Strategies for directing the differentiation of stem cells into the osteogenic lineage in vitro

Effects of osteogenic differentiation inducers on in vitro expanded adult mesenchymal stromal cells

PURPOSE

For bone regeneration therapy using stem cells, well-defined ex vivo protocols to expand mesenchymal stromal cells (MSC), as well as assays to show their potential differentiation into the osteogenic lineage, are needed. Aim of this study was to analyze the role of the biochemical osteogenic inducers, i.e. ascorbic acid, dexamethasone, and ß-glycerophosphate, employed in the current protocols for osteogenic differentiation of MSC in vitro, to address the requirements for reliable differentiation systems.

METHODS

MSC were isolated from the bone marrow of donors (46-73 years of age) undergoing total hip replacement, and expanded in vitro. At confluence, MSC were cultured under four different conditions: α-MEM plus serum (basal medium or C1), basal medium plus ascorbate (C2), basal medium plus ascorbate and dexamethasone (C3), or basal medium plus ascorbate, dexamethasone and ß-glycerophosphate (C4). Morphology, proliferation, mineralization, alkaline phosphatase, collagen and expression of bone-related genes of MSC under the different media were analyzed at fixed time points.

RESULTS

MSC proliferation and the number of colony forming units were increased by ascorbic acid, whereas dexamethasone enhanced the proportion of ALP-positive CFU and was critical for mineral deposition. Runx-2 and type I collagen gene expression decreased along with additive-induced MSC differentiation, i.e. from C1 to C4, while ALP and osteocalcin were differently regulated.

CONCLUSION

Our findings support the role of different inducers on the sequential stages of MSC expansion and osteogenic differentiation in vitro, suggesting the addition of DEX following proliferation to ensure mineralization, as an index of in vivo osteogenic potency of human mesenchymal cells.

Cellular and transcriptomic analysis of human mesenchymal stem cell response to plasma-activated hydroxyapatite coating

Atmospheric pressure plasma has recently emerged as a technique with a promising future in the medical field. In this work we used the technique as a post-deposition modification process as a means to activate hydroxyapatite (HA) coatings. Contact angle goniometry, optical profilometry, scanning electron microscopy morphology imaging and X-ray photoelectron spectroscopy analysis demonstrate that surface wettability is improved after treatment, without inducing any concomitant damage to the coating. The protein adsorption pattern has been found to be preferable for MSC, and this may result in greater cell attachment and adhesion to plasma-activated HA than to untreated samples. Cell cycle distribution analysis using flow cytometry reveals a faster transition from G(1) to S phase, thus leading to a faster cell proliferation rate on plasma-activated HA. This indicates that the improvement in surface wettability independently enhances cell attachment and cell proliferation, which is possibly mediated by FAK phosphorylation. Pathway-specific polymerase chain reaction arrays revealed that wettability has a substantial influence on gene expression during osteogenic differentiation of human MSC. Plasma-activated HA tends to enhance this process by systemically deregulating multiple genes. In addition, the majority of these deregulated genes had been appropriately translated, as confirmed by ELISA protein quantification. Lastly, alizarin red staining showed that plasma-activated HA is capable of improving mineralization for up to 3 weeks of in vitro culture. It was concluded from this study that atmospheric pressure plasma is a potent tool for modifying the biological function of a material without causing thermal damage, such that adhesion molecules and drugs might be deposited on the original coating to improve performance.

MR elastography monitoring of tissue-engineered constructs

The objective of tissue engineering (TE) is to create functional replacements for various tissues; the mechanical properties of these engineered constructs are critical to their function. Several techniques have been developed for the measurement of the mechanical properties of tissues and organs; however, current methods are destructive. The field of TE will benefit immensely if biomechanical models developed by these techniques could be combined with existing imaging modalities to enable noninvasive, dynamic assessment of mechanical properties during tissue growth. Specifically, MR elastography (MRE), which is based on the synchronization of a mechanical actuator with a phase contrast imaging pulse sequence, has the capacity to measure tissue strain generated by sonic cyclic displacement. The captured displacement is presented in shear wave images from which the complex shear moduli can be extracted or simplified by a direct measure, termed the shear stiffness. MRE has been extended to the microscopic scale, combining clinical MRE with high-field magnets, stronger magnetic field gradients and smaller, more sensitive, radiofrequency coils, enabling the interrogation of smaller samples, such as tissue-engineered constructs. The following topics are presented in this article: (i) current mechanical measurement techniques and their limitations in TE; (ii) a description of the MRE system, MRE theory and how it can be applied for the measurement of mechanical properties of tissue-engineered constructs; (iii) a summary of in vitro MRE work for the monitoring of osteogenic and adipogenic tissues originating from human adult mesenchymal stem cells (MSCs); (iv) preliminary in vivo studies of MRE of tissues originating from mouse MSCs implanted subcutaneously in immunodeficient mice with an emphasis on in vivo MRE challenges; (v) future directions to resolve current issues with in vivo MRE in the context of how to improve the future role of MRE in TE.

Does complement play a role in bone development and regeneration?

The skeletal and the immune system are not two independent systems, rather, there are multifaceted and complex interactions between the different cell types of both systems and there are several shared cytokines. As a part of the innate immunity, the complement system was found to be an important link between bone and immunity. Complement proteins appear to be involved in bone development and homeostasis, and specifically influence osteoblast and osteoclast activity. This review describes the complex mutual regulation of the two systems, and indicates some of the negative side effects as a result of inappropriate or excessive complement activation.

Osteogenic differentiation of stem cells alters vitamin D receptor expression

Pluripotent and multipotent stem cells adopt an osteoblastic phenotype when cultured in environments that enhance their osteogenic potential. Embryonic stem cells differentiated as embryoid bodies (EBs) in osteogenic medium containing β-glycerophosphate exhibit increased expression of bone markers, indicating that cells are osteoblastic. Interestingly, 1α,25-dihydroxyvitaminD3 (1,25D) enhances the osteogenic phenotype not just in EBs but also in multipotent adult mesenchymal stem cells (MSCs). 1,25D acts on osteoblasts via classical vitamin D receptors (VDR) and via a membrane 1,25D-binding protein [protein disulfide isomerase family A, member 3 (PDIA3)], which activates protein kinase C-signaling. The aims of this study were to determine whether these receptors are regulated during osteogenic differentiation of stem cells and if stem cells and differentiated progeny are responsive to 1,25D. mRNA and protein levels for VDR, PDIA3, and osteoblast-associated proteins were measured in undifferentiated cells and in cells treated with osteogenic medium. Mouse EBs expressed both VDR and PDIA3, but VDR increased as cells underwent osteogenic differentiation. Human MSCs expressed Pdia3 at constant levels throughout differentiation, but VDR increased in cells treated with osteogenic medium. These results suggest that both 1,25D signaling mechanisms are important, with PDIA3 playing a greater role during early events and VDR playing a greater role in later stages of differentiation. Understanding these coordinated events provide a powerful tool to control pluripotent and multipotent stem cell differentiation through induction medium.

