Homing of mesenchymal stem cells: mechanistic or stochastic? Implications for targeted delivery in arthritis

Mesenchymal stem cells (MSCs) are multipotent cells with the capacity to undergo chondrogenic differentiation. Systemically administered MSCs have been shown to preferentially accumulate at sites of tissue damage and inflammation, thus MSC-based therapy holds great promise for the treatment of inflammatory diseases such as RA. Modulation of MSC homing may allow targeted delivery of systemically administered MSCs to damaged articular cartilage, where they can suppress immune-mediated cartilage destruction and contribute to cartilage repair via a combination of chondrogenic differentiation and paracrine stimulation of intrinsic residual repair. To harness the potential of MSC homing, a thorough understanding of the mechanism is key. This review discusses current knowledge of the mechanism of MSC homing to injured/inflamed tissue and its implications for targeted MSC-based therapy in arthritis.

Articular Chondroprogenitor Cells Maintain Chondrogenic Potential but Fail to Form a Functional Matrix When Implanted Into Muscles of SCID Mice

OBJECTIVE

Articular cartilage is a complex tissue comprising phenotypically distinct zones. Research has identified the presence of a progenitor cell population in the surface zone of immature articular cartilage. The aim of the present study was to determine the in vivo plasticity of articular cartilage progenitor.

DESIGN

Chondropogenitor cells were isolated from bovine metacarpalphalangeal joints by differential adhesion to fibronectin. Cells were labeled with PKH26 and injected into the thigh muscle of severe-combined immunodeficient (SCID) mice. After 2 weeks, the muscles were dissected and cryosectioned. Sections were stained with safranin O and labeled for sox9 and collagen type II. Polymerase chain reaction analysis was carried out to determine plasticity for a number of tissue-specific markers. Full-depth chondrocytes acted as a control.

RESULTS

Fluorescent PKH26 labeled cells were detected after 2 weeks in all samples analyzed. A cartilage pellet was present after injection of freshly isolated chondrocytes. After injection with clonal and enriched populations of chondroprogenitors, no distinct pellet was detected, but diffuse cartilage nodules were found with regions of safranin O staining and Sox9. Low levels of collagen type II were also detected. Polymerase chain reaction analysis identified the presence of the endothelial cell marker PECAM-1 in one clonal cell line, demonstrating phenotypic plasticity into the phenotype of the surrounding host tissues.

CONCLUSIONS

The bovine articular cartilage progenitor cells were able to survive in vivo postimplantation, but failed to create a robust cartilage pellet, despite expressing sox9 and type II collagen. This suggests the cells require further signals for chondrogenic differentiation.

The Hippo transducer YAP1 transforms activated satellite cells and is a potent effector of embryonal rhabdomyosarcoma formation

The role of the Hippo pathway effector YAP1 in soft tissue sarcomas is poorly defined. Here we report that YAP1 activity is elevated in human embryonal rhabdomyosarcoma (ERMS). In mice, sustained YAP1 hyperactivity in activated, but not quiescent, satellite cells induces ERMS with high penetrance and short latency. Via its transcriptional program with TEAD1, YAP1 directly regulates several major hallmarks of ERMS. YAP1-TEAD1 upregulate pro-proliferative and oncogenic genes and maintain the ERMS differentiation block by interfering with MYOD1 and MEF2 pro-differentiation activities. Normalization of YAP1 expression reduces tumor burden in human ERMS xenografts and allows YAP1-driven ERMS to differentiate in situ. Collectively, our results identify YAP1 as a potent ERMS oncogenic driver and a promising target for differentiation therapy.

Prospective purification of a subpopulation of human synovial mesenchymal stem cells with enhanced chondro-osteogenic potency

OBJECTIVE

We previously reported the coexistence, within cultured mesenchymal stem cells (MSCs) from human synovial membrane, of single-cell-derived clonal cell populations with distinct differentiation potency. The aim of this study was to investigate marker sets for prospective purification of functionally distinct MSC subsets.

METHODS

Cells were enzymatically released from human synovium and culture expanded. Phenotype analysis was performed by flow cytometry using combinations of MSC markers. Sorting was carried out using the FACS DiVA cell sorter. Sorted cell populations were assessed for clonogenicity, kinetics of growth, cell senescence and chondro-osteogenic potency.

