Characterisation of human bone marrow stromal cell heterogeneity for skeletal regeneration strategies using a two-stage colony assay and computational modelling

Single-cell RNA-sequence analysis of human bone marrow reveals new targets for isolation of skeletal stem cells using spherical nucleic acids

There is a wealth of data indicating human bone marrow contains skeletal stem cells (SSC) with the capacity for osteogenic, chondrogenic and adipogenic differentiation. However, current methods to isolate SSCs are restricted by the lack of a defined marker, limiting understanding of SSC fate, immunophenotype, function and clinical application. The current study applied single-cell RNA-sequencing to profile human adult bone marrow populations from 11 donors and identified novel targets for SSC enrichment. Spherical nucleic acids were used to detect these mRNA targets in SSCs. This methodology was able to rapidly isolate potential SSCs found at a frequency of <1 in 1,000,000 in human bone marrow, with the capacity for tri-lineage differentiation in vitro and ectopic bone formation in vivo. The current studies detail the development of a platform to advance SSC enrichment from human bone marrow, offering an invaluable resource for further SSC characterisation, with significant therapeutic impact therein.

Guidelines for establishing a 3-D printing biofabrication laboratory

Advanced manufacturing and 3D printing are transformative technologies currently undergoing rapid adoption in healthcare, a traditionally non-manufacturing sector. Recent development in this field, largely enabled by merging different disciplines, has led to important clinical applications from anatomical models to regenerative bioscaffolding and devices. Although much research to-date has focussed on materials, designs, processes, and products, little attention has been given to the design and requirements of facilities for enabling clinically relevant biofabrication solutions. These facilities are critical to overcoming the major hurdles to clinical translation, including solving important issues such as reproducibility, quality control, regulations, and commercialization. To improve process uniformity and ensure consistent development and production, large-scale manufacturing of engineered tissues and organs will require standardized facilities, equipment, qualification processes, automation, and information systems. This review presents current and forward-thinking guidelines to help design biofabrication laboratories engaged in engineering model and tissue constructs for therapeutic and non-therapeutic applications.

Native-Osteoarthritic Joint Resident Stem and Progenitor Cells for Cartilage Cell-Based Therapies: A Quantitative Comparison With Respect to Concentration and Biological Performance

BACKGROUND

Cell-based therapy for cartilage repair is a promising approach and is becoming an established technique. Yet, there is no consensus on the optimal cell source.

PURPOSE

To provide a donor-matched quantitative comparison of the connective tissue progenitors (CTPs) derived from cartilage (Outerbridge grade 1-3 [G1-2-3]), bone marrow aspirate concentrate (BMC), infrapatellar fat pad (IPFP), synovium, and periosteum with respect to (1) cell concentration ([Cell], cells/mL), (2) CTP prevalence (P_CTP, colonies per million cells), and (3) biological performance based on in vitro proliferation potential (cells per colony) colony density, and differentiation potential (expression of negatively charged extracellular matrix: glycosaminoglycan-rich extra cellular matrix [GAG-ECM]).

STUDY DESIGN

Descriptive laboratory study.

METHODS

Tissues were obtained from 10 patients undergoing total knee arthroplasty (mean age, 59 years; women, n = 6). Automated quantitative colony-forming unit analysis was used to compare [Cell], P_CTP, and CTP biological performance across tissue sources.

RESULTS

[Cell] was highest in grade 3 cartilage (P = .002) and BMC (P = .001). Median P_CTP was highest in IPFP (P = .001), synovium (P = .003), and G1-2 cartilage (P = .02). Proliferation was highest in synovium-derived CTPs (P < .001). Median colony density was highest in G1-2-3 (P < .001). Median GAG-ECM was highest in G1-2-3 (P < .001). Within each patient, CTPs derived from all tissues were highly heterogeneous in biological performance as determined by cells per colony, density, and GAG-ECM.