Bone regeneration by stem cell and tissue engineering in oral and maxillofacial region

Clinical imperatives for the reconstruction of jaw bone defects or resorbed alveolar ridge require new therapies or procedures instead of autologous/allogeneic bone grafts. Regenerative medicine, based on stem cell science and tissue engineering technology, is considered as an ideal alternative strategy for bone regeneration. In this paper, we review the current choices of cell source and strategies on directing the osteogenic differentiation of stem cells. The preclinical animal models for bone regeneration and the key translational points to clinical success in oral and maxillofacial region are also discussed. We propose comprehensive strategies based on stem cell and tissue engineering researches, allowing for clinical application in oral and maxillofacial region.

Simvastatin promotes osteogenic differentiation of mouse embryonic stem cells via canonical Wnt/β-catenin signaling

Simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, has been known to reduce cholesterol biosynthesis. However, recent studies demonstrate that simvastatin shows diverse cholesterol-independent functions including cellular differentiation. In this study, we investigated the stimulatory effect of simvastatin on the osteogenic differentiation of mouse embryonic stem cells (ESCs). The osteogenic effect of simvastatin was observed at relatively low doses (ranging from 1 nM to 200 nM). Incubation of ESCs in simvastatin-supplemented osteogenic medium significantly increased alkaline phosphatase (ALP) activity at day 7. The matrix mineralization was also augmented and demonstrated pivotal levels after 14 days incubation of simvastatin. Osteogenic differentiation of ESCs by simvastatin was determined by upregulation of the mRNA expression of runtrelated gene 2 (Runx2), osterix (OSX), and osteocalcin (OCN) as osteogenic transcription factors. Moreover, the increased protein expression of OCN, osteopontin (OPN), and collagen type I (Coll I) was assessed using Western blot analysis and immunocytochemistry. However, the blockage of canonical Wnt signaling by DKK-1 downregulated simvastatin-induced ALP activity and the mRNA expression of each osteogenic transcription factor. Furthermore, the β-catenin specific siRNA transfection decreased the protein levels of OCN, OPN, and Coll I. Collectively, these findings suggest that simvastatin enhances the differentiation of ESCs toward osteogenic lineage through activation of canonical Wnt/β-catenin signaling.

Concise review: induced pluripotent stem cells and lineage reprogramming: prospects for bone regeneration

Bone tissue for transplantation therapies is in high demand in clinics. Osteodegenerative diseases, in particular, osteoporosis and osteoarthritis, represent serious public health issues affecting a respectable proportion of the elderly population. Furthermore, congenital indispositions from the spectrum of craniofacial malformations such as cleft palates and systemic disorders including osteogenesis imperfecta are further increasing the need for bone tissue. Additionally, the reconstruction of fractured bone elements after accidents and the consumption of bone parts during surgical tumor excisions represent frequent clinical situations with deficient availability of healthy bone tissue for therapeutic transplantations. Epigenetic reprogramming represents a powerful technology for the generation of healthy patient-specific cells to replace or repair diseased or damaged tissue. The recent generation of induced pluripotent stem cells (iPSCs) is probably the most promising among these approaches dominating the literature of current stem cell research. It allows the generation of pluripotent stem cells from adult human skin cells from which potentially all cell types of the human body could be obtained. Another technique to produce clinically interesting cell types is direct lineage reprogramming (LR) with the additional advantage that it can be applied directly in vivo to reconstitute a damaged organ. Here, we want to present the two technologies of iPSCs and LR, to outline the current states of research, and to discuss possible strategies for their implementation in bone regeneration.

Embryonic stem cells in scaffold-free three-dimensional cell culture: osteogenic differentiation and bone generation

Extracorporeal formation of mineralized bone-like tissue is still an unsolved challenge in tissue engineering. Embryonic stem cells may open up new therapeutic options for the future and should be an interesting model for the analysis of fetal organogenesis. Here we describe a technique for culturing embryonic stem cells (ESCs) in the absence of artificial scaffolds which generated mineralized miromasses. Embryonic stem cells were harvested and osteogenic differentiation was stimulated by the addition of dexamethasone, ascorbic acid, and ß-glycerolphosphate (DAG). After three days of cultivation microspheres were formed. These spherical three-dimensional cell units showed a peripheral zone consisting of densely packed cell layers surrounded by minerals that were embedded in the extracellular matrix. Alizarine red staining confirmed evidence of mineralization after 10 days of DAG stimulation in the stimulated but not in the control group. Transmission electron microscopy demonstrated scorching crystallites and collagenous fibrils as early indication of bone formation. These extracellular structures resembled hydroxyl apatite-like crystals as demonstrated by distinct diffraction patterns using electron diffraction analysis. The micromass culture technique is an appropriate model to form three-dimensional bone-like micro-units without the need for an underlying scaffold. Further studies will have to show whether the technique is applicable also to pluripotent stem cells of different origin.

Response of human mesenchymal stem cells (hMSCs) to the topographic variation of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) films

The influence of the topographic morphology of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) films on human mesenchymal stem cells (hMSCs) was investigated in this study. PHBHHx films with various surface characteristics were prepared by compression-molding, solvent-casting and electrospinning. The adhesion, proliferation and differentiation behaviors of hMSCs were significantly modulated by the surface characteristics of these films. HMSCs could aggregate and form cellular clusters on the cast PHBHHx films, and the time to form cellular aggregates increased as the surface roughness increased. The aggregated hMSCs on the cast films kept their original surface markers and presented much higher viability during the regular culture and lower differentiation ability upon osteogenic induction than the spread cells on the compression-molded films and TCPS. HMSCs spread well and showed a specific orientation on the surface of the random electrospun fibrous films, they were not able to migrate into the interior of electrospun fibrous films, and they revealed the highest viability during the regular culture but a lower differentiation activity upon osteogenic induction. The electrospun fibrous PHBHHx films could serve as a suitable substrate for large quantity culturing of hMSCs when undifferentiated hMSCs are desired.

Encapsulation of bone morphogenic protein-2 with Cbfa1-overexpressing osteogenic cells derived from human embryonic stem cells in hydrogel accelerates bone tissue regeneration

Bone tissue defects caused by trauma and disease are significant problems in orthopedic surgery. Human embryonic stem cells (hESCs) hold great promise for the treatment of bone tissue disease in regenerative medicine. In this study, we have established an effective method for the differentiation of osteogenic cells derived from hESCs using a lentiviral vector containing the transcription factor Cbfa1. Differentiation was initiated in embryoid body formation of Cbfa1-expressing hESCs, resulting in a highly purified population of osteogenic cells based on flow cytometric analysis. These cells also showed characteristics of osteogenic cells in vitro, as determined by reverse-transcription (RT)-polymerase chain reaction and immunocytochemistry using osteoblast-specific markers. We also evaluated the regenerative potential of Cbfa1-expressing cells derived from hESCs (hESC-CECs) compared with hESCs and the osteogenic effects of bone morphogenic protein-2 (BMP2) encapsulated in thermoreversible hydrogel in vivo. hESC-CECs were embedded in hydrogel constructs enriched with BMP2 to promote bone regeneration. We observed prominent mineralization and the formation of nodule-like structures using von Kossa and alizarin red S staining. In addition, the expression patterns of osteoblast-specific genes were verified by RT-polymerase chain reaction, and immunohistochemical analysis revealed that collagen type 1 and Cbfa1 were highly expressed in hESC-CECs compared with other cell types. Taken together, our results suggest that encapsulation of hESC-CECs with BMP2 in hydrogel constructs appears to be a promising method to enhance the in vitro osteoblastic differentiation and in vivo osteogenic activity of hESC-CECs.

