Measurement of total and local bone morphogenetic protein concentration in bone tumours

BMP-2 Delivery through Liposomes in Bone Regeneration

Bone regeneration is a central focus of maxillofacial research, especially when dealing with dental implants or critical sized wound sites. While bone has great regeneration potential, exogenous delivery of growth factors can greatly enhance the speed, duration, and quality of osseointegration, making a difference in a patient’s quality of life. Bone morphogenic protein 2 (BMP-2) is a highly potent growth factor that acts as a recruiting molecule for mesenchymal stromal cells, induces a rapid differentiation of them into osteoblasts, while also maintaining their viability. Currently, the literature data shows that the liposomal direct delivery or transfection of plasmids containing BMP-2 at the bone wound site often results in the overexpression of osteogenic markers and result in enhanced mineralization with formation of new bone matrix. We reviewed the literature on the scientific data regarding BMP-2 delivery with the help of liposomes. This may provide the ground for a future new bone regeneration strategy with real chances of reaching clinical practice.

[Study on the correlation between the content of bone morphogenetic protein 2 in demineralized bone matrix and its osteogenic activity in vitro and in vivo]

Objective

To investigate the correlation between the content of bone morphogenetic protein 2 (BMP-2) in demineralized bone matrix (DBM) and its osteogenic activity in vitro and in vivo, in order to choose a simple and convenient method to evaluate the osteogenic activity of DBM.

Methods

The left mid-femoral tissues of 9 donors were taken, and DBMs (S1-S9) were prepared by dynamic decalcification process, and inactivated DBM (control group) was prepared at the same time. Protease inhibitor method, collagenase method, guanidine hydrochloride/ethylene diamine tetraacetic acid (EDTA) method, and RIPA lysate method were used to extract BMP-2 in S1-S9 and inactivated DBMs. The BMP-2 content was measured and the differences between DBMs were compared. Then the S1-S9 and inactivated DBMs were co-cultured with mouse embryonic osteoblasts MC3T3-E1, respectively. The cell proliferation was detected by MTT method and fluorescence staining, and alkaline phosphatase (ALP) activity was detected at the same time. Thirty BALB/c male nude mice were divided into 10 groups, namely S1-S9 DBM groups (S1-S9 groups) and inactivated DBM group (control group), with 3 mice in each group. Muscle pockets of the middle thighs were prepared on both hindlimbs of mice in each group, and implanted corresponding DBM materials. At 4 weeks after operation, the samples were taken for HE staining observation and semi-quantitative evaluation, and the new bone formation score was calculated.

Results

The BMP-2 content of DBM derived from different donor bones was distinct. The BMP-2 content obtained by different extraction methods for DBM prepared from the same donor bone was also different, and the extraction efficiency of the guanidine hydrochloride/EDTA method was the highest. In vitro cell experiments, MTT test displayed that cell proliferations and ALP activity were significantly higher in S4 and S6 groups than in other groups at each time point after co-cultivation ( P<0.05). Moreover, the cell proliferation of S4 group was the most significant at 7 days ( P<0.05); fluorescence staining demonstrated that the osteoblasts of each group was in good condition, but the osteoblasts of S1, S2, S3, S4, and S6 groups were significantly more than other groups. In vivo ectopic osteogenesis experiments, the cartilage and new bone formation could be seen in the bone graft area of S1-S6 groups at 4 weeks after operation, and with the increase of BMP-2 content, the more new bone formation induced by the material, the higher the score of new bone formation of the material ( P<0.05). Among them, S4 and S6 groups contained a large number of chondrocytes and osteoblasts in the osteogenesis area.

Conclusion

The osteogenic activity of DBM can be evaluated through BMP-2 quantitative detection combined with in vitro osteoblast proliferation and differentiation experiments.

Microparticles for Sustained Growth Factor Delivery in the Regeneration of Critically-Sized Segmental Tibial Bone Defects

This study trialled the controlled delivery of growth factors within a biodegradable scaffold in a large segmental bone defect model. We hypothesised that co-delivery of vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) followed by bone morphogenetic protein-2 (BMP-2) could be more effective in stimulating bone repair than the delivery of BMP-2 alone. Poly(lactic-co-glycolic acid) (PLGA ) based microparticles were used as a delivery system to achieve a controlled release of growth factors within a medical-grade Polycaprolactone (PCL) scaffold. The scaffolds were assessed in a well-established preclinical ovine tibial segmental defect measuring 3 cm. After six months, mechanical properties and bone tissue regeneration were assessed. Mineralised bone bridging of the defect was enhanced in growth factor treated groups. The inclusion of VEGF and PDGF (with BMP-2) had no significant effect on the amount of bone regeneration at the six-month time point in comparison to BMP-2 alone. However, regions treated with VEGF and PDGF showed increased vascularity. This study demonstrates an effective method for the controlled delivery of therapeutic growth factors in vivo, using microparticles.

