Tissue-engineered bone regeneration

Genetic marking with the DeltaLNGFR-gene for tracing goat cells in bone tissue engineering

The use of bone marrow derived stromal cells (BMSC's) for bone tissue engineering has gained much attention as an alternative for autologous bone grafting. Little is known however, about the survival and differentiation of the cells, especially in the clinical application. The aim of this study was to develop a method to trace goat BMSC's in vivo. We investigated retroviral genetic marking, which allows stable expression of the label with cell division. Goat BMSC's were subjected to an amphotropic envelope containing a MoMuLV-based vector expressing the human low affinity nerve growth factor receptor (DeltaLNGFR). Labeling efficiency and effect on the cells were analyzed. Furthermore, transduced cells were seeded onto porous ceramic scaffolds, implanted subcutaneously in nude mice and examined after successive implantation periods. Flow cytometry indicated a transduction efficiency of 40-60%. Immunohistochemistry showed survival and subsequent bone formation of the gene-marked cells in vivo. Besides, marked cells were also found in cartilage and fibrous tissue. These findings indicate the maintenance of the precursor phenotype following gene transfer as well as the ability of the gene to be expressed following differentiation. We conclude that retroviral gene marking with DeltaLNGFR is applicable to trace goat BMSC's in bone tissue engineering research.

Ewing sarcoma: the pediatrician's point of view

BACKGROUND

Integrated multimodal care is needed for patients with Ewing sarcoma, which is the second most common primary bone malignancy in children and adolescence. Chemotherapy increases survival from less than 5% to 65-70% for patients with localized tumors and to 25-30% for those with metastases at diagnosis. Surgery is a major tool, whereas advances in imaging techniques have improved the indications for and the optimization of treatment. Radiotherapy remains useful, either alone or in addition to surgery, and new techniques (conformational RT and IMRT) will reduce short-term toxic effects. Pediatric oncologists do not outweigh surgeons or radiation therapists, but they are the ones who coordinate the medical team, which also includes pathologists and imaging specialists.

METHODS

The point of view of the pediatric oncologist was assessed as follows: the place of chemotherapy in Ewing tumor treatment, the place of radiotherapy in Ewing tumor treatment (including why avoid radiotherapy when technically possible in children), and how to proceed into the future? The place of surgery as local treatment for Ewing tumors was also evaluated.

RESULTS

These reviews show a dynamic and kaleidoscopic panorama of intense activity at the laboratory and clinical levels.

CONCLUSIONS

Though good survival rates have been achieved, improvements using entirely new approaches are needed.

Osteogenicity of autologous bone transplants in the goat

BACKGROUND

Little is known about the specific mechanisms that make autologous graft bone (AG) superior to the current alternatives. A potential mechanism is the active bone formation by the osteoprogenitor cells within the AG. However, whether these cells survive the transplantation is questionable, especially in nonvascularized, clinically sized grafts. In the present study, we investigated the role of viability in AG implanted ectopically and orthotopically in the goat.

METHODS

Eight goats were operated on twice. At the first operation, pieces of vital or devitalized autologous cortical bone were implanted in the paraspinal muscles. Eight weeks later, corticocancellous plugs were taken from the femoral condyles, morselized, and reimplanted as either vital or devitalized orthotopic grafts. The goats received fluorochrome labels at 5, 7, and 9 weeks after the first operation. At 12 weeks, the goats were killed, and the samples were examined histologically.

RESULTS

Ectopically, new bone had formed in both the vital and devitalized grafts. In the vital grafts, all three fluorochrome labels were present, indicating an early osteogenic mechanism. Within the devitalized grafts, only the 9-week label was observed. Histomorphometry indicated significantly more new bone in the vital grafts (10.3% vs. 1.7% in the devitalized grafts, P <0.01). Orthotopically, both vital and devitalized grafts showed new bone. Again, graft viability was advantageous in terms of new bone formation (14.5% vs. 9.3%, P <0.02).

CONCLUSION

The cells inside the autologous bone transplants most likely survived transplantation and were capable of initiating and sustaining new bone formation.

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.

Repair of calvarial defects with customized tissue-engineered bone grafts I. Evaluation of osteogenesis in a three-dimensional culture system

Bone generation by autogenous cell transplantation in combination with a biodegradable scaffold is one of the most promising techniques being developed in craniofacial surgery. The objective of this combined in vitro and in vivo study was to evaluate the morphology and osteogenic differentiation of bone marrow derived mesenchymal progenitor cells and calvarial osteoblasts in a two-dimensional (2-D) and three-dimensional (3-D) culture environment (Part I of this study) and their potential in combination with a biodegradable scaffold to reconstruct critical-size calvarial defects in an autologous animal model [Part II of this study; see Schantz, J.T., et al. Tissue Eng. 2003;9(Suppl. 1):S-127-S-139; this issue]. New Zealand White rabbits were used to isolate osteoblasts from calvarial bone chips and bone marrow stromal cells from iliac crest bone marrow aspirates. Multilineage differentiation potential was evaluated in a 2-D culture setting. After amplification, the cells were seeded within a fibrin matrix into a 3-D polycaprolactone (PCL) scaffold system. The constructs were cultured for up to 3 weeks in vitro and assayed for cell attachment and proliferation using phase-contrast light, confocal laser, and scanning electron microscopy and the MTS cell metabolic assay. Osteogenic differentiation was analyzed by determining the expression of alkaline phosphatase (ALP) and osteocalcin. The bone marrow-derived progenitor cells demonstrated the potential to be induced to the osteogenic, adipogenic, and chondrogenic pathways. In a 3-D environment, cell-seeded PCL scaffolds evaluated by confocal laser microscopy revealed continuous cell proliferation and homogeneous cell distribution within the PCL scaffolds. On osteogenic induction mesenchymal progenitor cells (12 U/L) produce significantly higher (p < 0.05) ALP activity than do osteoblasts (2 U/L); however, no significant differences were found in osteocalcin expression. In conclusion, this study showed that the combination of a mechanically stable synthetic framework (PCL scaffolds) and a biomimetic hydrogel (fibrin glue) provides a potential matrix for bone tissue-engineering applications. Comparison of osteogenic differentiation between the two mesenchymal cell sources revealed a similar pattern.

Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

Bone reconstruction of large defects using bone marrow derived autologous stem cells

Bone is a tissue that has the ability to heal itself when fractured. Occasionally, a critical defect can be formed when part of the bone is lost or excised, in this case the bone fails to heal and requires bone reconstruction to prevent a non-union defect. Autogenous cancellous bone is the current gold standard treatment in bone loss. Because the amount of autogenous cancellous bone that can be harvested is limited, the expanding need for bone reconstruction is paired by the growth of interest in the discipline of tissue engineering. Labs worldwide are working to provide the right carrier and the right set of cells that, once retransplanted, will ensure bone repair. Several investigators have focused their attention on a subset of autologous non-hematopoietic stem/progenitor cells contained in the adult bone marrow stroma, referred to as stromal stem cells (SSC), as the appropriate cells to be transplanted. The use of autologous cells is facilitated by less stringent ethical and regulatory issues and does not require the patient to be immunologically suppressed. In pre-clinical and clinical protocols of critical defects in which SSC are employed, two approaches are mainly used: in the first, SSC are derived from bone marrow and directly introduced at the lesion site, in the second, SSC are derived from several sites and are expanded ex vivo before being implanted. Both approaches, equally correct in principle, will have to demonstrate, with definitive evidence of their efficacy, their capability of solving a critical clinical problem such as non-union. In this report we outline the difficulties of working with SSC.

Novel Starch-Based Scaffolds for Bone Tissue Engineering: Cytotoxicity, Cell Culture, and Protein Expression

Starch-based biomaterials and scaffolds have been proposed for several biomedical applications. In the present work new scaffolds based on a 50/50 (wt%) blend of corn starch/ethylene-vinyl alcohol (SEVA-C) were studied. These scaffolds were processed by a melt-based technology, which has been used before with other starch-based materials but never with SEVA-C. Scanning electron microscopy (SEM) observation showed that the developed porous structures were 60% porous with pore size between 200 and 900 microm and a reasonable degree of interconnectivity. Moreover, scaffolds presented a compressive modulus of 117.50 +/- 3.7 MPa and a compressive strength of 20.8 +/- 2.4 MPa. Cytotoxicity evaluation was performed according to ISO/EN 10993 part 5 guidelines, and revealed that the developed scaffolds were nontoxic and did not inhibit cell growth. Direct contact assays were also carried out by use of a cell line of human osteoblast-like cells (SaOS-2). Cells were seeded (3 x 10(5) per scaffold) and allowed to grow for 4 weeks at 37 degrees C, in a humidified atmosphere containing 5% CO(2). Total protein assay showed that the cells were able to grow for the 4 weeks of the experiment. These data were further confirmed by SEM. Moreover, a cell viability assay (MTS test) demonstrated that cells were perfectly viable after the 4 weeks of culture, showing the adequacy of the developed structure in supporting them. Finally, Western blot analysis revealed that osteopontin was being actively expressed by the cells, which, in association with collagen deposition observed by SEM, seems to indicate that bone extracellular matrix was being deposited. Consequently it is believed that starch-based scaffolds should be considered as an alternative for bone tissue-engineering applications in the near future.

Bone regeneration via a mineral substrate and induced angiogenesis

Angiogenesis and biomineral substrates play major roles in bone development and regeneration. We hypothesized that macroporous scaffolds of biomineralized 85:15 poly(lactide-co-glycolide), which locally release vascular endothelial growth factor-165 (VEGF), would direct simultaneous regeneration of bone and vascular tissue. The presence of a bone-like biomineral substrate significantly increased regeneration of osteoid matrix (32 +/- 7% of total tissue area; mean +/- SD; p < 0.05) and mineralized tissue (14 +/- 2%; P < 0.05) within a rat cranium critical defect compared with a non-mineralized polymer scaffold (19 +/- 8% osteoid and 10 +/- 2% mineralized tissue). Further, the addition of VEGF to a mineralized substrate significantly increased the generation of mineralized tissue (19 +/- 4%; P < 0.05) compared with mineralized substrate alone. This appeared to be due to a significant increase in vascularization throughout VEGF-releasing scaffolds (52 +/- 9 vessels/mm(2); P < 0.05) compared with mineralized scaffolds without VEGF (34 +/- 4 vessels/mm(2)). Surprisingly, there was no significant difference in total osteoid between the two samples, suggesting that increased vascularization enhances mineralized tissue generation, but not necessarily osteoid formation. These results indicate that induced angiogenesis can enhance tissue regeneration, supporting the concept of therapeutic angiogenesis in tissue-engineering strategies.

Optimization of bone tissue engineering in goats: a peroperative seeding method using cryopreserved cells and localized bone formation in calcium phosphate scaffolds1

BACKGROUND

Bone tissue engineering by combining cultured bone marrow stromal cells with a porous scaffold is a promising alternative for the autologous bone graft. Drawbacks of the technique include the delay necessary for cell culture and the complicated logistics. We investigated methods to bypass these drawbacks. Furthermore, we investigated the localization of bone formation inside the scaffold.

METHODS

Bone marrow stromal cells from seven goats were culture expanded and cryopreserved. One week before surgery, some of the cells were thawed, cultured, and seeded on porous calcium phosphate scaffolds. The constructs were cultured for another week until implantation. The remaining cryopreserved cells were thawed just before implantation and peroperatively resuspended in plasma before combining with the scaffold. Scaffolds impregnated with fresh bone marrow, devitalized cultured constructs, and empty scaffolds served as controls. All samples were implanted in the back muscles of the goats for 9 weeks.

RESULTS

Histologic examination showed minimal (<1%) bone in the empty and devitalized scaffolds, 4.2 +/- 5.1 bone area percent in the bone marrow samples, and significantly more bone in both the cultured and peroperatively seeded constructs (11.7 +/- 2.5 and 14.0 +/- 2.0%). The peripheral 350 microm of the implants contained significantly less bone.

CONCLUSION

Peroperative preparation of osteogenic constructs with cryopreserved cells is feasible. These constructs yield substantially more bone than the scaffolds alone or scaffolds impregnated with fresh bone marrow. Bone deposition is much less on the scaffold periphery.

Tomorrow's skeleton staff: mesenchymal stem cells and the repair of bone and cartilage

Stem cells are regenerating medicine. Advances in stem cell biology, and bone marrow-derived mesenchymal stem cells in particular, are demonstrating that many clinical options once thought to be science fiction may be attainable as fact. The extra- and intra-cellular signalling used by stem cells as they differentiate into lineages appropriate to their destination are becoming understood. Thus, the growth stimuli afforded by LIF, FGF-2 and HGF, as well as the complementary roles of Wnt and Dickkopf-1 in stem cell proliferation are evident. The ability to direct multi-lineage mesenchymal stem sell (MSC) potential towards an osteogenic phenotype by stimulation with Menin and Shh are important, as are the modulatory roles of Notch-1 and PPARgamma. Control of chondrocytic differentiation is effected by interplay of Brachyury, BMP-4 and TGFbeta3. Smads 1, 4 and 5 also play a role in these phenotypic expressions. The ability to culture MSC has led to their use in tissue repair, both as precursor and differentiated cell substitutes, and with successful animal models of bone and cartilage repair using MSC, their clinical use is accelerating. However, MSC also suppress some T-cell functions in transplanted hosts, and could facilitate tumour growth, so a cautious approach is needed.