Comparison of ectopic bone formation of embryonic stem cells and cord blood stem cells in vivo

Cell-based reconstruction therapies promise new therapeutic opportunities for bone regeneration. Unrestricted somatic stem cells (USSC) from cord blood and embryonic stem cells (ESCs) can be differentiated into osteogenic cells. The purpose of this in vivo study was to compare their ability to induce ectopic bone formation in vivo. Human USSCs and murine ESCs were cultured as both monolayer cultures and micromasses and seeded on insoluble collagenous bone matrix (ICBM). One week and 1, 2, and 3 months after implanting the constructs in immune-deficient rats, computed tomography scans were performed to detect any calcification. Subsequently, the implanted constructs were examined histologically. The radiological examination showed a steep increase in the mineralized bone-like tissue in the USSC groups. This increase can be considered as statistically significant compared to the basic value. Moreover, the volume and the calcium portion measured by computed tomography scans were about 10 times higher than in the ESC group. The volume of mineralization in the ESC group increased to a much smaller extent over the course of time, and the control group (ICBM without cells) showed almost no alterations during the study. The histological examinations parallel the radiological findings. Cord blood stem cells in combination with ICBM-induced ectopic bone formation in vivo are stronger than ESCs.

Design of nano- and microfiber combined scaffolds by electrospinning of collagen onto starch-based fiber meshes: a man-made equivalent of natural extracellular matrix

Mimicking the structural organization and biologic function of natural extracellular matrix has been one of the main goals of tissue engineering. Nevertheless, the majority of scaffolding materials for bone regeneration highlights biochemical functionality in detriment of mechanical properties. In this work we present a rather innovative construct that combines in the same structure electrospun type I collagen nanofibers with starch-based microfibers. These combined structures were obtained by a two-step methodology and structurally consist in a type I collagen nano-network incorporated on a macro starch-based support. The morphology of the developed structures was assessed by several microscopy techniques and the collagenous nature of the nano-network was confirmed by immunohistochemistry. In addition, and especially regarding the requirements of large bone defects, we also successfully introduced the concept of layer by layer, as a way to produce thicker structures. In an attempt to recreate bone microenvironment, the design and biochemical composition of the combined structures also envisioned bone-forming cells and endothelial cells (ECs). The inclusion of a type I collagen nano-network induced a stretched morphology and improved the metabolic activity of osteoblasts. Regarding ECs, the presence of type I collagen on the combined structures provided adhesive support and obviated the need of precoating with fibronectin. It was also importantly observed that ECs on the nano-network organized into circular structures, a three-dimensional arrangement distinct from that observed for osteoblasts and resembling the microcappillary-like organizations formed during angiogenesis. By providing simultaneously physical and chemical cues for cells, the herein-proposed combined structures hold a great potential in bone regeneration as a man-made equivalent of extracellular matrix.

Stem cells in bone tissue engineering

Bone tissue engineering has been one of the most promising areas of research, providing a potential clinical application to cure bone defects. Recently, various stem cells including embryonic stem cells (ESCs), bone marrow-derived mesenchymal stem cells (BM-MSCs), umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs), adipose tissue-derived stem cells (ADSCs), muscle-derived stem cells (MDSCs) and dental pulp stem cells (DPSCs) have received extensive attention in the field of bone tissue engineering due to their distinct biological capability to differentiate into osteogenic lineages. The application of these stem cells to bone tissue engineering requires inducing in vitro differentiation of these cells into bone forming cells, osteoblasts. For this purpose, efficient in vitro differentiation towards osteogenic lineage requires the development of well-defined and proficient protocols. This would reduce the likelihood of spontaneous differentiation into divergent lineages and increase the available cell source for application to bone tissue engineering therapies. This review provides a critical examination of the various experimental strategies that could be used to direct the differentiation of ESC, BM-MSC, UCB-MSC, ADSC, MDSC and DPSC towards osteogenic lineages and their potential applications in tissue engineering, particularly in the regeneration of bone.

Non-resorbing osteoclasts induce migration and osteogenic differentiation of mesenchymal stem cells

Osteoclast activity has traditionally been regarded as restricted to bone resorption but there is some evidence that also non-resorbing osteoclasts might influence osteoblast activity. The aim of the present study was to further investigate the hypothesis of an anabolic function of non-resorbing osteoclasts by investigating their capability to recruit mesenchymal stem cells (MSC) and to provoke their differentiation toward the osteogenic lineage. Bone-marrow-derived human MSC were exposed to conditioned media (CM) derived from non-resorbing osteoclast cultures, which were generated from human peripheral blood monocytes. Osteogenic marker genes (transcription factor Runx2, bone sialoprotein, alkaline phosphatase (AP), and osteopontin) were significantly increased. Osteogenic differentiation (OD) was also proved by von Kossa and AP staining occurred in the same range as in MSC cultures stimulated with osteogenic supplements. Chemotactic responses of MSC were measured with a modified Boyden chamber assay. CM from osteoclast cultures induced a strong migratory response in MSC, which was greatly reduced in the presence of an anti-human platelet-derived growth factor (PDGF) receptor beta antibody. Correspondingly, significantly increased PDGF-BB concentrations were measured in the CM using a PDGF-BB immunoassay. CM derived from mononuclear cell cultures did not provoke MSC differentiation and had a significantly lower migratory effect on MSC suggesting that the effects were specifically mediated by osteoclasts. In conclusion, it can be suggested that human non-resorbing osteoclasts induce migration and OD of MSC. While effects on MSC migration might be mainly due to PDGF-BB, the factors inducing OD remain to be elucidated.

Effects of TGF-beta3 and preculture period of osteogenic cells on the chondrogenic differentiation of rabbit marrow mesenchymal stem cells encapsulated in a bilayered hydrogel composite

In this work, injectable, biodegradable hydrogel composites of crosslinked oligo(poly(ethylene glycol) fumarate) and gelatin microparticles (MPs) were used to fabricate a bilayered osteochondral construct. Rabbit marrow mesenchymal stem cells (MSCs) were encapsulated with transforming growth factor-beta3 (TGF-beta3)-loaded MPs in the chondrogenic layer and cocultured with cells of different periods of osteogenic preculture (0, 3, 6 and 12 days) in the osteogenic layer to investigate the effects of TGF-beta3 delivery and coculture on the proliferation and differentiation of cells in both layers. The results showed that, in the chondrogenic layer, TGF-beta3 significantly stimulated chondrogenic differentiation of MSCs. In addition, cells of various osteogenic preculture periods in the osteogenic layer, along with TGF-beta3, enhanced gene expression for MSC chondrogenic markers to different extents. In the osteogenic layer, cells maintained their alkaline phosphatase activity during the coculture; however, mineralization was delayed by the presence of TGF-beta3. Overall, this study demonstrated the fabrication of bilayered hydrogel composites which mimic the structure and function of osteochondral tissue, along with the application of these composites as cell and growth factor carriers, while illustrating that encapsulated cells of different degrees of osteogenic differentiation can significantly influence the chondrogenic differentiation of cocultured progenitor cells in both the presence and absence of chondrogenic growth factors.