RESULTS

During culture expansion, the co-localization of CD39 within the CD73(+) cell population identified a small cell subset that was maintained from passage 1 (P1) up to at least P12 in all donors tested. The CD73(+)CD39(+) cell subset displayed higher expression levels of Sox9 and Runx2 and a significantly greater chondro-osteogenic potency than the CD73(+)CD39(-) cell subset. In contrast, it was less clonogenic and proliferative. There was no difference in cell senescence between the sorted MSC subsets and the parental MSCs. Notably, there were no detectable differences in chondro-osteogenic potency between the CD73(+)CD39(-) and CD73(+)CD39(+) cell subsets purified from fresh synovial cell populations.

CONCLUSION

Our findings indicate that the combination of CD73 and CD39 allows the prospective purification from culture-expanded heterogeneous synovial MSC populations of a distinct MSC subset with greater chondro-osteogenic potency. We anticipate that such an approach will enhance the consistency of cell-based therapeutic protocols for the repair of osteochondral defects.

The Hippo pathway member Yap plays a key role in influencing fate decisions in muscle satellite cells

Satellite cells are the resident stem cells of skeletal muscle. Mitotically quiescent in mature muscle, they can be activated to proliferate and generate myoblasts to supply further myonuclei to hypertrophying or regenerating muscle fibres, or self-renew to maintain the resident stem cell pool. Here, we identify the transcriptional co-factor Yap as a novel regulator of satellite cell fate decisions. Yap expression increases during satellite cell activation and Yap remains highly expressed until after the differentiation versus self-renewal decision is made. Constitutive expression of Yap maintains Pax7(+) and MyoD(+) satellite cells and satellite cell-derived myoblasts, promotes proliferation but prevents differentiation. In contrast, Yap knockdown reduces the proliferation of satellite cell-derived myoblasts by ≈40%. Consistent with the cellular phenotype, microarrays show that Yap increases expression of genes associated with Yap inhibition, the cell cycle, ribosome biogenesis and that it represses several genes associated with angiotensin signalling. We also identify known regulators of satellite cell function such as BMP4, CD34 and Myf6 (Mrf4) as genes whose expression is dependent on Yap activity. Finally, we confirm in myoblasts that Yap binds to Tead transcription factors and co-activates MCAT elements which are enriched in the proximal promoters of Yap-responsive genes.

Immunostaining of skeletal tissues

Immunohistochemistry (IHC) is a routinely used technique in clinical diagnosis of pathological conditions and in basic research. It combines anatomical, immunological, and biochemical methods and relies on the specific binding of an antibody to an antigen. Using the technique with mineralised tissues is more complicated than with soft tissues. This can in most cases be overcome by demineralising the samples, which allows embedding in paraffin wax and a simpler work-up than for resin-embedded, or for frozen samples. This chapter describes methods for IHC on paraffin-embedded formaldehyde fixed sections to detect antigens in the musculoskeletal tissues.

Future dentistry: cell therapy meets tooth and periodontal repair and regeneration

Cell-based tissue repair of the tooth and – tooth-supporting – periodontal ligament (PDL) is a new attractive approach that complements traditional restorative or surgical techniques for replacement of injured or pathologically damaged tissues. In such therapeutic approaches, stem cells and/or progenitor cells are manipulated in vitro and administered to patients as living and dynamic biological agents. In this review, we discuss the clonogenic potential of human dental and periodontal tissues such as the dental pulp and the PDL and their potential for tooth and periodontal repair and/or regeneration. We propose novel therapeutic approaches using stem cells or progenitor cells, which are targeted to regenerate the lost dental or periodontal tissue.

Directed migration of human bone marrow mesenchymal stem cells in a physiological direct current electric field

At sites of bone fracture, naturally-occurring electric fields (EFs) exist during healing and may guide cell migration. In this study, we investigated whether EFs could direct the migration of bone marrow mesenchymal stem cells (BM-MSCs), which are known to be key players in bone formation. Human BM-MSCs were cultured in direct current EFs of 10 to 600 mV/mm. Using time-lapse microscopy, we demonstrated that an EF directed migration of BM-MSCs mainly to the anode. Directional migration occurred at a low threshold and with a physiological EF of ~25 mV/mm. Increasing the EF enhanced the MSC migratory response. The migration speed peaked at 300 mV/mm, at a rate of 42 ±1 µm/h, around double the control (no EF) migration rate. MSCs showed sustained response to prolonged EF application in vitro up to at least 8 h. The electrotaxis of MSCs with either early (P3-P5) or late (P7-P10) passage was also investigated. Migration was passage-dependent with higher passage number showing reduced directed migration, within the range of passages examined. An EF of 200 mV/mm for 2 h did not affect cell senescence, phenotype, or osteogenic potential of MSCs, regardless of passage number within the range tested (P3-P10). Our findings indicate that EFs are a powerful cue in directing migration of human MSCs in vitro. An applied EF may be useful to control or enhance migration of MSCs during bone healing.