CONCLUSION

Tissue sources differ in [Cell], P_CTP, and biological attributes. The data presented in this study suggest that cartilage (G1-2-3) is the preferred tissue source for cartilage repair based on P_CTP and GAG-ECM, followed by synovium, IPFP, BMC, and periosteum. However, due to the heterogeneous mixture of CTPs within each tissue source, there exists a subset of CTPs with biological performance similar to G1-2-3 cartilage, particularly in synovium and IPFP. Performance-based clonal selection and expansion of preferred CTPs and their progeny will potentially lead to improved cell population with predictive future.

CLINICAL RELEVANCE

Optimal tissue regeneration strategies will require informed decisions regarding which of the available tissue sources to use. Optimizing cell sourcing in any tissue may require separation of CTPs with preferred attributes from those with less desirable attributes. The heterogeneity manifest in the early stage of colony formation represents an opportunity for performance-based clone selection for clinical cell processing and manufacturing.

Primary Cells Isolated from Human Knee Cartilage Reveal Decreased Prevalence of Progenitor Cells but Comparable Biological Potential During Osteoarthritic Disease Progression

BACKGROUND

Current decisions on cellular therapies for osteoarthritis are based primarily on clinical experience or on assumptions about preferred cell sourcing. They have not been informed by rigorous standardized measurements of the chondrogenic connective-tissue progenitors (CTP-Cs) or their intrinsic diversity of chondrogenic potential. The goal of this study was to quantitatively define the CTP-Cs resident in cartilage of different grades of osteoarthritis and to compare their concentration, prevalence, and biological potential.

METHODS

Twenty-three patients who had varus malalignment of the knee and were scheduled to undergo elective total knee arthroplasty for idiopathic osteoarthritis and who had grade 1-2 osteoarthritis on the lateral femoral condyle and grade 3-4 osteoarthritis on the medial femoral condyle were recruited for study of the cartilage removed during surgery. CTP-Cs were assayed by a standardized colony-forming-unit assay using automated image-analysis software based on ASTM standard test method F2944-12.

RESULTS

Cell concentration was significantly greater (p < 0.001) in grade 3-4 cartilage than in grade 1-2 cartilage. The prevalence of CTP-Cs varied widely, but it trended lower in grade 3-4 cartilage than in grade 1-2 samples (p = 0.078). The biological performance of CTP-Cs from grade 1-2 and grade 3-4 cartilage was comparable. Increased cell concentration was a significant predictor of decreased CTP-C prevalence (p = 0.002).

CONCLUSIONS

Although grade 3-4 cartilage showed fewer CTP-Cs than grade 1-2 cartilage, the range of biological performance was comparable, which suggests that either may be used as a source for potent CTP-Cs. However, the biological reason for the heterogeneity of CTP-Cs in cartilage and the biological implications of that heterogeneity are not well understood and require further study.

CLINICAL RELEVANCE

In order to improve the efficacy of cartilage cell therapy procedures, it is key to characterize the quality and quantity of the cells and progenitors being administered. Additionally, understanding the heterogeneity in order to select appropriate subsets of populations will improve the rigor of decisions concerning cell sourcing and targeting for pharmacological and cellular therapies.

Re-using blood products as an alternative supplement in the optimisation of clinical-grade adipose-derived mesenchymal stem cell culture

Objectives

Adipose-derived mesenchymal stem cells (ADMSCs) are a promising strategy for orthopaedic applications, particularly in bone repair. Ex vivo expansion of ADMSCs is required to obtain sufficient cell numbers. Xenogenic supplements should be avoided in order to minimise the risk of infections and immunological reactions. Human platelet lysate and human plasma may be an excellent material source for ADMSC expansion. In the present study, use of blood products after their recommended transfusion date to prepare human platelet lysate (HPL) and human plasma (Hplasma) was evaluated for in vitro culture expansion and osteogenesis of ADMSCs.

Methods

Human ADMSCs were cultured in medium supplemented with HPL, Hplasma and a combination of HPL and Hplasma (HPL+Hplasma). Characteristics of these ADMSCs, including osteogenesis, were evaluated in comparison with those cultured in fetal bovine serum (FBS).