Severe mandibular atrophy treated with a subperiosteal implant and simultaneous graft with rhBMP-2 and mineralized allograft: a case report

A 71-year-old patient was successfully rehabilitated by means of a 3D model-derived, hydroxyapatite-coated titanium subperiosteal mandibular implant. The implant was specifically designed to allow bone augmentation. The deficient bone was simultaneously grafted with mineralized bone allograft and recombinant bone morphogenetic protein -2 (rhBMP-2). The 32-month postoperative cone beam computerized tomography follow-up showed vertical bone augmentation beneath the implant frame.

Point-counter-point debate: the association between recombinant human bone morphogenetic protein utilization and complications in spine surgery

Bone morphogenetic proteins (BMPs) have been utilized in spine surgery for over 10 years as a bone graft substitute. Potential BMP-related adverse effects including retrograde ejaculation and heterotopic neuroforaminal bone formation have been described. Additionally, some studies have suggested an association between BMP and cancer. Inconsistencies exist in the published spine literature with regards to the incidence and association of complications with BMP utilization. In a point-counterpoint format, this article discusses the current evidence concerning the relationship between the utilization of BMP in spinal fusion and the risk of cancer, retrograde ejaculation (RE), neuroforaminal bone formation, and its role in anterior cervical spine surgery and adolescents.

Complications with the use of bone morphogenetic protein 2 (BMP-2) in spine surgery

BACKGROUND CONTEXT

Recombinant human bone morphogenetic protein 2 (rhBMP-2) is a very potent osteogenic growth factor that has been used successfully in various spine fusions, obviating the need for autologous iliac crest bone graft harvest and therefore avoiding the associated morbidities.

PURPOSE

In the past few years, a tremendous increase in rhBMP-2 usage was noted, and concerns regarding costs, benefits, and safety issues were raised by many. The goal of this work was to provide a comprehensive review of the adverse events and complications associated with use of rhBMP-2.

STUDY DESIGN

Literature review.

METHODS

This is a review of the current literature on the reported adverse events, complications, and concerns associated with rhBMP-2 use.

RESULTS

This article discusses the wide spectrum of adverse outcomes related to rhBMP-2 use in the lumbar and the cervical spine; retrograde ejaculation, antibodies formation, postoperative radiculitis, postoperative nerve root injury, ectopic bone formation, vertebral osteolysis/edema, dysphagia and neck swelling, hematoma formation, interbody graft lucency, and wound healing complications are reviewed. Cost-related concerns, dosage considerations, carrier types, and theoretical carcinogenesis concerns were also presented.

CONCLUSIONS

Despite the excellent spinal fusion rates promoted by this powerful molecule, the increasingly reported adverse outcomes associated with bone morphogenetic protein usage have created real concerns. This article will provide the reader with a good understanding of the reported complications associated with rhBMP-2 use and ultimately help recognize its safety spectrum and limits for better clinical application.

Biomaterial scaffolds for treating osteoporotic bone

Healing fractures resulting from osteoporosis or cancer remains a significant clinical challenge. In these populations, healing is often impaired not only due to age and disease, but also by other therapeutic interventions such as radiation, steroids, and chemotherapy. Despite substantial improvements in the treatment of osteoporosis over the last few decades, osteoporotic fractures are still a major clinical challenge in the elderly population due to impaired healing. Similar fractures with impaired healing are also prevalent in cancer patients, especially those with tumor growing in bone. Treatment options for cancer patients are further complicated by the fact that bone anabolic therapies are contraindicated in patients with tumors. Therefore, many patients undergo surgery to repair the fracture, and bone grafts are often used to stabilize orthopedic implants and provide a scaffold for ingrowth of new bone. Both synthetic and naturally occurring biomaterials have been investigated as bone grafts for repair of osteoporotic fractures, including calcium phosphate bone cements, resorbable polymers, and allograft or autograft bone. In order to re-establish normal bone repair, bone grafts have been augmented with anabolic agents, such as mesenchymal stem cells or recombinant human bone morphogenetic protein-2. These developing approaches to bone grafting are anticipated to improve the clinical management of osteoporotic and cancer-induced fractures.