Osteogenic Differentiation of Human Bone Marrow Stromal Cells on Partially Demineralized Bone Scaffoldsin Vitro

Tissue engineering has been used to enhance the utility of biomaterials for clinical bone repair by the incorporation of an osteogenic cell source into a scaffold followed by the in vitro promotion of osteogenic differentiation before host implantation. In this study, three-dimensional, partially demineralized bone scaffolds were investigated for their ability to support osteogenic differentiation of human bone marrow stromal cells (BMSCs) in vitro. Dynamic cell seeding resulted in homogeneous cell attachment and infiltration within the matrix and produced significantly higher seeding efficiencies when compared with a conventional static seeding method. Dynamically seeded scaffolds were cultured for 7 and 14 days in the presence of dexamethasone and evaluated on biochemical, molecular, and morphological levels for osteogenic differentiation. Significant elevation in alkaline phosphatase activity was observed versus controls over the 14-day culture, with a transient peak indicative of early mineralization on day 7. On the basis of RT-PCR, dexamethasone-treated samples showed elevations in alkaline phosphatase and osteocalcin expression levels at 7 and 14 days over nontreated controls, while bone sialoprotein was produced only in the presence of dexamethasone at 14 days. Scanning electron microscopy evaluation of dexamethasone-treated samples at 14 days revealed primarily cuboidal cells indicative of mature osteoblasts, in contrast to nontreated controls displaying a majority of cells with a fibroblastic cell morphology. These results demonstrate that partially demineralized bone can be successfully used with human BMSCs to support osteogenic differentiation in vitro. This osseous biomaterial may offer new potential benefits as a tool for clinical bone replacement.

Bone Tissue Engineering for Spine Fusion: An Experimental Study on Ectopic and Orthotopic Implants in Rats

Alternatives to the use of autologous bone as a bone graft in spine surgery are needed. The purpose of this study was to examine tissue-engineered bone constructs in comparison with control scaffolds without cells in a posterior spinal implantation model in rats. Syngeneic bone marrow cells were cultured in the presence of bone differentiation factors and seeded on porous hydroxyapatite particles. Seven rats underwent a posterior surgical approach, in which scaffolds with (five rats) or without cells (two rats) were placed on both sides of the lumbar spine. In addition, separate scaffolds were inserted intramuscularly and subcutaneously during the surgical procedure. After 4 weeks, all rats were killed and examined radiographically, by manual palpation of the excised spine and histologically for signs of bone formation or spine fusion. All rats that received cell-seeded scaffolds showed newly formed bone in all three locations, whereas none of the locations in the control rats showed bone formation. The results of this study support the concept of developing tissue-engineering techniques in posterior spine fusion as an alternative to autologous bone.

Engineering bone-like tissuein vitro using human bone marrow stem cells and silk scaffolds

Porous biodegradable silk scaffolds and human bone marrow derived mesenchymal stem cells (hMSCs) were used to engineer bone-like tissue in vitro. Two different scaffolds with the same microstructure were studied: collagen (to assess the effects of fast degradation) and silk with covalently bound RGD sequences (to assess the effects of enhanced cell attachment and slow degradation). The hMSCs were isolated, expanded in culture, characterized with respect to the expression of surface markers and ability for chondrogenic and osteogenic differentiation, seeded on scaffolds, and cultured for up to 4 weeks. Histological analysis and microcomputer tomography showed the development of up to 1.2-mm-long interconnected and organized bonelike trabeculae with cuboid cells on the silk-RGD scaffolds, features still present but to a lesser extent on silk scaffolds and absent on the collagen scaffolds. The X-ray diffraction pattern of the deposited bone corresponded to hydroxyapatite present in the native bone. Biochemical analysis showed increased mineralization on silk-RGD scaffolds compared with either silk or collagen scaffolds after 4 weeks. Expression of bone sialoprotein, osteopontin, and bone morphogenetic protein 2 was significantly higher for hMSCs cultured in osteogenic than control medium both after 2 and 4 weeks in culture. The results suggest that RGD-silk scaffolds are particularly suitable for autologous bone tissue engineering, presumably because of their stable macroporous structure, tailorable mechanical properties matching those of native bone, and slow degradation.

Optimization of bone-tissue engineering in goats

Successful bone-tissue engineering (TE) has been reported for various strategies to combine cells with a porous scaffold. In particular, the period after seeding until implantation of the constructs may vary between hours and several weeks. Differences between these strategies can be reduced to (a) the presence of extracellular matrix, (b) the differentiation status of the cells, and (c) the presence of residual potentially immunogenic serum proteins. These parameters are investigated in two types of calcium phosphate scaffolds in a goat model of ectopic bone formation. Culture-expanded bone-marrow stromal cells from eight goats were seeded onto two types of hydroxyapatite granules: HA60/400 (60% porosity, 400-microm average pore size) and HA70/800. Scaffolds seeded with cells and control scaffolds were cultured for 6 days in medium containing autologous or semisynthetic serum, in the presence or absence of dexamethasone. Other scaffolds were seeded with cells just before implantation in medium with or without serum. All conditions were implanted autologously in the paraspinal muscles. After 12 weeks, bone had formed in 87% of all TE constructs, as demonstrated by histology. Histomorphometry indicated significantly more bone in the HA70/800 scaffolds. Furthermore, a significant advantage in bone formation was found when the constructs had been cultured for 6 days. In conclusion, both scaffold characteristics (porosity) and TE strategy (culturing of the constructs) were demonstrated to be important for bone TE.

Sustained release of ascorbate-2-phosphate and dexamethasone from porous PLGA scaffolds for bone tissue engineering using mesenchymal stem cells

The purpose of this research was to develop porous poly(D,L-lactide-co-glycolide) (PLGA) scaffolds from which ascorbate-2-phosphate (AsAP) and dexamethasone (Dex) are continuously released for a month for osteogenesis of mesenchymal stem cells for bone tissue engineering. Porous PLGA matrices containing AsAP and Dex were prepared by solvent casting/particulate leaching method. In vitro release and water uptake studies were performed in Dulbecco's phosphate buffered saline at 37 degrees C and 15 rpm. Drug loading and release rates were determined by high performance liquid chromatography. Release studies of Dex and AsAP showed that, after an initial burst release lasting 4 and 9 days, respectively, release rates followed zero order kinetics with high correlation coefficients at least until 35 days. Incorporation of AsAP into the scaffolds increased the release rates of Dex and AsAP, and the scaffold water uptake. When mesenchymal stem cells (MSCs) were cultured in the AsAP and Dex containing scaffolds in vitro, the amount of mineralization was significantly higher than in control scaffolds. In conclusion, AsAP and Dex were incorporated into porous PLGA scaffolds and continuously released over a month and osteogenesis of MSCs was increased by culture in these scaffolds.