Results

HPL and HPL+Hplasma had a significantly greater growth-promoting effect than FBS, while Hplasma exhibited a similar growth-promoting effect to that of FBS. ADMSCs cultured in HPL and/or Hplasma generated more colony-forming unit fibroblasts (CFU-F) than those cultured in FBS. After long-term culture, ADMSCs cultured in HPL and/or Hplasma showed reduced cellular senescence, retained typical cell phenotypes, and retained differentiation capacities into osteogenic and adipogenic lineages.

Conclusion

HPL and Hplasma prepared from blood products after their recommended transfusion date can be used as an alternative and effective source for large-scale ex vivo expansion of ADMSCs.

Cite this article: J. Phetfong, T. Tawonsawatruk, K. Seenprachawong, A. Srisarin, C. Isarankura-Na-Ayudhya, A. Supokawej. Re-using blood products as an alternative supplement in the optimisation of clinical-grade adipose-derived mesenchymal stem cell culture. Bone Joint Res 2017;6:414–422. DOI: 10.1302/2046-3758.67.BJR-2016-0342.R1.

Cell therapy for heart disease after 15 years: Unmet expectations

Over the past two decades cardiac cell therapy (CCT) has emerged as a promising new strategy to cure heart diseases at high unmet need. Thousands of patients have entered clinical trials for acute or chronic heart conditions testing different cell types, including autologous or allogeneic bone marrow (BM)-derived mononuclear or selected cells, BM- or adipose tissue-derived mesenchymal cells, or cardiac resident progenitors based on their potential ability to regenerate scarred or dysfunctional myocardium. Nowadays, the original enthusiasm surrounding the regenerative medicine field has been cushioned by a cumulative body of evidence indicating an inefficient or modest efficacy of CCT in improving cardiac function, along with the continued lack of indisputable proof for long-term prognostic benefit. In this review, we have firstly comprehensively outlined the positive and negative results of cell therapy studies in patients with acute myocardial infarction, refractory angina and chronic heart failure. Next, we have discussed cell therapy- and patient-related variables (e.g. cell intrinsic and extrinsic characteristics as well as criteria of patient selection and proposed methodologies) that might have dampened the efficacy of past cell therapy trials. Finally, we have addressed critical factors to be considered before embarking on further clinical trials.

Repair of Torn Avascular Meniscal Cartilage Using Undifferentiated Autologous Mesenchymal Stem Cells: From In Vitro Optimization to a First-in-Human Study

Meniscal cartilage tears are common and predispose to osteoarthritis (OA). Most occur in the avascular portion of the meniscus where current repair techniques usually fail. We described previously the use of undifferentiated autologous mesenchymal stem cells (MSCs) seeded onto a collagen scaffold (MSC/collagen‐scaffold) to integrate meniscal tissues in vitro. Our objective was to translate this method into a cell therapy for patients with torn meniscus, with the long‐term goal of delaying or preventing the onset of OA. After in vitro optimization, we tested an ovine‐MSC/collagen‐scaffold in a sheep meniscal cartilage tear model with promising results after 13 weeks, although repair was not sustained over 6 months. We then conducted a single center, prospective, open‐label first‐in‐human safety study of patients with an avascular meniscal tear. Autologous MSCs were isolated from an iliac crest bone marrow biopsy, expanded and seeded into the collagen scaffold. The resulting human‐MSC/collagen‐scaffold implant was placed into the meniscal tear prior to repair with vertical mattress sutures and the patients were followed for 2 years. Five patients were treated and there was significant clinical improvement on repeated measures analysis. Three were asymptomatic at 24 months with no magnetic resonance imaging evidence of recurrent tear and clinical improvement in knee function scores. Two required subsequent meniscectomy due to retear or nonhealing of the meniscal tear at approximately 15 months after implantation. No other adverse events occurred. We conclude that undifferentiated MSCs could provide a safe way to augment avascular meniscal repair in some patients. Registration: EU Clinical Trials Register, 2010‐024162‐22. Stem Cells Translational Medicine2017;6:1237–1248

Tumorigenicity analysis of heterogeneous dental stem cells and its self-modification for chromosome instability