Bone morphogenetic protein 2 inhibits hepatocellular carcinoma growth and migration through downregulation of the PI3K/AKT pathway

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide. Previous studies have suggested that abnormal expression of BMP-4, BMP-7, and BMP-9 is correlated with tumor progression in HCC, but the role played by BMP-2 in HCC has not yet been reported. To determine the role of BMP-2 in HCC, we first investigated the effect of exogenous BMP-2 on the growth of the cell lines HCC SK-Hep-1, Hep G2, and Hep 3B. Next, we studied the function of BMP-2 in SK-Hep-1 HCC cell line using a recombinant lentivirus vector to deliver BMP-2. We also used siRNA to silence endogenous BMP-2 expression in the HCC Hep 3B cell line. Then, cell growth and migration were assayed in vitro using WST-8, wound-healing, and transwell invasion assays. Cellular apoptosis and cell-cycle distribution were assessed using flow cytometry. We also investigated the effects of BMP-2 overexpression and knockdown on the expression of proliferating cell nuclear antigen (PCNA), matrix metallopeptidase-2 (MMP-2), phosphorylated AKT (p-AKT), phosphoinositide 3-kinase p85α (PI3Kp85α), Bax, Bcl-2, caspase-3, cleaved caspase-3, p21, and cyclin E. As a result, we observed that BMP-2 inhibited the proliferation of HCC cells. Furthermore, HCC cell proliferation and migration were significantly diminished by BMP-2 overexpression, as was indicated by WST-8, would healing, and transwell assays, while knockdown of BMP-2 led to an increase in proliferation and migration of Hep 3B cells. BMP-2 overexpression significantly increased the susceptibility of SK-Hep-1 cells to low-serum-induced apoptosis, while BMP-2 knockdown reduced the susceptibility of Hep 3B cells. Overexpression of BMP-2 induced G1 phase arrest through upregulation of p21. When BMP-2 expression was elevated in SK-Hep-1 cells, the expression of PI3Kp85α, p-AKT, PCNA, and MMP-2 declined. These results suggest that BMP-2 exerts an inhibitory effect on the growth and migration of HCC cells, possibly via a blockade of PI3K/AKT signaling.

Adenovirus-mediated overexpression of BMP-9 inhibits human osteosarcoma cell growth and migration through downregulation of the PI3K/AKT pathway

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily of signaling molecules and have previously been shown to be associated with the biological behavior of osteosarcoma. However, to date the effects and molecular mechanisms of BMP-9 on osteosarcoma progression are unknown. We performed real-time PCR and western blot analysis to characterize the endogenous expression of BMP-9 in osteosarcoma cell lines. We used a recombinant adenovirus expressing BMP-9 (adBMP-9) to infect osteosarcoma cell lines with relatively low endogenous BMP-9 expression to determine the functional relevance of BMP-9 overexpression to osteosarcoma cell growth and migration in vitro and in vivo, and further investigated the expression levels of Ki-67, matrix metallopeptidase-9 (MMP-9), phosphoinositide 3-kinase p85α (PI3Kp85α) and phosphorylated AKT (p-AKT). As a result, osteosarcoma cell proliferation and migration were significantly diminished by adBMP-9, indicated by MTT and wound-healing assays, and cell apoptosis was markedly induced, indicated by Hoechst 33342/PI assay and Annexin V-FITC apoptosis detection. When BMP-9 expression was enhanced, the expression of PI3Kp85α, p-AKT, Ki-67 and MMP-9 was downregulated in osteosarcoma cells. In addition, the tumor volumes in MG-63 and HOS subcutaneous nude mouse models treated with adBMP-9 were significantly smaller compared to those of the ad-GFP group. These results suggested that the enhanced expression of BMP-9 in osteosarcoma cells by adBMP-9 exerted inhibitory effects on growth and migration of osteosarcoma cells possibly via blockade of the PI3K/AKT signaling pathway.

