70%-44.97%) more MSCs in the same culture period when the nonadherent cell populations were collected. The findings confirmed that the nonadherent cell population in the bone marrow culture is a complementary source of MSCs, collecting these cells is a simple and cost-effective way to increase MSCs numbers and reduce the time required for culturing MSCs for clinical applications.

Angiogenesis and pericytes in the initiation of ectopic calcification

Ectopic calcification of blood vessels, heart valves, and skeletal muscle is a major clinical problem. There is now good evidence that angiogenesis is associated with ectopic calcification in these tissues and that it is necessary, but not sufficient, for calcification to occur. Angiogenesis may regulate ectopic calcification in several ways. First, many angiogenic factors are now known to exert both direct and indirect effects on bone and cartilage formation. Second, cytokines released by endothelial cells can induce the differentiation of osteoprogenitor cells. Third, the new blood vessels provide oxygen and nutrients to support the growing bone. Finally, the new blood vessels can serve as a conduit for osteoprogenitor cells. These osteoprogenitor cells may be derived from the circulation or from pericytes that are present in the neovessels themselves. Indeed, there is now compelling evidence that pericytes can differentiate into osteoblasts and chondrocytes both in vitro and in vivo. Other vascular cells, including adventitial myofibroblasts, calcifying vascular cells, smooth muscle cells, and valvular interstitial cells, have also been shown to exhibit multilineage potential in vitro. Although these cells share many properties with pericytes, the precise relationship between them is not known. Furthermore, it still remains to be determined whether all or some of these cells contribute to the ectopic calcification observed in vivo. A better understanding of the underlying mechanisms that link angiogenesis, pericytes, and ectopic calcification should provide a basis for development of therapeutic strategies to treat or arrest this clinically significant condition.

How does pulsed low-intensity ultrasound enhance fracture healing?

Pulsed low-energy ultrasound, a non-invasive therapeutic treatment modality, may improve callus formation and enhance fracture healing by initiating enhanced angioneogenesis.

Cryopreservation and in vitro expansion of chondroprogenitor cells isolated from the superficial zone of articular cartilage

Understanding the proliferation mechanisms of chondroprogenitor cells and their influence on cell differentiation is crucial in order to develop large-scale expansion processes for tissue engineering applications. Proliferation control mechanisms were mainly attributed to substrate limitation and cell-cell contact inhibition. The limiting substrates were found to be components of the FCS, with an optimal proliferation rate achieved in the presence of 40% FCS. In addition, the medium supply rate was found to be essential in reducing substrate limitation. In terms of FCS, 10 microL FCS cm(-2) h(-1) was the threshold feed rate required to prevent substrate limitation. Above this rate, maximum cell densities of 5.3 x 10(5) cells/cm2 were achieved, representing a 53-fold expansion. To reduce the need for high supply rates, the effect of specific growth factors was also investigated. Cell densities of 3.3 x 10(5) cells/cm2 were achieved in batch cultures using 40% FCS and 1 ng/mL TGF-beta1. Chondroprogenitor cells were expanded in this medium up to three passages without compromising their ability to differentiate and produce cartilage-like matrix in pellet cultures. In addition to substrate limitation, cell-cell contact, even at very sparse subconfluent densities, appeared capable of exerting some degree of growth inhibition. The cells exhibited deceleratory growth kinetics, characterized by a decrease of specific growth rates over time.

Dexamethasone stimulates differentiation of odontoblast-like cells in human dental pulp cultures

Regenerative dental pulp strategies require the identification of precursors able to differentiate into odontoblast-like cells that secrete reparative dentin after injury. Pericytes have the ability to give rise to osteoblasts, chondrocytes, and adipocytes, a feature that has led to the suggestion that odontoblast-like cells could derive from these perivascular cells. In order to gain new insights into this hypothesis, we investigated the effects of dexamethasone (Dex), a synthetic glucocorticoid employed to induce osteogenic differentiation in vitro, in a previously reported model of human dental pulp cultures containing pericytes as identified by their expression of smooth muscle actin (SMA) and their specific ultrastructural morphology. Our data indicated that Dex (10(-8) M) significantly inhibited cell proliferation and markedly reduced the proportion of SMA-positive cells. Conversely, Dex strongly stimulated alkaline phosphatase (ALP) activity and induced the expression of the transcript encoding the major odontoblastic marker, dentin sialophosphoprotein. Nevertheless, parathyroid hormone/parathyroid hormone-related peptide receptor, core-binding factor a1/osf 2, osteonectin, and lipoprotein lipase mRNA levels were not modified by Dex treatment. Dex also increased the proportion of cells expressing STRO-1, a marker of multipotential mesenchymal progenitor cells. These observations indicate that glucocorticoids regulate the commitment of progenitors derived from dental pulp cells to form odontoblast-like cells, while reducing the proportion of SMA-positive cells. These results provide new perspectives in deciphering the cellular and molecular mechanisms leading to reparative dentinogenesis.