Prospects of stem cell therapy in osteoarthritis

Osteoarthritis is a common disorder in which there is not only extensive degeneration but also an aberrant attempt at repair in joints. Stem cell therapy could provide a permanent, biological solution, with all sources of stem cells (embryonic, fetal and adult) showing some degree of potential. Mesenchymal stromal/stem cells, however, appear to be the leading candidates because of their ability to be sourced from many or all joint tissues. They may also modulate the immune response of individuals, in a manner influenced by local factors. This biological behavior of stem cells renders the application of regulatory standardizations challenging in comparison to pharmaceutical therapies. However, this would not be an issue if endogenous stem cells were activated to effect repair of an arthritic joint.

Functional mesenchymal stem cell niches in adult mouse knee joint synovium in vivo

OBJECTIVE

We previously reported that human synovium contains cells that, after culture expansion, display properties of mesenchymal stem cells (MSCs). The objective of this study was to identify MSCs in native synovium in vivo.

METHODS

To identify stem cells in the synovium in vivo, a double nucleoside analog cell-labeling scheme was used in a mouse model of joint-surface injury. For labeling of slow-cycling cells, mice received iododeoxyuridine (IdU) for 30 days, followed by a 40-day washout period. For labeling of cells that proliferate after injury, mice underwent knee surgery to produce an articular cartilage defect and received chlorodeoxyuridine (CIdU) for 4 days, starting at multiple time points after surgery. Unoperated and sham-operated joints served as controls. Knee joint paraffin sections were analyzed by double and triple immunostaining to detect nucleoside analogs, conventional MSC markers, and chondrocyte-lineage markers.

RESULTS

Long-term-retaining, slow-cycling IdU-positive cells were detected in the synovium. At 4 days and 8 days after injury, there was marked proliferation of IdU-positive cells, which costained for CIdU. IdU-positive cells were nonhematopoietic, nonendothelial stromal cells, were distinct from pericytes, and stained positive for MSC markers. MSCs were phenotypically heterogeneous and located in topographically distinct niches in the lining layer and the subsynovial tissue. Twelve days after injury, double nucleoside-labeled cells within synovium were embedded in cartilage-specific metachromatic extracellular matrix and costained positive for the chondrocyte-lineage markers Sox9 and type II collagen.

CONCLUSION

Our findings provide the first evidence of the existence of resident MSCs in the knee joint synovium that undergo proliferation and chondrogenic differentiation following injury in vivo.

WNT-3A modulates articular chondrocyte phenotype by activating both canonical and noncanonical pathways

A single Wnt can simultaneously activate different pathways with distinct and independent outcomes and reciprocal regulation in human articular chondrocytes.

Activation and disruption of Wnt/β-catenin signaling both result in cartilage breakdown via unknown mechanisms. Here we show that both WNT-3A and the Wnt inhibitor DKK1 induced de-differentiation of human articular chondrocytes through simultaneous activation of β-catenin–dependent and independent responses. WNT-3A activates both the β-catenin–dependent canonical pathway and the Ca²⁺/CaMKII noncanonical pathways, with distinct transcriptional targets. WNT-3A promotes cell proliferation and loss of expression of the chondrocyte markers COL2A1, Aggrecan, and SOX9; however, proliferation and AXIN2 up-regulation are downstream of the canonical pathway and are rescued by DKK1, whereas the loss of differentiation markers is CaMKII dependent. Finally, we showed that in chondrocytes, the Ca²⁺/CaMKII-dependent and β-catenin–dependent pathways are reciprocally inhibitory, thereby explaining why DKK1 can induce loss of differentiation through de-repression of the CaMKII pathway. We propose a novel model in which a single WNT can simultaneously activate different pathways with distinct and independent outcomes and with reciprocal regulation. This offers an opportunity for selective pharmacological targeting.