A comprehensive review of the safety profile of bone morphogenetic protein in spine surgery

WE REVIEW OUR current understanding of the development and potential clinical applications of bone morphogenetic protein (BMP) in spine surgery. We also review the evidence for adverse events associated with the use of BMP and suggest potential reasons for these events and means of complication avoidance. Bone morphogenetic protein 2 (rhBMP-2) is approved by the Food and Drug Administration for anterior lumbar interbody fusion; rhBMP-7, on the other hand, is approved for long bone defects and has received a humanitarian device exemption for revision posterolateral lumbar operations and recalcitrant long bone unions. Nevertheless, "off-label" use in various spinal procedures has been reported and is increasing in frequency. Specific guidelines for rhBMP-2 and rhBMP-7 use are lacking because of the limited availability of randomized controlled clinical trials and its diverse use in many spinal applications. Mechanisms of delivery, carrier type, graft position, surgical location, and variations in BMP concentration may differ from one surgery to the next. Adverse events linked to either rhBMP-2 or rhBMP-7 use include ectopic bone formation, bone resorption or remodeling at the graft site, hematoma, neck swelling, and painful seroma. Other potential theoretical concerns include carcinogenicity and teratogenic effects. In this review, we provide the reader with a historical perspective on BMP, current and past research to support its use in spinal procedures, and a critical analysis of the complications reported thus far.

Quantification of growth factors in allogenic bone grafts extracted with three different methods

BACKGROUND

Bony allografts are used for defect filling. A reliable sterilization method is the peracetic acid-ethanol sterilization procedure (PES). Several studies showed the antimicrobiological efficacy of this method. Aim of this study was the quantification of growth factors necessary for bone formation in PES sterilized allografts (n = 9).

METHODS

To extract the growth factors from the tissue three different methods were used: (a) use of collagenase 1 for extraction, (b) incubation of the material in a proteinase inhibitor cocktail (Complete), and (c) extraction with guanidine HCl. The supernatants from the different methods were analyzed for the total protein concentration and different growth factors.

RESULTS

The extraction with guanidine HCl resulted in the highest amount of protein measurable in the supernatants of the samples. For comparison of the individual growth factor values the results were normalized to the protein content. The highest growth factor amount/protein was detectable for BMP-2 using the GndHCL method followed by FGFa, IGF-I, TGF-beta1, VEGF, and PDGF. Comparing the three extraction methods, significant differences were measured for the individual growth factor content.

CONCLUSION

PES sterilized bony allografts contain several growth factors. Depending on the extraction method, the quantity of the analyzed growth factors varies.

Actions of TGF-beta as tumor suppressor and pro-metastatic factor in human cancer

Transforming growth factor-beta (TGF-beta) is a secreted polypeptide that signals via receptor serine/threonine kinases and intracellular Smad effectors. TGF-beta inhibits proliferation and induces apoptosis in various cell types, and accumulation of loss-of-function mutations in the TGF-beta receptor or Smad genes classify the pathway as a tumor suppressor in humans. In addition, various oncogenic pathways directly inactivate the TGF-beta receptor-Smad pathway, thus favoring tumor growth. On the other hand, all human tumors overproduce TGF-beta whose autocrine and paracrine actions promote tumor cell invasiveness and metastasis. Accordingly, TGF-beta induces epithelial-mesenchymal transition, a differentiation switch that is required for transitory invasiveness of carcinoma cells. Tumor-derived TGF-beta acting on stromal fibroblasts remodels the tumor matrix and induces expression of mitogenic signals towards the carcinoma cells, and upon acting on endothelial cells and pericytes, TGF-beta regulates angiogenesis. Finally, TGF-beta suppresses proliferation and differentiation of lymphocytes including cytolytic T cells, natural killer cells and macrophages, thus preventing immune surveillance of the developing tumor. Current clinical approaches aim at establishing novel cancer drugs whose mechanisms target the TGF-beta pathway. In conclusion, TGF-beta signaling is intimately implicated in tumor development and contributes to all cardinal features of tumor cell biology.

Effect of recombinant human bone morphogenetic protein-2 in an experimental model of spinal fusion in a radiated area

STUDY DESIGN

An animal model of posterolateral intertransverse process spine fusion was used.

OBJECTIVES

To investigate whether recombinant human bone morphogenetic protein-2 (rhBMP-2) can overcome the adverse effects of radiation treatment (RT) on spine fusion.

SUMMARY OF BACKGROUND DATA

Spinal metastases are common. Some of these patients are candidates for spinal cord decompression and vertebral reconstruction; however, radiation has significant adverse effects on bone healing.