Complications encountered while using thin-wire-hybrid-external fixation modular frames for fracture fixation. A retrospective clinical analysis and possible support for "Damage Control Orthopaedic Surgery"

One hundred ninety eight adult patients who had sustained long bone fractures were treated by external fixation from admission to bone healing and consolidation. Of these, 135 had sustained high-energy injuries, 39 of them had suffered multi-system injuries. Superficial pin track infection was the most common complication, occurring predominantly in pins located in the femur, upper tibia and upper humerus. There were no cases of deep infection or osteomyelitis. One patient with a femoral shaft fracture developed a DVT although he was on preventive low molecular weight heparin, i.e. sc Clexane 40 mg daily. There were no cases of PE or ARDS. External fixation systems are a minimal invasive surgical modality, which allow three-dimensional fracture fixation after closed or minimal open reduction. They require a good command of surgical anatomy, but provide an optimal preservation of the fracture's soft tissue envelope, the critical biological factor for new bone formation and fracture healing. Recent publications have suggested that in the critically ill patient, minimally invasive fracture fixation surgery may prevent the perpetuation of a reactive, life threatening inflammatory reaction (the "second hit") which may induce the development of multiple organ dysfunction (MODS).

Expression of Runx2/Cbfa1/Pebp2alphaA during angiogenesis in postnatal rodent and fetal human orofacial tissues

Transient expression of Runx2 is reported in endothelial cells and vascular smooth muscle cells during vessel formation in skin, stroma of forming bones and developing periodontal ligament, developing skeletal muscle cells, and fat tissue. The data suggest that Runx2 is expressed in a multipotential mesenchymal cell population that gives rise to various osseous and nonosseous cell lineages.

INTRODUCTION

Runx2/Cbfa1 is a transcription factor essential for cells of osteogenic and dentinogenic lineages. Here we examined expression of Runx2/Cbfa1 (all isotypes) in several nonskeletal cell types present in developing orofacial tissues of neonatal rodents and human fetuses with special emphasis on vessel formation.

MATERIALS AND METHODS

Sections obtained from heads or jaws of postnatal mice, hamster, and human fetuses were immunostained with monoclonal anti-Pebp2aA antibody. Mouse and human tissues were also examined by in situ hybridization. Sections of Runx2 null mutant mice with a LacZ reporter construct inserted in the Runx2 locus were stained for Runx2 promoter activity with anti-galactosidase.

RESULTS

We found transient mRNA and protein expression in endothelial cells and in vascular smooth muscle cells of forming vessels in skin, alveoli of forming bone, and forming periodontal ligament. We also noticed weak and variable expression in some fibroblasts of embryonic skin, early differentiating cross-striated muscle cells, and differentiating fat cells.

CONCLUSION

Runx2 is not an exclusive marker for chondrogenic, osteogenic, and dentinogenic tissues, but is much more widely present in an early multipotential mesenchymal cell population that gives rise to several other lineages.