Cryopreservation of tissue engineered constructs for bone

The large-scale clinical use of tissue engineered constructs will require provisions for its mass availability and accessibility. Therefore, it is imperative to understand the effects of low temperature (-196 degrees C) on the tissue engineered biological system. Initial studies used samples of the osteoblast-like cell line (SaOS-2) adhered to a two-dimensional poly(lactide-co-glycolide) thin film (2D-PLAGA) or a three-dimensional poly(lactide-co-glycolide) sintered microsphere matrix (3D-PLAGA) designed for bone tissue engineering. Experimental samples were tested for their ability to maintain cell viability, following low temperature banking for one week, in solutions of the penetrating cryoprotective agents, dimethylsulfoxide (DMSO), ethylene glycol, and glycerol. Results indicated the DMSO solution yielded the greatest percent cell survival for SaOS-2 cells adhered to both the 2D- and 3D-PLAGA scaffolds; therefore, DMSO was used to cryopreserve mineralizing primary rabbit osteoblasts cells adhered to 2D-PLAGA matrices for 35 days. Results indicated retention of the extracellular matrix architecture as no statistically significant difference in the pre- and post-thaw mineralized structures was measured. Percent cell viability of the mineralized constructs following low temperature storage was approximately 50%. These are the first studies to address the issue of preservation techniques for tissue engineered constructs. The ability to successfully cryopreserve mineralized tissue engineered matrices for bone may offer an unlimited and readily available source of bone-like materials for orthopaedic applications.

Bioengineered human bone tissue using autogenous osteoblasts cultured on different biomatrices

Surgical treatment of critical-size posttraumatic bone defects is still a challenging problem, even in modern bone and joint surgery. Progress in cellular and molecular biology during the last decade now permits novel approaches in bone engineering. Recent conceptual and technical advances have enabled the use of mitotically expanded, bone-derived cells as a therapeutic approach for tissue repair. Using three different tissue carrier systems, we successfully cultivated human osteoblasts in a newly developed perfusion chamber. We studied cell proliferation and the expression of osteocalcin, osteopontin, bone morphogenetic protein-2A, alkaline phosphatase, and vascular endothelial growth factor as parameters for osteoblast function and viability. Adherence of highly enriched human osteoblasts had already started after 1 h and resulted in completely overgrown human bone pieces after 10 days. Expression analysis of bone-specific alkaline phosphatase indicated differentiating osteoblasts, whereas the high mRNA expression of osteocalcin and osteopontin revealed terminally differentiated osteoblasts and the process of mineralization. Additionally, gene expression was significantly higher when demineralized bone was used as biomatrix, compared to autoclaved bone and hydroxyapatite ceramics. We conclude that with our newly developed perfusion culture system, vital autogenous bone implants of clinically applicable size can be generated within 17 days in order to manage critical-size bone defects.

Skeletal homeostasis in tissue-engineered bone

Tissue-engineering strategies to stimulate bone regeneration may offer an alternative approach to conventional orthopaedic and maxillofacial surgical therapies. Over the last decade, significant advances have been accomplished in developing biomimetic matrices, growth factors, cell transplantation and gene delivery therapeutics to support new bone growth. However, it is not known if tissue-engineered bone recapitulates the biology of normal skeletal tissue in response to physiologic cues. Here, we report that bone formed by the differentiation of transplanted murine bone marrow stromal cells (BMSCs) responds to a systemically delivered calciotropic hormone. Ectopic ossicles in mice exposed to catabolic doses of parathyroid hormone (PTH) had increased numbers of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts as compared to control mice. In contrast, treatment with anabolic doses of PTH promoted a marked increase in trabecular bone mass as analyzed by microcomputed tomography and histomorphometry. Our findings demonstrate that bone formed from transplanted BMSCs is responsive to normal physiologic signals, and can be augmented by the addition of a systemic anabolic agent. Because multiple and distinct ossicles can be generated in a single animal, this versatile system may be used to: (a) elucidate cellular/molecular mechanisms in bone regeneration; (b) study cell-to-cell interactions in the bone marrow microenvironment in health and disease; and (c) evaluate the efficacy of osteotropic agents that modulate bone turnover in vivo.

A cyclo peptide activates signaling events and promotes growth and the production of the bone matrix

The interaction of bone cells and their underlying extracellular matrix impacts biological processes such as maintenance of tissue integrity. The biological recognition of the extracellular matrix by attached cells is mediated by the activity of integrins that recognize adhesive-specific domains. The most widely recognized adhesive motif is the RGD sequence, common to many of the adhesive matrix molecules. Here, we show that cyclo DFKRG which was previously selected to increase cell adhesion of human bone marrow stromal cells (HBMSC), increases both cell differentiation and mineralization through activation of tyrosine kinases, focal adhesion kinase (p(125)FAK) and Mitogen Activated Protein (MAP) kinases.

Electrophoretic deposition of porous hydroxyapatite scaffold

Bioactive porous hydroxyapatite (HA) scaffold was fabricated using electrophoretic deposition (EPD) technique in the present work. Bulk HA scaffold was achieved by repeated deposition. The green scaffold was sintered at 1200 degrees C to 82% of the theoretical density. Scanning electron microscopy examination and mercury porosimetry measurement have shown that the porosity remains interconnected and a range of pore size from several microns to hundreds of microns was obtained. X-ray diffraction analysis was performed and confirmed that there is no HA decomposition during the sintering process. Mechanical characterization has also shown that the EPD scaffold possesses excellent properties. Cell culturing experiment was carried out and the result shows that the scaffold bioactivity is not only dependent on the interconnectivity of the pores, but also the pore size.

Platelet-derived growth factors enhance proliferation of human stromal stem cells

Studies on new procedures for bone reconstruction suggest that autologous cells seeded on a resorbable scaffold can improve the treatment of bone defects. It is important to develop culture conditions for ex vivo expansion of stromal stem cells (SSC) that do not compromise their self-renewing and differentiation capability. Bone marrow SSC and platelet gel (PG) obtained by platelet-rich plasma provide an invaluable source for autologous progenitor cells and growth factors for bone reconstruction. In this study the effect of platelet-rich plasma (PRP) released by PG on SSC proliferation and differentiation was investigated. MTT assay was used to investigate the effect of PRP on proliferation: results showed that PRP induced SSC proliferation. The effect was dose dependent and 10% PRP is sufficient to induce a marked cell proliferation. Untreated cells served as controls. Upon treatment with 10% PRP, cells entered logarithmic growth. Removal of PRP restored the characteristic proliferation rate. Because SSC can gradually lose their capability to differentiate along the chondrogenic and osteogenic lineage during subculture in vitro, we tested whether 10% PRP treatment affected SSC ability to mineralize. SSC were first exposed to 10% PRP for five passages, at passage 6 PRP was washed away and plated cells were treated with dexamethasone (DEX). DEX induced a three-fold increase in the number of alkaline phosphatase positive cells and induced mineralization that is consistent with the differentiation of osteochondroprogenitor cells. In conclusion, 10% PRP promotes SSC proliferation; cells expanded with 10% PRP can mineralize the extracellular matrix once PRP is withdrawn.