METHODS

A posterolateral fusion model was used with rhBMP-2 or iliac crest bone graft (ICBG). Eighty one-year-old rabbits were divided into eight groups: 1) RT 14 days before surgery, rhBMP-2; 2) RT 14 days before surgery, ICBG; 3) RT 2 days after surgery, rhBMP-2; 4) RT 2 days after surgery, ICBG; 5) RT 14 days after surgery, rhBMP-2; 6) RT 14 days after surgery, ICBG; 7) no RT, rhBMP-2; 8) no RT, ICBG. Animals were killed approximately 35 days after surgery. Manual palpation was the definitive test of fusion. Biomechanical and histologic assessments were also performed.

RESULTS

All rhBMP-2 groups had significantly greater fusion rates versus respective ICBG control groups: 1 (86%) versus 2 (0%) (P = 0.005), 3 (100%) versus 4 (0%) (P < 0.0001), 5 (100%) versus 6 (0%) (P < 0.0001), and 7 (100%) versus 8 (60%) (P = 0.003). Stiffness and ultimate strength did not differ significantly between the experimental and control groups. Histologic assessment confirmed new bone formation in the fusion masses from rhBMP-2 groups.

CONCLUSIONS

Use of rhBMP-2 produced a significantly greater rate of fusion compared with ICBG in a previously radiated area in an animal model, without the morbidity of ICBG harvesting and without the risk of inadvertently using autograft contaminated by micrometastases.

Safety profile for the clinical use of bone morphogenetic proteins in the spine

STUDY DESIGN

A review was conducted.

OBJECTIVE

To determine the safety profiles of human recombinant bone morphogenetic protein-2 (rhBMP-2) and osteogenic protein-1 (OP-1) used clinically in spine applications.

SUMMARY OF BACKGROUND DATA

Safety issues associated with the use of bone morphogenetic proteins in spine applications include the possibility of bony overgrowth, interaction with exposed dura, cancer risk, systemic toxicity, reproductive toxicity, immunogenicity, local toxicity, osteoclastic activation, and effects on distal organs. These issues have been given detailed examination in both human and animal studies, and safety data are available for both rhBMP-2 and OP-1. The safety data available for OP-1 are less detailed.

METHODS

The study involved reviews of published reports and the safety data submitted to the Food and Drug Administration (rhBMP-2 and OP-1) and to the European Agency for the Evaluation of Medicinal Products (OP-1), as well as personal communication with the manufacturers of rhBMP-2 (Medtronic Sofamore Danek, Memphis, TN) and OP-1 (Stryker Biotech, Hopkinton, MA).

RESULTS

Application of either rhBMP-2 or OP-1 to raw decorticated bony surfaces leads to new bone formation, which is desirable in the intertransverse or interbody regions. However, new bone formation also may occur if rhBMP-2 or OP-1 comes in contact with laminectomy sites or decompressed neuroforamina, and may lead to restenosis. Inadvertent placement of either rhBMP-2 or OP-1 in the spinal canal leads to formation of bone. Leakage of rhBMP-2 or OP-1 outside the fusion area may lead to adjacent-level fusion. Accurate placement of these factors and adequate retention by their carrier are highly important factors in minimizing these problems. Subdural bone formation occurs if OP-1 is implanted directly beneath the dura. Osteoclastic overstimulation does not appear to be a significant problem with rhBMP-2. However, bone resorption has been associated with OP-1 used in the setting of thoracolumbar fractures. Findings show that RhBMP-2 has an antiproliferative effect on many cancer cells, and no evidence exists that it is carcinogenic. It is unlikely that OP-1 has carcinogenic potential, although fewer data are available. Systemic and local toxicity, significant adverse effects, and harmful effects on distant organs have not been observed in either human or animal studies on rhBMP-2 and OP-1. The benign safety profile of rhBMP-2 may result from its rapid systemic clearance, which results in very little systemic exposure. Systemic exposure to OP-1 also is low. No reproductive toxicity has been observed with either rhBMP-2 or OP-1. However, there is no human safety data. Subclinical immune responses in human subjects to collagen carriers have been reported. Antibody responses to rhBMP-2 have been detected in less than 1% of spine patients. Low titer immune responses have been observed in 38% of patients treated with OP-1. There were no associated clinical adverse effects.

CONCLUSIONS

Given the available data, both rhBMP-2 and OP-1 appear to be safe provided they are used appropriately, placed accurately, not allowed to come into contact with decompressed areas, and contained in the region of fusion. They must be used with caution in the presence of dural defects.