Expansion and characterization of human embryonic stem cell-derived osteoblast-like cells

Human embryonic stem cells (hESCs) have the potential to serve as a repository of cells for the replacement of damaged or diseased tissues and organs. However, to use hESCs in clinically relevant scenarios, a large number of cells are likely to be required. The aim of this study was to demonstrate an alternative cell culture method to increase the quantity of osteoblast-like cells directly derived from hESCs (hESCs-OS). Undifferentiated hESCs were directly cultivated and serially passaged in osteogenic medium (hESC-OS), and exhibited similar expression patterns of osteoblast-related genes to osteoblast-like cells derived from mesenchymal stem cells derived from hESCs (hESCs-MSCs-OS) and human bone marrow stromal cells (hBMSCs-OS). In comparison to hESCs-MSCs-OS, the hESCs-OS required a shorter expansion time to generate a homogenous population of osteoblast-like cells that did not contain contaminating undifferentiated hESCs. Identification of human specific nuclear antigen (HuNu) in the newly formed bone in calvarial defects verified the role of the transplanted hESCs-OS as active bone forming cells in vivo. Taken together, this study suggests that osteoblast-like cells directly derived from hESCs have the potential to serve as an alternative source of osteoprogenitors for bone tissue engineering strategies.

Engineering embryonic stem-cell aggregation allows an enhanced osteogenic differentiation in vitro

Pluripotent embryonic stem (ES) cells hold great promise for the field of tissue engineering, with numerous studies investigating differentiation into various cell types including cardiomyocytes, chondrocytes, and osteoblasts. Previous studies have detailed osteogenic differentiation via dissociated embryoid body (EB) culture in osteoinductive media comprising of ascorbic acid, beta-glycerophosphate, and dexamethasone. It is hoped that these osteogenic cultures will have clinical application in bone tissue repair and regeneration and pharmacological testing. However, differentiation remains highly inefficient and generates heterogeneous populations. We have previously reported an engineered three-dimensional culture system for controlled ES cell-ES cell interaction via the avidin-biotin binding complex. Here we investigate the effect of such engineering on ES cell differentiation. Engineered EBs exhibit enhanced osteogenic differentiation assessed by cadherin-11, Runx2, and osteopontin expression, alkaline phosphatase activity, and bone nodule formation. Results show that cultures produced from intact EBs aggregated for 3 days generated the greatest levels of osteogenic differentiation when cultured in osteoinductive media. However, when cultured in control media, only engineered samples appeared to exhibit bone nodule formation. In addition, polymerase chain reaction analysis revealed a decrease in endoderm and ectoderm expression within engineered samples. This suggests that engineered ES cell aggregation has increased mesoderm homogeneity, contributing to enhanced osteogenic differentiation.

Skeletal tissue engineering using embryonic stem cells

Various cell types have been investigated as candidate cell sources for cartilage and bone tissue engineering. In this review, we focused on chondrogenic and osteogenic differentiation of mouse and human embryonic stem cells (ESCs) and their potential in cartilage and bone tissue engineering. A decade ago, mouse ESCs were first used as a model to study cartilage and bone development and essential genes, factors and conditions for chondrogenesis and osteogenesis were unravelled. This knowledge, combined with data from the differentiation of adult stem cells, led to successful chondrogenic and osteogenic differentiation of mouse ESCs and later also human ESCs. Next, researchers focused on the use of ESCs for skeletal tissue engineering. Cartilage and bone tissue was formed in vivo using ESCs. However, the amount, homogeneity and stability of the cartilage and bone formed were still insufficient for clinical application. The current protocols require improvement not only in differentiation efficiency but also in ESC-specific hurdles, such as tumourigenicity and immunorejection. In addition, some of the general tissue engineering challenges, such as cell seeding and nutrient limitation in larger constructs, will also apply for ESCs. In conclusion, there are still many challenges, but there is potential for ESCs in skeletal tissue engineering.

Ultrastructural observation on cells meeting the histological criteria for preosteoblasts--a study in the mouse tibial metaphysis

Preosteoblasts are currently defined as the precursors of mature osteoblasts. These cells are morphologically diverse and may represent a continuum during osteoblast differentiation. We have attempted to categorize the different preosteoblastic phenotypes in vivo by examining bone cells expressing the runt-related transcription factor 2, alkaline phosphatase and BrdU incorporation - histological traits of a preosteoblast - under transmission electron microscopy (TEM). TEM observations demonstrated, at least, in part two preosteoblastic subtypes: (i) a cell rich in cisterns of rough endoplasmic reticulum (rER) with vesicles and vacuoles and (ii) a subtype featuring extended cytoplasmic processes that connect with distant cells, with a small amount of scattered cisterns of rER and with many vesicles and vacuoles. ER-rich cells, whose cellular machinery is similar to that of an osteoblast, were often seen adjacent to mature osteoblasts, and therefore, may be ready for terminal differentiation. In contrast, ER-poor and vesicle-rich cells extended their cytoplasmic processes to mature osteoblasts and, frequently, to bone-resorbing osteoclasts. The abundant vesicles and vacuoles identified in this cell type indicate that this cell is involved in vesicular transport rather than matrix synthesis activity. In summary, our study verified the morphological diversity and the ultrastructural properties of osteoblastic cells in vivo.

Regulation of embryonic stem cell self-renewal and differentiation by TGF-beta family signaling

Embryonic stem (ES) cells are characterized by their ability to indefinitely self-renew and potential to differentiate into all the cell lineages of the body. ES cells are considered to have potential applications in regenerative medicine. In particular, the emergence of an ES cell analogue - induced pluripotent stem (iPS) cells via somatic cell reprogramming by co-expressing a limited number of critical stemness-related transcriptional factors has solved the problem of obtaining patient-specific pluripotent cells, encouraging researchers to develop more specific and functional cell lineages from ES or iPS cells for broad therapeutic applications. ES cell fate choice is delicately controlled by a core transcriptional network, epigenetic modification profiles and complex signaling cascades both intrinsically and extrinsically. Of these signals, transforming growth factor beta (TGF-beta) family members, including TGF-beta, bone morphogenetic protein (BMP), Activin and Nodal, have been reported to influence cell self-renewal and a broad spectrum of lineage differentiation in ES cells, in accordance with the key roles of TGF-beta family signaling in early embryo development. In this review, the roles of TGF-beta family signals in coordinating ES cell fate determination are summarized.

Biocompatibility of osteogenic predifferentiated human cord blood stem cells with biomaterials and the influence of the biomaterial on the process of differentiation

Modern cell-based bone reconstruction therapies offer new therapeutic opportunities and tissue engineering represents a more biological-oriented approach to heal bone defects of the skeleton. Human unrestricted somatic stem cells (USSCs) derived form umbilical cord blood offer new promising aspects e.g., can differentiate into osteogenetic cells. Furthermore these cells have fewer ethical and legal restrictions compared to embryonic stem cells (ESCs). The purpose of this study was to evaluate the compatibility of osteogenic pre-differentiated USSCs with various biomaterials and to address the question, whether biomaterials influence the process of differentiation of the USSCs. After osteogenic differentiation with DAG USSCs were cultivated with various biomaterials. To asses the biocompatibility of USSCs the attachment and the proliferation of the cells on the biomaterial were measured by a CyQUANT(®) assay, the morphology was analyzed by scanning electron microscopy and the influence of the gene expression was analyzed by real time PCR. Our results provide evidence that insoluble collagenous bone matrix followed by β-tricalciumphosphate is highly suitable for bone tissue engineering regarding cell attachment and proliferation. The gene expression analysis indicates that biomaterials influence the gene expression of USSCs. These results are in concordance with our previous study with ESCs.