Bone and cartilage formation by skeletal muscle derived cells

In adult individuals when most tissues have progressively lost the ability to regenerate, bone maintains the potential for a continuous self remodeling. The bone marrow has been so far the main recognized source of osteoprogenitor cells that contribute to the turnover of the skeletal scaffold. The possibility though exists that a pool of osteoprogenitor cells resides within other adult tissues and in particular, as reported previously, in other connective tissues such as fat and skeletal muscle. In an attempt to identify an alternative source of osteoprogenitor cells other than bone marrow we looked into the skeletal muscle. A plastic adhering cell population, from now on referred to as skeletal muscle derived cells (SMDCs), was obtained from biopsies of human skeletal muscle. SMDCs were clonogenic and displayed a fibroblast-like morphology. The isolated cell population had a mesenchymal origin as indicated by abundant expression of type I collagen, fibronectin, and vimentin and appeared heterogeneous. SMDCs were positive for alpha smooth actin, and to a lesser extent for desmin and alpha sarcomeric myosin, two specific markers of the myogenic phenotype. Surprisingly though SMDCs expressed early markers of an osteogenic commitment as indicated by positive staining for alkaline phosphatase, osteopontin, and osteonectin. Under the appropriate stimuli, these cells deposited in vitro a mineralized bone matrix and a proteoglycan rich matrix. In addition, SMDCs cultured in the presence of low serum and insulin differentiated towards adipocytes developing abundant lipid droplets in the cytoplasm. Furthermore SMDCs formed three-dimensional bone tissue in vivo when implanted in an immunodeficient mouse, and a mature cartilage rudiment when maintained as a pellet culture. In summary, we report the isolation and characterization of a cell population from the human skeletal muscle not only able to express in vitro specific markers of distinct mesenchymal lineages (adipogenic, chondrogenic, and osteogenic), but most importantly, able to complete the differentiation pathway leading to the formation of bone and cartilage. In this respect SMDCs resemble bone marrow stromal cells (BMSCs).

Cell therapy for bone disease: a review of current status

Bone marrow is a reservoir of pluripotent stem/progenitor cells for mesenchymal tissues. Upon in vitro expansion, in vivo bone-forming efficiency of bone marrow stromal cells (BMSCs) is dramatically lower in comparison with fresh bone marrow, and their in vitro multidifferentiation potentials are gradually lost. Nevertheless, when BMSCs are isolated and expanded in the presence of fibroblast growth factor 2, the percentage of cells able to differentiate into the osteogenic, chondrogenic, and adipogenic lineages is greater. Osteogenic progenitors are not exclusive to skeletal tissues. We could also think of cells in different adult tissues as potentially capable of following an osteochondrogenic differentiation pathway, but, under normal physiological conditions, they are inhibited in this process by the environment and/or the adjacent cell populations. When, for some reason such as pathology, the environment changes dramatically and the inhibiting condition is removed, these cells could become osteoblasts. Bone is repaired via local delivery of cells within a scaffold. Bone formation was first assessed in small animal models. Large animal models were successively developed to prove the feasibility of the tissue engineering approach in a model closer to a real clinical situation. Eventually, pilot clinical studies were performed. Extremely appealing is the possibility of using mesenchymal progenitors in the therapy of genetic bone diseases via systemic infusion. There is experimental evidence to suggest that mesenchymal progenitors delivered by this route engraft with a very low efficiency and do not produce relevant and durable clinical effects. Under some conditions, where the local microenvironment is either altered (i.e., injury) or under important remodeling processes (i.e., fetal growth), engraftment of stem and progenitor cells seems to be enhanced. A better understanding of their engraftment mechanisms will, hopefully, extend the field of therapeutic applications of mesenchymal progenitors.

Muscle-derived stem cells for musculoskeletal tissue regeneration and repair

Muscle recently has been identified as a good source of adult stem cells that can differentiate into cells of different lineages. The most well-known muscle progenitor cells are satellite cells, which not only contribute to the replenishment of the myogenic cell pool but also can become osteoblasts, adipocytes and chondrocytes. Other populations of stem cells that appear to be distinct from satellite cells also have been discovered recently. Muscle-derived stem cells (MDSCs) can be divided into two major categories based on these cells' varied abilities to differentiate into myogenic lineages. Interestingly, MDSCs that can differentiate readily into myogenic cells are usually CD45-. In contrast, MDSCs with less myogenic potential are CD45+. Various lines of evidence suggest that different populations of MDSCs are closely related. Furthermore, MDSCs appear to be closely related to endothelial cells or pericytes of the capillaries surrounding myofibers. When used in tissue engineering applications, MDSCs--particularly those genetically engineered to express growth factors--have been demonstrated to possess great potential for the regeneration and repair of muscle, bone and cartilage. Further research is necessary to delineate the relationship between different populations of MDSCs and between MDSCs and other adult stem cells, to investigate their developmental origin, and to determine the regulatory pathways and factors that control stem cell self-renewal, proliferation and differentiation. This knowledge could greatly enhance the usefulness of muscle-derived stem cells, as well as other adult stem cells, for tissue repair and regeneration applications.