Molecular profiling of human chondrosarcomas for matrix production and cancer markers

Chondrosarcoma is the second most common malignant bone tumor, characterized by production of abundant extracellular matrix resembling hyaline cartilage. To better understand the molecular pathogenesis of chondrosarcoma, we analyzed 12 chondrosarcomas for their production of connective tissue components and SOX9, a key regulator of normal chondrocyte differentiation. Furthermore, 10 chondrosarcoma samples were screened for additional changes in gene expression using cDNA array analysis. In Northern analysis, several tumors were found to express type II collagen mRNA at levels comparable to fetal cartilage used as a control. Interestingly, the highest levels of type II collagen mRNA were seen in 2 of the 3 grade 3 chondrosarcomas, which also exhibited the highest mRNA levels of SOX9 and "prechondrogenic" pro alpha 1(IIA) collagen. Expression of SOX9 in human chondrosarcomas is novel and suggests that chondrosarcomas originate from a multipotent stem cell committed to differentiation along the chondrogenic pathway. Results of the cDNA array analyses emphasize the heterogeneous nature of chondrosarcoma as no single transcript was systematically up- or downregulated in all tumors analyzed. Among the interesting changes observed was upregulation of decorin mRNA in 7 of the 10 tumors analyzed. Further studies are needed to determine whether decorin plays a role in the pathogenesis of chondrosarcoma. The cDNA arrays also revealed discrepancies from Northern and RNase protection analyses in transcript levels of matrix components, emphasizing the need to validate cDNA array data with other techniques.

Expression of bone morphogenetic protein-6 (BMP-6) in myoepithelial cells in canine mammary gland tumors

Seventy-three mammary tumors and three mammary tissue specimens were examined to elucidate the expression of bone morphogenetic protein (BMP)-6 in the myoepithelial cells of canine mammary gland tumors. Morphologically, the myoepithelial cells were classified into four types: resting and proliferating cells inside the basement membrane, and spindle- and star-shaped cells proliferating in the outer area of the basement membrane. The characteristics of these myoepithelial cells were confirmed by immunohistochemistry using antibodies raised against keratin, cytokeratin 19, alpha-smooth muscle actin, and vimentin. In simple adenoma, a small number of resting myoepithelial cells was immunopositive for BMP-6. In complex adenomas and benign mixed tumors, all types of myoepithelial cells, depending in some cases on their specific location within the tumor, were immunopositive for BMP-6, but almost all of the tubular epithelial cells were immunonegative. Foci consisting of a proliferation of BMP-6-positive star- and spindle-shaped cells had mucinous stroma with marked hyaline and chondroid changes. In contrast, the foci with BMP-6-negative spindle- and star-shaped cells tended to have mucinous stroma without chondroid change. Several types of mesenchymal cells including chondrocytes, osteoblasts, and fibroblastlike cells in the mixed tumors, showed an intense immunopositive reaction for the BMP-6 antibody, and were located close to the ectopic cartilage and bone matrix. No significant immunoreactivity for BMP-6 was observed in most of the malignant mammary tumors; only one malignant mixed tumor was examined. All of these findings indicate that BMP-6 expression in myoepithelial cells may increase in complex adenomas and benign mixed tumors in canine mammary glands, and that BMP-6 expression is most intense in the vicinity of chondroid matrix in these tumors.

Osteoprogenitor cells within skeletal muscle

The formation of ectopic bone within skeletal muscle is a widely observed phenomenon. However, the source of the osteoprogenitor cells responsible for ectopic bone formation remains unknown. This study was designed to test for osteogenic differentiation among cells isolated from skeletal muscle tissue. Different subpopulations of cells derived from an adult mouse skeletal muscle were tested for induction of alkaline phosphatase activity after exposure to bone morphogenetic protein-2 in vitro. A responsive subpopulation was identified, transduced with a retrovirus encoding for beta-galactosidase (Rv-lacZ) and an adenoviral construct encoding for one bone morphogenetic protein-2, and injected into the hindlimb of immune compromised (severe combined immunodeficient, or SCID) mice. The injected cells appeared to actively participate in the ectopic bone formation. The existence of lacZ-positive muscle-derived cells colocalized with osteocalcin-producing cells within lacunae of newly formed bone matrix suggests osteoblast and osteocyte differentiation. Although a specific cell was not isolated, these data support the contentions that osteoprogenitor cells reside within skeletal muscle and that muscle may represent a source other than bone marrow for the harvest of these cells.