Modification of gene expression induced in human osteogenic and osteosarcoma cells by culture on a biphasic calcium phosphate bone substitute

Bone hybrids made of bioceramics seeded with mesenchymal or osteoblastic cells are very promising alternatives to autologous bone graft. Along this line, the development of in vitro models, dedicated to analyze the influence of these biomaterials on osteogenic cells, will help to improve the performance of these bone substitutes. In the present work we analyzed the effects of a macroporous biphasic calcium phosphate ceramic (BCP, Triosite) on three different human osteosarcoma cell lines and on human primary osteogenic cells and compared this culture substratum to traditional culture on plastic. We showed that all these osteoblastic cells adhere and proliferate on the trabecular BCP blocks, with a different spatial organization for osteosarcoma cells compared to normal osteogenic cells. We also demonstrated that osteoblastic marker genes such as Cbfa1, type I collagen, osteonectin, osteopontin, and osteocalcin were expressed at similar levels by these cells cultured on either substratum, suggesting that adhesion to BCP does maintain the osteoblastic phenotype of these cells. Next, we provided the first evidence of differences of cytokine expression profiles revealed on this Ca-P ceramic as compared to expression in classical culture. These modifications affected the expression of cytokines such as TGF-beta1, G-CSF, and IL-3 and were quantitatively different between osteosarcoma cells and normal osteogenic cells. Given the role of these cytokines in bone biology and in hematopoiesis, these results obtained in vitro suggest that the BCP ceramic studied here could stimulate osteogenesis in vivo by activating cellular processes during bone formation and healing. This study highlights the notion that the nature of the culture substratum must be taken into account when studying bone cell biology in vitro. Owing to the nature and spatial organization of the BCP, our hypothesis is that culture on BCP is closer to the physiological situation than culture on plastic.

Macroporous biphasic calcium phosphate scaffold with high permeability/porosity ratio

Macroporous biphasic calcium phosphate (BCP) with channel-shaped pores was produced by a novel dual-phase mixing method. The processing route includes mixing water-based BCP slurry and polymethylmethacrylate resin; shaping in a mold; and polymerization, drying, pyrolyzing, and sintering. After comparison with two other commercial macroporous BCP materials, which were produced along different routes, it was found that conventional parameters such as porosity and pore size cannot describe a macroporous structure precisely enough for the application as tissue-engineering scaffold. Instead, permeability can be seen as an intrinsic and quantitative parameter to describe the macroporous structure of various scaffolds, because it is independent of sample size and fluid used in the test. Another parameter, the permeability/porosity ratio, provides an indication of the percolative efficiency per unit porous volume of a scaffold. Structural characterizations and permeability studies of other macroporous scaffold materials were also performed, and it was found that permeability could reflect a combination of five important parameters for scaffold: (1) porosity, (2) pore size and distribution, (3) interconnectivity, (4) fenestration size and distribution, and (5) orientation of pores. Finally, the implications of relating permeability with biological performances are also discussed.

Scaffolds and biomaterials for tissue engineering: a review of clinical applications

Tissue engineering is a multidisciplinary area of research aimed at regeneration of tissues and restoration of organ function. This is achieved through implantation of cells/tissues grown outside the body or by stimulating cells to grow into an implanted matrix. In this short review, we discuss the use of biomaterials, in the form of scaffolds, for tissue engineering and review clinical applications to otorhinolaryngology-head and neck surgery.

In vivo bone engineering in a rabbit femur

The repair of bone defects in reconstructive surgery has significant limitations. Donor site morbidity, limited supply of autograft, and risks and complications associated with allografting and synthetic bone substitutes are among the most significant. In an effort to address these problems, the search for an ideal bone replacement has led to the development of a new method of poly(lactide-co-glycolide) (PLGA) foam processing, enabling the production of a biodegradable scaffold with similar porosity to human trabecular bone. In this study, these scaffolds were evaluated for bone repair in vivo in a femoral critical-sized segmental defect in New Zealand White (NZW) rabbits. Three groups of nine animals were investigated. In the first group, the critical-sized defects were empty. Scaffolds alone were implanted in the second group, whereas autologous bone marrow cell-loaded scaffolds were implanted in the third group. Animals ambulated freely for 8 weeks after surgery, and bone formation throughout the defects was serially assessed radiographically and quantified using a bone formation index (BFI) measure. Postmortem radiography and histology were also undertaken to examine bone formation. There was a significant effect of applying this technology to the amount of bone formed in the defects as determined by the BFI (F = 3.41, P < 0.05). The mean BFI for the cell-loaded scaffolds was greater than for the control group at all measured time points (2-, 4-, 6-, and 8-week radiographs). This difference was significant for the 2- and 8-week radiographs (P < 0.05). Qualitative histological assessment confirmed these findings. We concluded from these findings that these PLGA scaffolds loaded with marrow-derived progenitor cells yield significant bone formation in a critical-sized rabbit femoral defect. This technology comprising a novel scaffold design and autologous cells may provide an alternative to current strategies for reconstruction of bony defects.

Viable osteogenic cells are obligatory for tissue-engineered ectopic bone formation in goats

In this study we investigated the bone-forming capacity of tissue-engineered (TE) constructs implanted ectopically in goats. As cell survival is questionable in large animal models, we investigated the significance of vitality, and thus whether living cells instead of only the potentially osteoinductive extracellular matrix are required to achieve bone formation. Vital TE constructs of porous hydroxyapatite (HA) covered with differentiated bone marrow stromal cells (BMSCs) within an extracellular matrix (ECM) were compared with identical constructs that were devitalized before implantation. The devitalized implants did contain the potentially osteoinductive ECM. Furthermore, we evaluated HA impregnated with fresh bone marrow and HA only. Two different types of HA granules with a volume of approximately 40 microm were investigated: HA70/800, a microporous HA with 70% interconnected macroporosity and an average pore size of 800 microm, and HA60/400, a smooth HA with 60% interconnected macropores and an average size of 400 microm. Two granules of each type were combined and then treated as a single unit for cell seeding, implantation, and histology. The tissue-engineered samples were obtained by seeding culture-expanded goat BMSCs on the HA and subsequently culturing these constructs for 6 days to allow cell differentiation and ECM formation. To devitalize, TE constructs were frozen in liquid nitrogen according to a validated protocol. Fresh bone marrow impregnation was performed perioperatively (4 mL per implant unit). All study groups were implanted in bilateral paraspinal muscles. Fluorochromes were administered at three time points to monitor bone mineralization. After 12 weeks the units were explanted and analyzed by histology of nondecalcified sections. Bone formation was present in all vital tissue-engineered implants. None of the other groups showed any bone formation. Histomorphometry indicated that microporous HA70/800 yielded more bone than did HA60/400. Within the newly formed bone, the fluorescent labels showed that mineralization had occurred before 5 weeks of implantation and was directed from the HA surface toward the center of the pores. In conclusion, tissue-engineered bone formation in goats can be achieved only with viable constructs of an appropriate scaffold and sufficient BMSCs.