A novel collagen scaffold supports human osteogenesis--applications for bone tissue engineering

Collagen glycosaminoglycan (CG) scaffolds have been clinically approved as an application for skin regeneration. The goal of this study has been to examine whether a CG scaffold is a suitable biomaterial for generating human bone tissue. Specifically, we have asked the following questions: (1) can the scaffold support human osteoblast growth and differentiation and (2) how might recombinant human transforming growth factor-beta (TGF-beta(1)) enhance long-term in vitro bone formation? We show human osteoblast attachment, infiltration and uniform distribution throughout the construct, reaching the centre within 14 days of seeding. We have identified the fully differentiated osteoblast phenotype categorised by the temporal expression of alkaline phosphatase, collagen type 1, osteonectin, bone sialo protein, biglycan and osteocalcin. Mineralised bone formation has been identified at 35 days post-seeding by using von Kossa and Alizarin S Red staining. Both gene expression and mineral staining suggest the benefit of introducing an initial high treatment of TGF-beta(1) (10 ng/ml) followed by a low continuous treatment (0.2 ng/ml) to enhance human osteogenesis on the scaffold. Osteogenesis coincides with a reduction in scaffold size and shape (up to 70% that of original). A notable finding is core degradation at the centre of the tissue-engineered construct after 49 days of culture. This is not observed at earlier time points. Therefore, a maximum of 35 days in culture is appropriate for in vitro studies of these scaffolds. We conclude that the CG scaffold shows excellent potential as a biomaterial for human bone tissue engineering.

Type I collagen gel in seeding medium improves murine mesencymal stem cell loading onto the scaffold, increases their subsequent proliferation, and enhances culture mineralization

Collagen I as a major organic component of bone matrix may be important for establishment and maintenance of mesenchymal stem cells (MSCs) in osteogenic 3D culture. To explore this subject, murine marrow-derived MSCs were seeded onto hybrid scaffolds of alginate/gelatin/beta-tricalcium phosphate in a medium either with or without collagen I gel. The cultures were then provided with osteogenic medium and incubated for three weeks during which loading efficiency, cell proliferation and the culture mineralization were quantified and statistically compared. According to the findings, in culture with collagen, although about 60% of the cells left the scaffolds, the remaining cells, however, proliferated extensively with a population doubling number (PDN) equivalent to 2.46 +/- 0.31 and organized as cell aggregations that were heavily mineralized (calcium concentration = 1.017 +/- 0.141 mM per scaffold), whereas in the culture without collagen, about 75% of the cells left the scaffolds, less cell proliferation occurred (PDN = 1.48 +/- 0.29) and no cell aggregation was observed. The calcium concentration in this culture was 0.185 +/- 0.029 mM per scaffold. All these differences were statistically significant (p < 0.001). Taken together, these data suggested that using the collagen I in seeding medium could help mMSCs loading into the scaffold, enhance their subsequent proliferation, and increase calcium deposition in 3D culture system.

Mesenchymal stem cell modification of endothelial matrix regulates their vascular differentiation

Mesenchymal stem cells (MSCs) respond to a variety of differentiation signal provided by their local environments. A large portion of these signals originate from the extracellular matrix (ECM). At the same time, MSCs secrete various matrix-altering agents, including proteases, that alter ECM-encoded differentiation signals. Here we investigated the interactions between MSC and ECM produced by endothelial cells (EC-matrix), focusing not only on the differentiation signals provided by EC-matrix, but also on MSC-alteration of these signals and the resultant affects on MSC differentiation. MSCs were cultured on EC-matrix modified in one of three distinct ways. First, MSCs cultured on native EC-matrix underwent endothelial cell (EC) differentiation early during the culture period and smooth muscle cell (SMC) differentiation at later time points. Second, MSCs cultured on crosslinked EC-matrix, which is resistant to MSC modification, differentiated towards an EC lineage only. Third, MSCs cultured on EC-matrix pre-modified by MSCs underwent SMC-differentiation only. These MSC-induced matrix alterations were found to deplete the factors responsible for EC-differentiation, yet activate the SMC-differentiation factors. In conclusion, our results demonstrate that the EC-matrix contains factors that support MSC differentiation into both ECs and SMCs, and that these factors are modified by MSC-secreted agents. By analyzing the framework by which EC-matrix regulates differentiation in MSCs, we have uncovered evidence of a feedback system in which MSCs are able to alter the very matrix signals acting upon them.

Concurrent differentiation of marrow stromal cells to osteogenic and vasculogenic lineages

When rat bone marrow stromal (BMS) cells were seeded on aligned type I collagen scaffolds and cultured in osteogenic media, they underwent simultaneous maturation and differentiation into osteogenic and vascular cell lineages. In addition, these cells produced mineralized matricellular deposits. BMS cells were seeded in Petri dish or the collagen scaffold, cultured in osteogenic media for 3, 6, and 9 d and subsequently processed for immunohistochemical and cytochemical analysis. Immunolocalization of lineage-specific proteins were visualized using confocal microscopy and mRNA transcript analysis was performed by real-time quantitative polymerase chain reaction (RT-qPCR). The alkaline phosphatase activity and calcium content significantly increased over the observed period of time in an osteogenic medium. Sheets of abundant Pecam (CD31), Flk-1 (VEGFR-2), tomato lectin (TL/LEL), and alpha-smooth muscle actin (alpha-SMA) positive cells were observed in the collagen scaffolds. Nascent capillary-like vessels were also seen amidst the osteoblasts in osteogenic culture, augmenting the maturation and differentiation of BMS cells into osteoblasts. In our in vitro study, concurrent differentiation of BMS cells, a heterogeneous cell population with multilineage differentiation potential, to osteogenic and vascular lineages demonstrated that the substrates (three-dimensional (3-D), collagen type I, aligned fibrils) had a profound effect on guiding the differentiation pathway of BMS cells.

Phenotypic characterization, osteoblastic differentiation, and bone regeneration capacity of human embryonic stem cell-derived mesenchymal stem cells

To enhance the understanding of differentiation patterns and bone formation capacity of hESCs, we determined (1) the temporal pattern of osteoblastic differentiation of human embryonic stem cell-derived mesenchymal stem cells (hESC-MSCs), (2) the influence of a three-dimensional matrix on the osteogenic differentiation of hESC-MSCs in long-term culture, and (3) the bone-forming capacity of osteoblast-like cells derived from hESC-MSCs in calvarial defects. Incubation of hESC-MSCs in osteogenic medium induced osteoblastic differentiation of hESC-MSCs into mature osteoblasts in a similar chronological pattern to human bone marrow stromal cells and primary osteoblasts. Osteogenic differentiation was enhanced by culturing the cells on three-dimensional collagen scaffolds. Fluorescent-activated cell sorting of alkaline phosphatase expressing cells was used to obtain an enriched osteogenic cell population for in vivo transplantation. The identification of green fluorescence protein and expression of human-specific nuclear antigen in osteocytes in newly formed bone verified the role of transplanted human cells in the bone regeneration process. The current cell culture model and osteogenic cell enrichment method could provide large numbers of osteoprogenitor cells for analysis of differentiation patterns and cell transplantation to regenerate skeletal defects.