Heterogeneity demonstrates that stem cells are constituted by several sub-clones in various differentiation states. The heterogeneous state is maintained by cross-talk among sub-clones, thereby ensuring stem cell adaption. In this study, we investigated the roles of heterogeneity on genetic stability. Three sub-clones (DF2, DF8 and DF18) were isolated from heterogeneous dental stem cells (DSCs), and were proved to be chromosome instability (CIN) after long term expansion. Cell apoptosis were not detected in sub-clones, which exhibited strong tumorigenesis tendency, coupled with weak expression of p53 and aberrant ultra-structure. However, 3 sub-clones did not overexpress tumor related markers or induce tumorigenesis in vivo. The mixed-culture study suggested that 3-clone-mixed culturing cells (DF1) presented apparent decrease in the ratio of aneuploidy. The screening experiment further proved that 3 sub-clones functioned separately in this modification procedure but only mixed culturing all 3 sub-clones, simulated heterogeneous microenvironment, could achieve complete modification. Additionally, osteogenesis capability of 3 sub-clones was partially influenced by CIN while DSCs still kept stronger osteogenesis than sub-clones. These results suggested aberrant sub-clones isolated from heterogeneous DSCs were not tumorigenesis and could modify CIN by cross-talk among themselves, indicating that the heterogeneity played a key role in maintaining genetic stability and differentiation capability in dental stem cells.

Study of the involvement of allogeneic MSCs in bone formation using the model of transgenic mice

Mesenchymal stem cells (MSCs) are thought to be the most attractive type of cells for bone repair. However, much still remains unknown about MSCs and needs to be clarified before this treatment can be widely applied in the clinical practice. The purpose of this study was to establish the involvement of allogeneic MSCs in the bone formation in vivo, using a model of transgenic mice and genetically labeled cells. Polylactide scaffolds with hydroxyapatite obtained by surface selective laser sintering were used. The scaffolds were sterilized and individually seeded with MSCs from the bone marrow of 5-week-old GFP(+) transgenic C57/Bl6 or GFP(-)C57/Bl6 mice. 4-mm-diameter critical-size defects were created on the calvarial bone of mice using a dental bur. Immediately after the generation of the cranial bone defects, the scaffolds with or without seeded cells were implanted into the injury sites. The cranial bones were harvested at either 6 or 12 weeks after the implantation. GFP(+) transgenic mice having scaffolds with unlabeled MSCs were used for the observation of the host cell migration into the scaffold. GFP(-) mice having scaffolds with GFP(+)MSCs were used to assess the functioning of the seeded MSCs. The obtained data demonstrated that allogeneic MSCs were found on the scaffolds 6 and 12 weeks post-implantation. By week 12, a newly formed bone tissue from the seeded cells was observed, without an osteogenic pre-differentiation. The host cells did not appear, and the control scaffolds without seeded cells remained empty. Besides, a possibility of vessel formation from seeded MSCs was shown, without a preliminary cell cultivation under controlled conditions.

Transient Canonical Wnt Stimulation Enriches Human Bone Marrow Mononuclear Cell Isolates for Osteoprogenitors

Activation of the canonical Wnt signaling pathway is an attractive anabolic therapeutic strategy for bone. Emerging data suggest that activation of the Wnt signaling pathway promotes bone mineral accrual in osteoporotic patients. The effect of Wnt stimulation in fracture healing is less clear as Wnt signaling has both stimulatory and inhibitory effects on osteogenesis. Here, we tested the hypothesis that transient Wnt stimulation promotes the expansion and osteogenesis of a Wnt‐responsive stem cell population present in human bone marrow. Bone marrow mononuclear cells (BMMNCs) were isolated from patients undergoing hip arthroplasty and exposed to Wnt3A protein. The effect of Wnt pathway stimulation was determined by measuring the frequency of stem cells within the BMMNC populations by fluorescence‐activated cell sorting and colony forming unit fibroblast (CFU‐F) assays, before determining their osteogenic capacity in in vitro differentiation experiments. We found that putative skeletal stem cells in BMMNC isolates exhibited elevated Wnt pathway activity compared with the population as whole. Wnt stimulation resulted in an increase in the frequency of skeletal stem cells marked by the STRO‐1^bright/Glycophorin A⁻ phenotype. Osteogenesis was elevated in stromal cell populations arising from BMMNCs transiently stimulated by Wnt3A protein, but sustained stimulation inhibited osteogenesis in a concentration‐dependent manner. These results demonstrate that Wnt stimulation could be used as a therapeutic approach by transient targeting of stem cell populations during early fracture healing, but that inappropriate stimulation may prevent osteogenesis. Stem Cells2016;34:418–430