Tissue engineering and cell therapy of cartilage and bone

Trauma and disease of bones and joints, frequently involving structural damage to both the articular cartilage surface and the subchondral bone, result in severe pain and disability for millions of people worldwide and represent major challenges for the orthopedic surgeons. Therapeutic repair of skeletal tissues by tissue engineering has raised the interest of the scientific community, providing very promising results in preclinical animal models and clinical pilot studies. In this review, we discuss this approach. The choice of a proper cell type is addressed. The use of terminally differentiated cells, as in the case of autologous chondrocyte implantation, is compared with the advantages/disadvantages of using more undifferentiated cell types, such as stem cells or early mesenchymal progenitors that retain multi-lineage and self-renewal potentials. The need for proper scaffold matrices is also examined, and we provide a brief overview of their fundamental properties. A description of the natural and biosynthetic materials currently used for reconstruction purposes, either of cartilage or bone, is given. Finally, we highlight the positive aspects and the remaining problems that will drive future research in articular cartilage and bone repair.

Application and limitations of chloromethyl-benzamidodialkylcarbocyanine for tracing cells used in bone Tissue engineering

Bone tissue engineering has the potential to provide us with an autologous bone substitute. Despite extensive research to optimize the technique, little is known about the survival and function of the cells after implantation. To monitor the cells, in vivo labeling is the method of choice. In this study we investigated the use of the fluorescent membrane marker chloromethyl-benzamidodialkylcarbocyanine (CM-Dil) to label cells used in bone tissue engineering. When applying label concentrations up to 50 microM, cells could be labeled efficiently without negative effects on cell vitality, proliferation, or bone-forming capacity. Porous hydroxyapatite scaffolds were seeded with labeled cells, and up to 6 weeks after implantation in nude mice cells could be traced inside tissue-engineered bone. However, contrary to other reports concerning intramembranous labels, transfer of the label from labeled to unlabeled cells was detected. Transfer occurred both in vitro and in vivo between vital cells and between dead and living cells. To determine when in vivo label transfer happened, devitalized, labeled constructs were implanted for various time periods in nude mice. The presence of vital labeled cells inside these constructs, when evaluated at different implantation periods, indicated transfer of the label. Transfer occurred at 7 days postimplantation when 40 microM label was applied, whereas 10 microM labeled constructs showed transfer 10 days after implantation. These findings indicate that CM-Dil label is useful for in vivo tracing of cells for follow-up periods up to 10 days. This makes the label particularly useful for cell survival studies in tissue-engineered implants.

Bone formation by rat bone marrow cells cultured on titanium fiber mesh: effect of in vitro culture time

The objective of this study was to examine the effect of cell culture time on bone formation by rat bone marrow cells seeded in titanium fiber mesh. As a seeding technique, a high cell suspension was used (3 x 10(6) cells/mL). Therefore, 30 meshes were repeatedly rotated in a 10 mL tube (containing 30 x 10(6) cells) on a rotation plate (2 rpm) for 3 h. Osteogenic cells were cultured for 1, 4, and 8 days on titanium fiber mesh and finally implanted subcutaneously in rats. Meshes without cells were also implanted subcutaneously in rats. DNA and scanning electron microscopy (SEM) analyses and calcium measurements determined cellular proliferation and differentiation during the in vitro incubation period of the mesh implants. Four weeks after implant insertion, the animals were sacrificed. The implants, with their surrounding tissue, were retrieved and prepared for histologic evaluation and calcium measurements. DNA analysis of the in vitro experiment showed a lag phase from day 1 through day 4, but a 42% increase in DNA between days 4 and 8. SEM and calcium measurements indicated an increase in calcium from day 1 to day 4, yet only a small but significant increase from days 4 to 8. Histologic analysis demonstrated that bone was formed in all day 1 and day 4 implants, and that the bone-like tissue was present uniformly through the meshes. The bony tissue was morphologically characterized by osteocytes embedded in a mineralized matrix, with a layer of osteoid and osteoblasts at the surface. The day 8 implants showed only calcium phosphate deposition in the titanium fiber mesh. Calcium measurements of the implants revealed that calcification in day 1 implants was significantly higher (p < 0.05) compared to day 4 and day 8 implants. No significant difference in calcium content existed between day 4 and day 8 implants. On the basis of our results, we conclude that 1) bone formation was generated more effectively in osteogenic cells by a short culture time after seeding in titanium fiber mesh; 2) dynamic cell seeding is probably more effective than static cell seeding; and 3) selection of the right cells from the heterogenous bone marrow population remains a problem.

Immunoselection and adenoviral genetic modulation of human osteoprogenitors: in vivo bone formation on PLA scaffold

The aim of this study was to examine the potential of immunoselected genetically modified human osteoprogenitors to form bone in vivo on porous PLA scaffolds. Human osteoprogenitors from bone marrow were selected using the antibody STRO-1 utilising a magnetically activated cell separation system. The STRO-1(+) fraction isolated 7% of nucleated marrow cells and increased fibroblastic colony formation by 300% and alkaline phosphatase activity by 190% over unselected marrow cell cultures. To engineer bone tissue, STRO-1(+) culture-expanded cells were transduced with AxCAOBMP-2, an adenovirus carrying the human BMP-2 gene, injected into diffusion chambers containing porous PLA scaffolds, and implanted in vivo. After 11 weeks the presence of bone mineral was observed by X-ray analysis and confirmed for mineral by von Kossa, as well as bone matrix composition by Sirius red staining, birefringence, and type I collagen immunohistochemistry. Bone formation in vivo indicates the potential of using immunoselected progenitor cells and ex vivo gene transfer with biodegradable scaffolds, for the development of protocols for the treatment of a wide variety of musculo-skeletal disorders.