Osteogenic differentiation of bone marrow stromal cells induced by coculture with chondrocytes encapsulated in three-dimensional matrices

Endochondral ossification implicates chondrocyte signaling as an important factor in directing the osteogenic differentiation of mesenchymal stem cells in vivo. In this study, the osteoinductive capabilities of articular chondrocytes suspended in alginate hydrogels were analyzed via coculture with bone marrow stromal cells (BMSCs). In particular, the effect of chondrocyte coculture time on the mechanism underlying this osteogenic induction was examined. Chondrocytes were suspended in alginate beads and cultured above BMSCs in monolayer. Beads containing chondrocytes were removed after 1, 10, or 21 days of coculture. Quantitative reverse transcriptase polymerase chain reaction was used to assess the expression of alkaline phosphatase, bone morphogenetic protein-2, and osteocalcin by BMSCs after days 1, 8, 14, and 21. Calcium deposition was also assayed to characterize the extent of mineralization within cultures. Results indicate that osteogenic differentiation of BMSCs is initiated upon brief exposure to chondrocyte signaling, but requires continued exposure in order to progress fully and maintain an osteoblastic phenotype.

In vitro generation of an osteochondral construct using injectable hydrogel composites encapsulating rabbit marrow mesenchymal stem cells

Injectable, biodegradable hydrogel composites of crosslinked oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs) were utilized to fabricate a bilayered osteochondral construct consisting of a chondrogenic layer and an osteogenic layer, and to investigate the differentiation of rabbit marrow mesenchymal stem cells (MSCs) encapsulated in both layers in vitro. The results showed that MSCs in the chondrogenic layer were able to undergo chondrogenic differentiation, especially in the presence of TGF-beta1-loaded MPs. In the osteogenic layer, cells maintained their osteoblastic phenotype. Although calcium deposition in the osteogenic layer was limited, cells in the osteogenic layer significantly enhanced chondrogenic differentiation of MSCs in the chondrogenic layer. The greatest effect was observed when MSCs were encapsulated with TGF-beta1-loaded MPs and cultured with osteogenic cells in the bilayered constructs. Overall, this study demonstrates the fabrication of bilayered hydrogel composites that mimic the structure and function of osteochondral tissue, along with the application of these composites as cell and growth factor carriers.

Immunohistochemical characterization of nanocrystalline hydroxyapatite silica gel (NanoBone(r)) osteogenesis: a study on biopsies from human jaws

OBJECTIVES

Bone substitute biomaterials may be osteogenic, osteoconductive or osteoinductive. To test for these probable characteristics in a new nanoporous grafting material consisting of nanocrystalline hydroxyapatite embedded in a porous silica gel matrix (NanoBone(s)), applied in humans, we studied biopsies from 12 patients before dental implantation following various orofacial augmentation techniques with healing times of between 3.5 and 12 months.

MATERIAL AND METHODS

Sections from decalcified specimens were investigated using histology, histochemistry [periodic acid Schiff, alcian blue staining and tartrate-resistant acid phosphatase (TRAP)] and immunohistochemistry, with markers for osteogenesis, bone remodelling, resorption and vessel walls (alkaline phosphatase, bone morphogenetic protein-2, collagen type I, ED1, osteocalcin, osteopontin, runx2 and Von-Willebrand factor).

RESULTS

Histologically, four specific stages of graft transformation into lamellar bone could be characterized. During early stages of healing, bone matrix proteins were absorbed by NanoBone(s) granules, forming a proteinaceous matrix, which was invaded by small vessels and cells. We assume that the deposition of these molecules promotes early osteogenesis in and around NanoBone(s) and supports the concomitant degradation probably by osteoclast-like cells. TRAP-positive osteoclast-like cells were localized directly on the granular surfaces. Runx2-immunoreactive pre-osteoblasts, which are probably involved in direct osteogenesis forming woven bone that is later transformed into lamellar bone, were attracted. Graft resorption and bone apposition around the graft granules appear concomitantly.

CONCLUSIONS

We postulate that NanoBone(s) has osteoconductive and biomimetic properties and is integrated into the host's physiological bone turnover at a very early stage.

Comparative transcriptional analysis of embryoid body versus two-dimensional differentiation of murine embryonic stem cells

Understanding the process of ex vivo embryonic stem (ES) cell differentiation is important for generating higher yields of desirable cell types or lineages and for understanding fundamental aspects of ES differentiation. We used DNA microarray analysis to investigate the differentiation of mouse ES cells cultured under three differentiation conditions. Embryoid body (EB) formation was compared to differentiation on surfaces coated with either gelatin (GEL) or matrigel (MAT). Based on the transcriptional patterns of a list of literature-based "stemness" genes, ES cell differentiation on the two coated surfaces appeared similar but not identical to EB differentiation. A notable difference was the GEL and MAT upregulation but EB downregulation of nine such stemness genes, which are related to cell adhesion and epithelial differentiation. Further, GEL and MAT differentiation showed higher expression of bone formation-related genes (Spp1, Csf1, Gsn, Bmp8b, Crlf1). Gene ontology analysis shows an increase in the expression of genes related to migration and cell structure in all three conditions. Overall, GEL and MAT conditions resulted in a more similar to each other transcriptional profile than to the EB condition, and such differences are apparently related to higher nutrient and metabolite gradients and limitations in the EB versus the GEL or MAT cultures.

Cultivation of Human Embryonic Stem Cells Without the Embryoid Body Step Enhances Osteogenesis In Vitro

Osteogenic cultures of embryonic stem cells (ESCs) are predominately derived from three-dimensional cell spheroids called embryoid bodies (EBs). An alternative method that has been attempted and merits further attention avoids EBs through the immediate separation of ESC colonies into single cells. However, this method has not been well characterized and the effect of omitting the EB step is unknown. Herein, we report that culturing human embryonic stem cells (hESCs) without the EB stage leads to a sevenfold greater number of osteogenic cells and to spontaneous bone nodule formation after 10-12 days. In contrast, when hESCs were differentiated as EBs for 5 days followed by plating of single cells, bone nodules formed after 4 weeks only in the presence of dexamethasone. Furthermore, regardless of the inclusion of EBs, bone matrix formed, including cement line matrix and mineralized collagen, which displayed apatitic mineral (PO4) with calcium-to-phosphorous ratios similar to those of hydroxyapatite and human bone. Together these results demonstrate that culturing hESCs without an EB step can be used to derive large quantities of functional osteogenic cells for bone tissue engineering.