Dual origin of mesenchymal stem cells contributing to organ growth and repair

In many adult tissues, mesenchymal stem cells (MSCs) are closely associated with perivascular niches and coexpress many markers in common with pericytes. The ability of pericytes to act as MSCs, however, remains controversial. By using genetic lineage tracing, we show that some pericytes differentiate into specialized tooth mesenchyme-derived cells--odontoblasts--during tooth growth and in response to damage in vivo. As the pericyte-derived mesenchymal cell contribution to odontoblast differentiation does not account for all cell differentiation, we identify an additional source of cells with MSC-like properties that are stimulated to migrate toward areas of tissue damage and differentiate into odontoblasts. Thus, although pericytes are capable of acting as a source of MSCs and differentiating into cells of mesenchymal origin, they do so alongside other MSCs of a nonpericyte origin. This study identifies a dual origin of MSCs in a single tissue and suggests that the pericyte contribution to MSC-derived mesenchymal cells in any given tissue is variable and possibly dependent on the extent of the vascularity.

The regulation of differentiation in mesenchymal stem cells

Mesenchymal stromal/stem cells (MSCs) are a population of stromal cells present in the bone marrow and most connective tissues, capable of differentiation into mesenchymal tissues such as bone and cartilage. MSCs are attractive candidates for biological cell-based tissue repair approaches because of their extensive proliferative ability in culture while retaining their mesenchymal multilineage differentiation potential. In addition to its undoubted scientific interest, the prospect of monitoring and controlling MSC differentiation is a crucial regulatory and clinical requirement. Hence, the molecular regulation of MSC differentiation has been extensively studied. Most of the studies are in vitro, because the identity of MSCs in their tissues of origin in vivo remains undefined. This review addresses the current knowledge of the molecular basis of differentiation of cultured MSCs, with a particular focus on chondrogenesis and osteogenesis. Building on the information coming from developmental biology studies of embryonic skeletogenesis, several signaling pathways and transcription factors have been investigated and shown to play critical roles in MSC differentiation. In particular, the Wnt and transforming growth factor-β/bone morphogenetic protein signaling pathways are well known to modulate in MSCs the molecular differentiation into cartilage and bone. Relevant to the emerging concept of stem cell niches is the demonstration that physical factors can also participate in the regulation of MSC differentiation. Knowledge of the regulation of MSC differentiation will be critical in the design of three-dimensional culture systems and bioreactors for automated bioprocessing through mathematical models applied to systems biology and network science.

Mesenchymal stem cells: a perspective from in vitro cultures to in vivo migration and niches

Mesenchymal Stromal Progenitor/Stem Cells (MSCs) are a rare population of non-hematopoietic stromal cells, present in the bone marrow and most connective tissues of the body. They are capable of differentiation into mesenchymal tissues such as bone, cartilage, adipose tissue and muscle. In the absence of specific markers, MSCs have been defined following isolation and culture expansion, by their expression of various molecules including CD90, CD105 and CD73 and absence of markers like CD34, CD45, and CD14. MSCs have extensive proliferative ability in culture in an uncommitted state while retaining their multilineage differentiation potential, which make them attractive candidates for biological cell-based tissue repair approaches. However, their identity in their tissues of origin is not clear and the niches in which they reside are not defined. This review addresses the current state of MSC research including the differentiation potency of culture expanded MSCs, expression of chemokines and their receptors in MSCs--both relevant issues for the advocated use of MSCs for tissue repair and their systemic delivery to the affected tissues. It also reviews current knowledge of MSC niches in their native tissues, addressing the relationship with pericytes. Finally, it provides a scientific basis for the requirement of a thorough characterisation of the endogenous MSC niches within their native tissues in vivo. The knowledge of MSC niches will instruct development of innovative therapeutic measures such as producing pharmacological substances that target endogenous MSCs and their niches in order to activate and guide intrinsic repair and to improve disease outcomes.

Toward cell therapy using placenta-derived cells: disease mechanisms, cell biology, preclinical studies, and regulatory aspects at the round table