Formation of three-dimensional cell/polymer constructs for bone tissue engineering in a spinner flask and a rotating wall vessel bioreactor

The aim of this study is to investigate the effect of the cell culture conditions of three-dimensional polymer scaffolds seeded with rat marrow stromal cells (MSCs) cultured in different bioreactors concerning the ability of these cells to proliferate, differentiate towards the osteoblastic lineage, and generate mineralized extracellular matrix. MSCs harvested from male Sprague-Dawley rats were culture expanded, seeded on three-dimensional porous 75:25 poly(D,L-lactic-co-glycolic acid) biodegradable scaffolds, and cultured for 21 days under static conditions or in two model bioreactors (a spinner flask and a rotating wall vessel) that enhance mixing of the media and provide better nutrient transport to the seeded cells. The spinner flask culture demonstrated a 60% enhanced proliferation at the end of the first week when compared to static culture. On day 14, all cell/polymer constructs exhibited their maximum alkaline phosphatase activity (AP). Cell/polymer constructs cultured in the spinner flask had 2.4 times higher AP activity than constructs cultured under static conditions on day 14. The total osteocalcin (OC) secretion in the spinner flask culture was 3.5 times higher than the static culture, with a peak OC secretion occurring on day 18. No considerable AP activity and OC secretion were detected in the rotating wall vessel culture throughout the 21-day culture period. The spinner flask culture had the highest calcium content at day 14. On day 21, the calcium deposition in the spinner flask culture was 6.6 times higher than the static cultured constructs and over 30 times higher than the rotating wall vessel culture. Histological sections showed concentration of cells and mineralization at the exterior of the foams at day 21. This phenomenon may arise from the potential existence of nutrient concentration gradients at the interior of the scaffolds. The better mixing provided in the spinner flask, external to the outer surface of the scaffolds, may explain the accelerated proliferation and differentiation of marrow stromal osteoblasts, and the localization of the enhanced mineralization on the external surface of the scaffolds.

In situ forming lactic acid based orthopaedic biomaterials: influence of oligomer chemistry on osteoblast attachment and function

The ability of osteoblasts to attach and function normally on scaffolds fabricated from synthetic materials is essential for musculoskeletal tissue engineering applications. In this study, the osteoconductivity of polymer networks formed from multifunctional lactic acid oligomers was assessed. These oligomers form highly crosslinked networks via a photoinitiated polymerization, which provides potential advantages for many orthopaedic applications. Depending on the initial oligomer chemistry and the resultant polymer hydrophobicity, protein adsorption and osteoblast function varied significantly between the various lactic acid based polymer chemistries. Results were compared to control polymers of tissue culture polystyrene (TCPS) and 50:50 poly(lactic-co-glycolic acid) (PLGA). The viability of osteoblasts attached to poly(2EG10LA) and poly(2EG6LA) was close to the TCPS and PLGA after 7 and 14 days of culture, whereas cell viability was approximately 50% lower on poly(8EG6LA). Additionally, the alkaline phosphatase activity and mineralization of attached osteoblasts were similar on poly(2EG10LA) and PLGA, whereas these markers of bone formation were significantly lower for poly(2EG6LA) and poly(8EG6LA). For example, the alkaline phosphatase activity of rat calvarial osteoblasts attached to poly(2EG10LA) was 0.048 +/- 0.006 micromol mg(-1) protein-min, but only 0.030 +/- 0.003 micromol mg(-1) protein-min for osteoblasts attached to poly(8EG6LA) after 14 days of culture. Finally, osteoblasts were seeded onto three-dimensional scaffolds to demonstrate the applicability of the scaffolds for bone tissue engineering.

The potential of biomimesis in bone tissue engineering: lessons from the design and synthesis of invertebrate skeletons

Synthetic bone replacement materials are now widely used in orthopedics. However, to date, replication of trabecular bone structure and mechanical competence has proved elusive. Maximization of bone tissue attachment to replacement materials requires a highly organized porous structure for tissue integration and a template for assembly, combined with structural properties analogous to living bone. Natural structural biomaterials provide an abundant source of novel bone replacements. Animal skeletons have been designed through optimization by natural selection to physically support and physiologically maintain diverse tissue types encompassing a variety of functions. These skeletons possess structural properties that provide support for the complete reconstruction and regeneration of ectodermal, mesodermal, and bone tissues derived from animal and human and are thus suited to a diversity of tissue engineering applications. Increased understanding of biomineralization has initiated developments in biomimetic synthesis with the generation of synthetic biomimetic materials fabricated according to biological principles and processes of self-assembly and self-organization. The synthesis of complex inorganic forms, which mimic natural structures, offers exciting avenues for the chemical construction of macrostructures and a new generation of biologically and structurally related bone analogs for tissue engineering.

Bone formation by human postnatal bone marrow stromal stem cells is enhanced by telomerase expression

Human postnatal bone marrow stromal stem cells (BMSSCs) have a limited life-span and progressively lose their stem cell properties during ex vivo expansion. Here we report that ectopic expression of human telomerase reverse transcriptase (hTERT) in BMSSCs extended their life-span and maintained their osteogenic potential. In xenogenic transplants, hTERT-expressing BMSSCs (BMSSC-Ts) generated more bone tissue, with a mineralized lamellar bone structure and associated marrow, than did control BMSSCs. The enhanced bone-forming ability of BMSSC-Ts was correlated with a higher and sustained expression of the early pre-osteogenic stem cell marker STRO-1, indicating that telomerase expression helped to maintain the osteogenic stem cell pool during ex vivo expansion. These results show that telomerase expression can overcome critical technical barriers to the ex vivo expansion of BMSSCs, and suggest that telomerase therapy may be a useful strategy for bone regeneration and repair.

Bone tissue engineering: hope vs hype

The requirement for new bone to replace or restore the function of traumatised, damaged, or lost bone is a major clinical and socioeconomic need. Bone formation strategies, although attractive, have yet to yield functional and mechanically competent bone. Bone tissue engineering has been heralded as the alternative strategy to regenerate bone. In essence, the discipline aims to combine progenitor or mature cells with biocompatible materials or scaffolds, with or without appropriate growth factors, to initiate repair and regeneration. This brief review outlines the concepts, challenges, and limitations in bone tissue engineering and the potential that could improve the quality of life for many as a result of interdisciplinary collaboration.

Novel peptide-based biomaterial scaffolds for tissue engineering

Biomaterial scaffolds are components of cell-laden artificial tissues and transplantable biosensors. Some of the most promising new synthetic biomaterial scaffolds are composed of self-assembling peptides that can be modified to contain biologically active motifs. Peptide-based biomaterials can be fabricated to form two- and three-dimensional structures. Recent studies show that biomaterial promotion of multi-dimensional cell-cell interactions and cell density are crucial for proper cellular differentiation and for subsequent tissue formation. Other refinements in tissue engineering include the use of stem cells, cell pre-selection and growth factor pre-treatment of cells that are used for seeding scaffolds. These cell-culture technologies, combined with improved processes for defining the dimensions of peptide-based scaffolds, might lead to further improvements in tissue engineering. Novel peptide-based biomaterial scaffolds seeded with cells show promise for tissue repair and for other medical applications.