Gene expression analysis in osteoblastic differentiation from peripheral blood mesenchymal stem cells

MSCs are known to have an extensive proliferative potential and ability to differentiate in various cell types. Osteoblastic differentiation from mesenchymal progenitor cells is an important step of bone formation, though the pattern of gene expression during differentiation is not yet well understood. Here, to investigate the possibility to obtain a model for in vitro bone differentiation using mesenchymal stem cells (hMSCs) from human subjects non-invasively, we developed a method to obtain hMSCs-like cells from peripheral blood by a two step method that included an enrichment of mononuclear cells followed by depletion of unwanted cells. Using these cells, we analyzed the expression of transcription factor genes (runt-related transcription factor 2 (RUNX2) and osterix (SP7)) and bone related genes (osteopontin (SPP1), osteonectin (SPARC) and collagen, type I, alpha 1 (COLIA1)) during osteoblastic differentiation. Our results demonstrated that hMSCs can be obtained from peripheral blood and that they are able to generate CFU-F and to differentiate in osteoblast and adipocyte; in this study, we also identified a possible gene expression timing during osteoblastic differentiation that provided a powerful tool to study bone physiology.

Biomimetism, biomimetic matrices and the induction of bone formation

Bone formation by induction initiates by invocation of osteogenic soluble molecular signals of the transforming growth factor-β (TGF-β) superfamily; when combined with insoluble signals or substrata, the osteogenic soluble signals trigger the ripple-like cascade of cell differentiation into osteoblastic cell lines secreting bone matrix at site of surgical implantation. A most exciting and novel strategy to initiate bone formation by induction is to carve smart self-inducing geometric concavities assembled within biomimetic constructs. The assembly of a series of repetitive concavities within the biomimetic constructs is endowed with the striking prerogative of differentiating osteoblast-like cells attached to the biomimetic matrices initiating the induction of bone formation as a secondary response. Importantly, the induction of bone formation is initiated without the exogenous application of the osteogenic soluble molecular signals of the TGF-β superfamily. This manuscript reviews the available data on this fascinating phenomenon, i.e. biomimetic matrices that arouse and set into motion the mammalian natural ability to heal thus constructing biomimetic matrices that in their own right set into motion inductive regenerative phenomena initiating the cascade of bone differentiation by induction biomimetizing the remodelling cycle of the primate cortico-cancellous bone.

Compatibility of Embryonic Stem Cells with Biomaterials

Periodontal bone defects and atrophy of the jaws in an aging population are of special concern. Tissue engineering using embryonic stem cells (ESCs) and biomaterials may offer new therapeutic options. The purpose of this study is to evaluate the compatibility of ESCs with biomaterials and the influence of biomaterials on the osteogenic gene expression profile.

Therefore, ESCs are cultured with various biomaterials. The cytocompatibility of murine ESCs is measured regarding the proliferation of the cells on the materials by CyQUANT ® assay, the morphology by scanning electron microscopy, and the influence on the gene expression by real time PCR.

The results show that insoluble collagenous bone matrix, followed by β-tricalciumphosphate, is most suitable for bone tissue engineering regarding cell proliferation, and phenotype. The gene expression analysis indicates that biomaterials do influence the gene expression of ESCs.

Our results provide new insight into the cytocompatibility of ESCs on different scaffolds.

Efficient differentiation of human embryonic stem cells into a homogeneous population of osteoprogenitor-like cells

The use of human embryonic stem cells (hESC) in both research and therapeutic applications requires relatively large homogeneous populations of differentiated cells. The differentiation of three hESC lines into highly homogeneous populations of osteoprogenitor-like (hESC-OPL) cells is reported here. These cells could be expanded in a defined culture system for more than 18 passages, and showed a fibroblast-like morphology and a normal stable karyotype. The cells were strongly positive for the same antigenic markers as mesenchymal stem cells but negative for markers of haematopoetic stem cells. The hESC-OPL cells were able to differentiate into the osteogenic, but not into the chondrogenic or adipogenic, lineage and were positive for markers of early stages of osteogenic differentiation. When cultured in the presence of osteogenic supplements, the cells indicated the capacity to achieve, under inductive conditions, a mature osteoblast phenotype. The differentiation protocol is based on a monolayer approach, and does not require any exogenous factors other than fetal calf serum, or coculture systems of animal or human origin. This method is likely to be amenable to large-scale production of homogeneous osteoprogenitor-like cells and thus overcomes one of the major problems of differentiation of hESC, with important relevance for further cell therapy studies.

Differentiation of adipose stem cells

The broad definition of a stem cell is a cell that has the ability to self-renew and differentiate into one or more specialized terminally differentiated cell types. It has become evident that stem cells persist in, and can be isolated from, many adult tissues. Adipose tissue has been shown to contain a population of cells that retain a high proliferation capacity in vitro and the ability to undergo extensive differentiation into multiple cell lineages. These cells are referred to as adipose stem cells and are biologically similar, although not identical, to mesenchymal stem cells derived from the bone marrow. Differentiation causes stem cells to adopt the phenotypic, biochemical, and functional properties of more terminally differentiated cells. This chapter will provide investigators with some background on stem cells derived from adipose tissue and then provide details on adipose stem cell multilineage differentiation along osteogenic, adipogenic, chondrogenic, and neurogenic lineages.

Bone differentiation of marrow-derived mesenchymal stem cells using beta-tricalcium phosphate-alginate-gelatin hybrid scaffolds

The aim of the present study was to establish a 3D culture system for bone differentiation of mesenchymal stem cells (MSCs), using a new hybrid sponge. To manufacture the scaffold, a composite of beta-tricalcium phosphate-alginate-gelatin was prepared and cast as pellets of 1 cm diameter. The sponge was then fabricated by drying in freeze-dryer for 12 h. The porosity, mean pore size, compressive modulus and strength of the composite sponge fabricated in this study were 89.7%, 325.3 microm, 1.82 and 0.196 MPa, respectively. To establish a 3D culture system, the rat bone marrow-derived MSCs were suspended in 500 microl diluted collagen gel, loaded into the porous sponge and provided with medium with or without osteogenic supplements for 3 weeks. The day after loading, the cells appeared in the scaffold's internal spaces, where later some of them from either culture survived by anchoring on the surfaces. At the end of cultivation period, individually adhered cells from both cultures were observed to be replaced by cell aggregates, in which mineralized matrix was detected by alizarin red staining. Furthermore, RT-PCR analysis indicated that the bone-specific gene osteocalcin was expressed in cultures in both the presence and absence of the osteogenic supplements. Taken together, it seems that the studied scaffolds are cell-compatible and, more importantly, possess some osteo-inductive properties.

Skeletal stem cells: phenotype, biology and environmental niches informing tissue regeneration

Advances in our knowledge of the biology of skeletal stem cells, together with an increased understanding of the regeneration of normal tissue offer exciting new therapeutic approaches in musculoskeletal repair. Skeletal stem cells from various adult tissues such as bone marrow can be identified and isolated based on their expression of a panel of markers associated with smooth muscle cells, pericytes and endothelial cells. Thus, skeletal stem cell-like populations within bone marrow may share a common perivascular stem cell niche within the microvascular network. To date, the environmental niche that nurtures and maintains the stromal stem cell at different anatomical sites remains poorly understood. However, an understanding of the osteogenic and perivascular niches will inform identification of the key growth factors, matrix constituents and physiological conditions that will enhance the ex vivo amplification and differentiation of osteogenic stem cells to mimic native tissue critical for tissue repair. This review will examine skeletal stem cell biology, the advances in our understanding of the skeletal and perivascular niche and interactions therein and the opportunities to harness that knowledge for musculoskeletal regeneration.