Among the many cell types that may prove useful to regenerative medicine, mounting evidence suggests that human term placenta-derived cells will join the list of significant contributors. In making new cell therapy-based strategies a clinical reality, it is fundamental that no a priori claims are made regarding which cell source is preferable for a particular therapeutic application. Rather, ongoing comparisons of the potentiality and characteristics of cells from different sources should be made to promote constant improvement in cell therapies, and such comparisons will likely show that individually tailored cells can address disease-specific clinical needs. The principle underlying such an approach is resistance to the notion that comprehensive characterization of any cell type has been achieved, neither in terms of phenotype nor risks-to-benefits ratio. Tailoring cell therapy approaches to specific conditions also requires an understanding of basic disease mechanisms and close collaboration between translational researchers and clinicians, to identify current needs and shortcomings in existing treatments. To this end, the international workshop entitled "Placenta-derived stem cells for treatment of inflammatory diseases: moving toward clinical application" was held in Brescia, Italy, in March 2009, and aimed to harness an understanding of basic inflammatory mechanisms inherent in human diseases with updated findings regarding biological and therapeutic properties of human placenta-derived cells, with particular emphasis on their potential for treating inflammatory diseases. Finally, steps required to allow their future clinical application according to regulatory aspects including good manufacturing practice (GMP) were also considered. In September 2009, the International Placenta Stem Cell Society (IPLASS) was founded to help strengthen the research network in this field.

Human single-chain variable fragment that specifically targets arthritic cartilage

Objective

To demonstrate that posttranslational modification of type II collagen (CII) by reactive oxygen species (ROS), which are known to be present in inflamed arthritic joints, can give rise to epitopes specific to damaged cartilage in rheumatoid arthritis (RA) and osteoarthritis (OA) and to establish a proof of concept that antibodies specific to ROS-modified CII can be used to target therapeutics specifically to inflamed arthritic joints.

Methods

We used a semisynthetic phage display human antibody library to raise single-chain variable fragments (scFv) specific to ROS-modified CII. The specificity of anti–ROS-modified CII scFv to damaged arthritic cartilage was assessed in vitro by immunostaining articular cartilage from RA and OA patients and from normal controls. The in vivo targeting potential was tested using mice with antigen-induced arthritis, in which localization of anti–ROS-modified CII scFv in the joints was determined. The therapeutic effect of anti–ROS-modified CII scFv fused to soluble murine tumor necrosis factor receptor II–Fc fusion protein (mTNFRII-Fc) was also investigated.

Results

The anti–ROS-modified CII scFv bound to damaged arthritic cartilage from patients with RA and OA but not to normal preserved cartilage. When systemically administered to arthritic mice, the anti–ROS-modified CII accumulated selectively at the inflamed joints. Importantly, when fused to mTNFRII-Fc, it significantly reduced inflammation in arthritic mice, as compared with the effects of mTNFRII-Fc alone or of mTNFRII-Fc fused to an irrelevant scFv.

Conclusion

Our findings indicate that biologic therapeutics can be targeted specifically to arthritic joints and suggest a new approach for the development of novel treatments of arthritis.

Distinct mesenchymal progenitor cell subsets in the adult human synovium

OBJECTIVE

To analyse the heterogeneity at the single-cell level of human mesenchymal progenitor cells from SM.

METHODS

Cell populations were enzymatically released from the knee joint synovium of adult human individuals. Single cell-derived clonal populations were obtained by limiting dilution and serially passaged to determine growth rates. Phenotypic analysis was carried out by flow cytometry. Replicative senescence was assessed by the senescence-associated beta-galactosidase staining. Telomere lengths were determined semiquantitatively by Southern blotting. Telomerase activity was measured using a real-time quantitative telomerase repeat amplification procedure. Culture-expanded clonal populations were subjected to in vitro differentiation assays to investigate their mesenchymal multipotency.

RESULTS

The 50 clones analysed displayed wide variations in the proliferation rates, even within the same donor sample. The time taken to reach 20 population doublings ranged from 44 to 130 days. The phenotype of the clones tested was compatible with that of mesenchymal stem cells. Mean telomere lengths ranged from 5.2 to 10.9 kb with positive linear trend with telomerase activity, but no correlation with proliferative rates or cell senescence. All clones tested were capable of chondrogenic and osteogenic differentiation, though with large variability in potency. In contrast, only 30% of the clones were adipogenic.

CONCLUSIONS

We report for the first time the co-existence, within the synovium, of progenitor cell subsets with distinct mesenchymal differentiation potency. Our findings further emphasize the need for strategies to purify cell populations with the clinically desired tissue formation potentials.