Tissue-engineered bone repair of sheep cranial defects with autologous bone marrow stromal cells

Cranial bone defect remains a major challenge to craniofacial surgeons because of limited availability of autologous bone graft to repair the defects and the donor site defects secondary to tissue harvesting. In contrast, tissue-engineering technique can generate a large bone tissue using small amount of autologous cells and therefore avoid these problems. Bone Marrow Stromal Cells (MSCs) have the potential of multi-lineage (including osteogenic) differentiation. The objective of this study was to investigate the potential of using autologous MSCs to repair cranial bone defects by a tissue-engineering approach. Autologous MSCs were isolated from eight adult sheep respectively and were in vitro expanded and induced to become osteogenic cells. Bilateral full-thickness defects (20 mm in diameter) of parietal bones were created in animals and the bone defects were either repaired with the bone implants constituted with MSCs and calcium alginate at the experimental side (n = 8) or treated with calcium alginate only without MSCs (n = 4) or left unrepaired (n = 4) at the control side. New bone tissues were observed either grossly or histologically at the defects of experimental group as early as 6 weeks post-repairing, but not in control groups. The engineered bone tissue became more mature at 18 weeks post-repairing. Three-dimensional computerized tomography (CT) scan revealed an almost complete repair of the defect of experimental group at 18 weeks. This study may provide insight for future clinical repair of cranial defect.

Microenvironment and stem properties of bone marrow-derived mesenchymal cells

Adult stem cells are self-renewing, pluripotent, and able to repopulate the tissue in which they reside. Cells endowed with these properties have been isolated from several tissues and an increasing number of reports provide evidence of their ability, following transplantation, to engraft host tissues other than those of their origin. In this setting, interest in the well-documented capacity of bone marrow stromal cells to undergo multilineage differentiation is growing. Neural and cardiomyogenic lineages have recently been proposed as additional differentiative pathways of these cells. However, culture conditions and inductive molecules can alter the behavior of bone marrow stromal cells and the microenvironment is critical for proper in vivo delivery. The maintenance of their stem properties and the possibility of reprogramming their commitment is a field of primary interest given the potential use of these cells in regenerative medicine. We discuss here how the microenvironmental cues, and the growth factors that physiologically govern commitment and subsequent differentiation, influence the properties of bone marrow stromal cells and modulate their engraftment into host tissues.

Stem cells in tissue engineering

The concept of producing 'spare parts' of the body for replacement of damaged or lost organs lies at the core of the varied biotechnological practices referred to generally as tissue engineering. Use of postnatal stem cells has the potential to significantly alter the perspective of tissue engineering. Successful long-term restoration of continuously self-renewing tissues such as skin, for example, depends on the use of extensively self-renewing stem cells. The identification and isolation of stem cells from a number of tissues provides appropriate targets for prospective gene therapies.

Born again bone: tissue engineering for bone repair

Destruction of bone tissue due to disease and inefficient bone healing after traumatic injury may be addressed by tissue engineering techniques. Growth factor, cytokine protein, and gene therapies will be developed, which, in conjunction with suitable carriers, will regenerate missing bone or help in cases of defective healing.

Three-dimensional cell-scaffold constructs promote efficient gene transfection: implications for cell-based gene therapy

To date, introduction of gene-modified cells in vivo is still a critical limitation for cell-based gene therapy. In this study, based on tissue engineering techniques, we developed a three-dimensional (3-D) transfection system to be cell-based gene delivery vehicle. Human trophoblast-like ED(27) and fibroblastic NIH3T3 cells were used as model cell lines. Cells were seeded onto PET fibrous matrices and plated on polyethylene terephathalate (PET) films as 2-D transfection control. The cell-matrices and cell-films were transfected with pCMV-betagal and pEGFP (green fluorescent protein) reporter gene vectors using LipofectAmine reagent. Gene expression on 3-D versus 2-D growth surface were investigated. The effects of seeding method, seeding density, porosity of the PET matrix, and culturing time of the cell-matrix complex on cDNA transfection and expression in the 3-D cell-matrix complex were also investigated. The beta-gal assay and GFP detection showed that 3-D transfection promoted a higher gene expression level and longer expression time as compared to 2-D transfection. There existed an optimal initial cell seeding density for gene transfection of 3-D cell-matrix complex. Cells seeded on PET matrices with a lower porosity ( approximately 87%) had higher gene expression activities than cells in the matrices with a higher porosity ( approximately 90%). Also, Higher gene expression levels of beta-gal were obtained for the more uniformly seeded matrices that were seeded with a depth-filtration method. The results from this study demonstrate the potential utility of cells seeded onto 3-D fibrous matrices as cell-based gene delivery vehicle for in vitro study of gene expression or in vivo gene therapy.

Osteogenesis and bone-marrow-derived cells

This paper addresses some of the important aspects of stem cell commitment to the bone cell lineage examining the various types of precursor cells, their responses to cytokines and other extracellular influences, and recent observations on the biochemical and molecular control of lineage-specific gene expression. The process of osteopoiesis involves the proliferation and maturation of primitive precursor cells into functional osteoblasts. The bone cells purportedly originate from mesenchymal stem cells that commit to the osteogenic cell lineage becoming osteoprogenitor cells, preosteoblasts, osteoblasts, and osteocytes. Further understanding of this developmental process requires that lineage-specific markers be identified for the various populations of bone cells and their precursors, that cell separation techniques be established so that cells of the osteogenic lineage can be purified at different stages of differentiation, and that these isolated cells are studied under serum-free, chemically defined conditions.

Exogenously regulated stem cell-mediated gene therapy for bone regeneration

Regulated expression of transgene production and function is of great importance for gene therapy. Such regulation can potentially be used to monitor and control complex biological processes. We report here a regulated stem cell-based system for controlling bone regeneration, utilizing genetically engineered mesenchymal stem cells (MSCs) harboring a tetracycline-regulated expression vector encoding the osteogenic growth factor human BMP-2. We show that doxycycline (a tetracycline analogue) is able to control hBMP-2 expression and thus control MSC osteogenic differentiation both in vitro and in vivo. Following in vivo transplantation of genetically engineered MSCs, doxycycline administration controlled both bone formation and bone regeneration. Moreover, our findings showed increased angiogenesis accompanied by bone formation whenever genetically engineered MSCs were induced to express hBMP-2 in vivo. Thus, our results demonstrate that regulated gene expression in mesenchymal stem cells can be used as a means to control bone healing.