A three-dimensional tubular scaffold that modulates the osteogenic and vasculogenic differentiation of rat bone marrow stromal cells

Bone marrow stromal cells (BMSCs) or mesenchymal stem cells (MSCs) are a heterogeneous population of cells that are multipotent. When rat BMSCs were seeded onto a 3-dimensional (3-D) tubular scaffold engineered from aligned type I collagen strands and cultured in osteogenic medium, they simultaneously matured and differentiated into osteoblastic and vascular cell lineages. In addition, these osteoblasts produced mineralized matricellular deposits. BMSCs were seeded at a density of 2 x 10(6) cells/15 mm tube and cultured in basal or osteogenic medium for 3, 6, and 9 days. These cells were subsequently processed for real-time reverse-transcriptase polymerase chain reaction (RT-qPCR), immunohistochemical, cytochemical, and biochemical analyses. Immunolocalization of lineage-specific proteins was visualized using confocal microscopy. In the present study, the expression pattern of key osteogenic markers significantly differed in response to basal and osteogenic media. Alkaline phosphatase activity and calcium content increased significantly over the observed period of time in osteogenic medium. The observed up-regulation of transcripts coding for osteoblastic phenotypic markers is reminiscent of in vivo expression patterns. Abundant sheets of Pecam (CD31) -, Flk-1 (vascular endothelial growth factor receptor-2) -, CD34-, tomato lectin-, and alpha-smooth muscle actin-positive cells were observed in these tube cultures. Moreover, nascent capillary-like vessels were also seen amid the osteoblasts in osteogenic cultures. Our 3-D culture system augmented the maturation and differentiation of BMSCs into osteoblasts. Thus, our in vitro model provides an excellent opportunity to study the concurrent temporal and spatial regulation of osteogenesis and vasculogenesis during bone development.

Induction of osteogenic markers in differentially treated cultures of embryonic stem cells

Background

Facial trauma or tumor surgery in the head and face area often lead to massive destruction of the facial skeleton. Cell-based bone reconstruction therapies promise to offer new therapeutic opportunities for the repair of bone damaged by disease or injury. Currently, embryonic stem cells (ESCs) are discussed to be a potential cell source for bone tissue engineering. The purpose of this study was to investigate various supplements in culture media with respect to the induction of osteogenic differentiation.

Methods

Murine ESCs were cultured in the presence of LIF (leukemia inhibitory factor), DAG (dexamethasone, ascorbic acid and β-glycerophosphate) or bone morphogenetic protein-2 (BMP-2). Microscopical analyses were performed using von Kossa staining, and expression of osteogenic marker genes was determined by real time PCR.

Results

ESCs cultured with DAG showed by far the largest deposition of calcium phosphate-containing minerals. Starting at day 9 of culture, a strong increase in collagen I mRNA expression was detected in the DAG-treated cells. In BMP-2-treated ESCs the collagen I mRNA induction was less increased. Expression of osteocalcin, a highly specific marker for osteogentic differentiation, showed a double-peaked curve in DAG-treated cells. ESCs cultured in the presence of DAG showed a strong increase in osteocalcin mRNA at day 9 followed by a second peak starting at day 17.

Conclusion

Supplementation of ESC cell cultures with DAG is effective in inducing osteogenic differentiation and appears to be more potent than stimulation with BMP-2 alone. Thus, DAG treatment can be recommended for generating ESC populations with osteogenic differentiation that are intended for use in bone tissue engineering.

Manufacture of degradable polymeric scaffolds for bone regeneration

Many innovative technology platforms for promoting bone regeneration have been developed. A common theme among these is the use of scaffolds to provide mechanical support and osteoconduction. Scaffolds can be either ceramic or polymer-based, or composites of both classes of material. Both ceramics and polymers have their own merits and drawbacks, and a better solution may be to synergize the advantageous properties of both materials within composite scaffolds. In this current review, after a brief introduction of the anatomy and physiology of bone, different strategies of fabricating polymeric scaffolds for bone regeneration, including traditional and solid free-form fabrication, are critically discussed and compared, while focusing on the advantages and disadvantages of individual techniques.

Principles of cartilage tissue engineering in TMJ reconstruction

Diseases and defects of the temporomandibular joint (TMJ), compromising the cartilaginous layer of the condyle, impose a significant treatment challenge. Different regeneration approaches, especially surgical interventions at the TMJ's cartilage surface, are established treatment methods in maxillofacial surgery but fail to induce a regeneration ad integrum. Cartilage tissue engineering, in contrast, is a newly introduced treatment option in cartilage reconstruction strategies aimed to heal cartilaginous defects. Because cartilage has a limited capacity for intrinsic repair, and even minor lesions or injuries may lead to progressive damage, biological oriented approaches have gained special interest in cartilage therapy. Cell based cartilage regeneration is suggested to improve cartilage repair or reconstruction therapies. Autologous cell implantation, for example, is the first step as a clinically used cell based regeneration option. More advanced or complex therapeutical options (extracorporeal cartilage engineering, genetic engineering, both under evaluation in pre-clinical investigations) have not reached the level of clinical trials but may be approached in the near future. In order to understand cartilage tissue engineering as a new treatment option, an overview of the biological, engineering, and clinical challenges as well as the inherent constraints of the different treatment modalities are given in this paper.

Efficiency of the elongation factor-1alpha promoter in mammalian embryonic stem cells using lentiviral gene delivery systems

The establishment of new technology for genetic modification in human embryonic stem (ES) cell lines has raised great hopes for achieving new ground in basic and clinical research. Recently, lentiviral vector technology has been shown to be highly effective and therefore could emerge as a popular tool for human ES cell genetic modification. The objectives of this study were to evaluate the efficiency of promoters in lentiviral gene delivery systems in mammalian ES cells, including mouse, monkey, and human, and to construct efficient and optimized conditions for lentivirus-mediated transfection systems. Mammalian ES cells were transfected with self-inactivating (SIN) human immunodeficiency virus type-1 (HIV-1)-based lentiviral vectors containing the human polypeptide chain elongation factor-1alpha (EF-1alpha) promoter or cytomegalovirus (CMV) promoter and analyzed by fluorescence-activated cell sorting (FACS) analysis for the expression of the enhanced green fluorescent protein (eGFP) reporter gene. The efficiency of the EF-1alpha promoter was higher than that of the CMV promoter in all ES cells tested. The EF-1alpha promoter efficiently drove gene expression (14.74%) compared with CMV promoter (3.69%) in human ES cells. We generated a stable eGFP+ human ES cell line (CHA3-EGFP human ES cells) that continuously expressed high levels of EGFP ( approximately 95%) from the EF-1alpha promoter and was maintained for up to 60 weeks with undifferentiated proliferation. The established CHA3-EGFP human ES cell lines were characterized as being negative for nondifferentiation markers and teratoma formation. These results imply that genetic modification by lentiviral vectors with specific promoters in ES cells constitute a powerful tool for guided differentiation as well as gene therapy.