Deterministic and stochastic approaches in the clinical application of mesenchymal stromal cells (MSCs)

Mesenchymal stromal cells (MSCs) have enormous intrinsic clinical value due to their multi-lineage differentiation capacity, support of hemopoiesis, immunoregulation and growth factors/cytokines secretion. MSCs have thus been the object of extensive research for decades. After completion of many pre-clinical and clinical trials, MSC-based therapy is now facing a challenging phase. Several clinical trials have reported moderate, non-durable benefits, which caused initial enthusiasm to wane, and indicated an urgent need to optimize the efficacy of therapeutic, platform-enhancing MSC-based treatment. Recent investigations suggest the presence of multiple in vivo MSC ancestors in a wide range of tissues, which contribute to the heterogeneity of the starting material for the expansion of MSCs. This variability in the MSC culture-initiating cell population, together with the different types of enrichment/isolation and cultivation protocols applied, are hampering progress in the definition of MSC-based therapies. International regulatory statements require a precise risk/benefit analysis, ensuring the safety and efficacy of treatments. GMP validation allows for quality certification, but the prediction of a clinical outcome after MSC-based therapy is correlated not only to the possible morbidity derived by cell production process, but also to the biology of the MSCs themselves, which is highly sensible to unpredictable fluctuation of isolating and culture conditions. Risk exposure and efficacy of MSC-based therapies should be evaluated by pre-clinical studies, but the batch-to-batch variability of the final medicinal product could significantly limit the predictability of these studies. The future success of MSC-based therapies could lie not only in rational optimization of therapeutic strategies, but also in a stochastic approach during the assessment of benefit and risk factors.

Are MSCs angiogenic cells? New insights on human nestin-positive bone marrow-derived multipotent cells

Recent investigations have made considerable progress in the understanding of tissue regeneration driven by mesenchymal stromal cells (MSCs). Data indicate the anatomical location of MSC as residing in the “perivascular” space of blood vessels dispersed across the whole body. This histological localization suggests that MSCs contribute to the formation of new blood vessels in vivo. Indeed, MSCs can release angiogenic factors and protease to facilitate blood vessel formation and in vitro are able to promote/support angiogenesis. However, the direct differentiation of MCSs into endothelial cells is still matter of debate. Most of the conflicting data might arise from the presence of multiple subtypes of cells with heterogeneous morpho functional features within the MSC cultures. According to this scenario, we hypothesize that the presence of the recently described Mesodermal Progenitor Cells (MPCs) within the MSCs cultures is responsible for their variable angiogenic potential. Indeed, MPCs are Nestin-positive CD31-positive cells exhibiting angiogenic potential that differentiate in MSC upon proper stimuli. The ISCT criteria do not account for the presence of MPC within MSC culture generating confusion in the interpretation of MSC angiogenic potential. In conclusion, the discovery of MPC gives new insight in defining MSC ancestors in human bone marrow, and indicates the tunica intima as a further, and previously overlooked, possible additional source of MSC.

Assessing the potential of colony morphology for dissecting the CFU-F population from human bone marrow stromal cells

Mesenchymal stem cells (MSCs) provide an ideal cell source for bone tissue engineering strategies. However, bone marrow stromal cell (BMSC) populations that contain MSCs are highly heterogeneous expressing a wide variety of proliferative and differentiation potentials. Current MSC isolation methods employing magnetic-activated and fluorescent-activated cell sorting can be expensive and time consuming and, in the absence of specific MSC markers, fail to generate homogeneous populations. We have investigated the potential of various colony morphology descriptors to provide correlations with cell growth potential. Density-independent colony forming unit-fibroblastic (CFU-F) capacity is a MSC prerequisite and resultant colonies display an array of shapes and sizes that might be representative of cell function. Parent colonies were initially categorised according to their diameter and cell density and grouped before passage for the subsequent assessment of progeny colonies. Whereas significant morphological differences between distinct parent populations indicated a correlation with immunophenotype, enhanced CFU-F capacity was not observed when individual colonies were isolated according to these morphological parameters. Colony circularity, an alternative morphological measure, displayed a strong correlation with subsequent cell growth potential. The current study indicates the potential of morphological descriptors for predicting cell growth rate and suggests new directions for research into dissection of human BMSC CFU-F populations.