Efficient Lentiviral Transduction and Improved Engraftment of Human Bone Marrow Mesenchymal Cells

Human bone marrow (BM) mesenchymal stem/progenitor cells are potentially attractive targets for ex vivo gene therapy. The potential of lentiviral vectors for transducing BM mesenchymal cells was examined using a self-inactivating vector that expressed the green fluorescent protein (GFP) from an internal cytomegalovirus (CMV) promoter. This vector was compared with oncoretroviral vectors expressing GFP from the CMV promoter or a modified long-terminal repeat that had been optimized for long-term expression in stem cells. The percentage of GFP-positive cells was consistently higher following lentiviral versus oncoretroviral transduction, consistent with increased GFP mRNA levels and increased gene transfer efficiency measured by polymerase chain reaction and Southern blot analysis. In vitro GFP and FVIII expression lasted for several months post-transduction, although expression slowly declined. The transduced cells retained their stem/progenitor cell properties since they were still capable of differentiating along adipogenic and osteogenic lineages in vitro while maintaining high GFP and FVIII expression levels. Implantation of lentivirally transduced human BM mesenchymal cells using collagen scaffolds into immunodeficient mice resulted in efficient engraftment of gene-engineered cells and long-term transgene expression in vivo. These biocompatible BM mesenchymal implants represent a reversible, safe, and versatile protein delivery approach because they can be retrieved in the event of an unexpected adverse reaction or when expression of the protein of interest is no longer required. In conclusion, efficient gene delivery with lentiviral vectors in conjunction with the use of bioengineered reversible scaffolds improves the therapeutic prospects of this novel approach for gene therapy, protein delivery, or tissue engineering.

Identification of the molecular response of articular cartilage to injury, by microarray screening: Wnt-16 expression and signaling after injury and in osteoarthritis

OBJECTIVE

To characterize the molecular response of adult human articular cartilage to acute mechanical injury.

METHODS

An established ex vivo model was used to compare gene expression of adult human articular cartilage explants 24 hours after mechanical injury with that of uninjured controls by microarray analysis of gene expression. Confirmation for selected genes was obtained by real-time polymerase chain reaction and immunohistochemical analysis. Expression of selected genes was also investigated in preserved and osteoarthritic (OA) cartilage.

RESULTS

Six hundred ninety genes were significantly regulated at least 2-fold following mechanical injury. They included genes previously reported to be differentially expressed in OA versus normal cartilage or having allelic variants genetically linked to OA. Significant functional clusters included genes associated with wound healing, developmental processes, and skeletal development. The transforming growth factor beta, fibroblast growth factor, and Wnt pathways were modulated. A systematic analysis of the Wnt signaling pathway revealed up-regulation of Wnt-16, down-regulation of FRZB, up-regulation of Wnt target genes, and nuclear localization of beta-catenin in injured cartilage. In addition, in OA, Wnt-16 and beta-catenin were barely detectable in preserved cartilage areas, but were dramatically up-regulated in areas of the same joint with moderate to severe OA damage.

CONCLUSION

Our findings indicate that mechanical injury to adult human articular cartilage results in the activation of a signaling response, with reactivation of morphogenetic pathways. Therapeutic targeting of such pathways may improve current protocols of joint surface defect repair and/or prevent the evolution of such lesions into posttraumatic OA.

Comparative osteogenic transcription profiling of various fetal and adult mesenchymal stem cell sources

Human mesenchymal stem cells (MSC) from adult and fetal tissues are promising candidates for cell therapy but there is a need to identify the optimal source for bone regeneration. We have previously characterized MSC populations in first trimester fetal blood, liver, and bone marrow and we now evaluate their osteogenic differentiation potential in comparison to adult bone marrow MSC. Using quantitative real-time RT-PCR, we demonstrated that 16 osteogenic-specific genes (OC, ON, BSP, OP, Col1, PCE, Met2A, OPG, PHOS1, SORT, ALP, BMP2, CBFA1, OSX, NOG, IGFII) were expressed in both fetal and adult MSC under basal conditions and were up-regulated under osteogenic conditions both in vivo and during an in vitro 21-day time-course. However, under basal conditions, fetal MSC had higher levels of osteogenic gene expression than adult MSC. Upon osteogenic differentiation, fetal MSC produced more calcium in vitro and reached higher levels of osteogenic gene up-regulation in vivo and in vitro. Second, we observed a hierarchy within fetal samples, with fetal bone marrow MSC having greater osteogenic potential than fetal blood MSC, which in turn had greater osteogenic potential than fetal liver MSC. Finally, we found that the level of gene expression under basal conditions was positively correlated with both calcium secretion and gene expression after 21 days in osteogenic conditions. Our findings suggest that stem cell therapy for bone dysplasias such as osteogenesis imperfecta may benefit from preferentially using first trimester fetal blood or bone marrow MSC over fetal liver or adult bone marrow MSC.