Therapeutic applications of mesenchymal stem cells: current outlook

The past decade has seen tremendous growth in the clinical application of cell-based therapies, and the number of planned human clinical trials to evaluate these therapies continues to increase in number and scope at a rapid pace. A considerable effort on this front has been devoted to evaluating the therapeutic potential of mesenchymal stem cells (MSCs), which were initially characterized as connective tissue progenitors resident in bone marrow. MSCs are now known to possess potent tissue reparative properties that have been linked to secretion of paracrine-acting angiogenic, trophic, anti-inflammatory, and immunomodulatory factors. Accordingly, MSC-based therapies are being evaluated for the treatment of a broad array of ischemic, inflammatory, and immunological disorders. Nevertheless, knowledge regarding how the wide-ranging activities of MSCs vary between and are specified within populations remains largely unexplored. Lack of such knowledge makes it difficult to predict and/or control how sampling bias and ex vivo expansion of populations alters their biological activity and therapeutic potency. Herein, we discuss how heterogeneity of MSC populations may explain, in part, disparate outcomes in both experimental animal and human clinical trial data, and discuss several strategies to achieve more reproducible and efficacious outcomes for MSC-based therapies.

Functional heterogeneity of mesenchymal stem cells: implications for cell therapy

The term mesenchymal stem cell (MSCs) was adopted in the 1990s to describe a population of bone-marrow-derived cells that demonstrated the capacity for tri-lineage differentiation at a clonal level. Research conducted during the ensuing decades has demonstrated that MSCs fulfill many functions in addition to connective tissue progenitors including contributing to the HSC niche and regulating the function of immune effector cells of both the innate and adaptive immune system. Despite these advances, fundamental aspects of MSC biology remain indeterminate. For example, the embryonic origin of MSCs and their niche in vivo remains a highly debated topic. More importantly, the mechanisms that regulate self-renewal and lineage specification have also been largely unexplored. The later is significant in that MSC population's exhibit considerable donor-to-donor and intra-population heterogeneity but knowledge regarding how different functional attributes of MSCs are specified at the population level is unknown. This poses significant obstacles in research and in efforts to develop clinical manufacturing protocols that reproducibly generate functionally equivalent MSC populations. Herein, I discuss data demonstrating that MSC populations are intrinsically heterogeneous, elaborate on the molecular basis for this heterogeneity, and discuss how heterogeneity impacts clinical manufacturing and the therapeutic potency of MSCs.

Time-lapse microscopy and classification of 2D human mesenchymal stem cells based on cell shape picks up myogenic from osteogenic and adipogenic differentiation

Current methods to characterize mesenchymal stem cells (MSCs) are limited to CD marker expression, plastic adherence and their ability to differentiate into adipogenic, osteogenic and chondrogenic precursors. It seems evident that stem cells undergoing differentiation should differ in many aspects, such as morphology and possibly also behaviour; however, such a correlation has not yet been exploited for fate prediction of MSCs. Primary human MSCs from bone marrow were expanded and pelleted to form high-density cultures and were then randomly divided into four groups to differentiate into adipogenic, osteogenic chondrogenic and myogenic progenitor cells. The cells were expanded as heterogeneous and tracked with time-lapse microscopy to record cell shape, using phase-contrast microscopy. The cells were segmented using a custom-made image-processing pipeline. Seven morphological features were extracted for each of the segmented cells. Statistical analysis was performed on the seven-dimensional feature vectors, using a tree-like classification method. Differentiation of cells was monitored with key marker genes and histology. Cells in differentiation media were expressing the key genes for each of the three pathways after 21 days, i.e. adipogenic, osteogenic and chondrogenic, which was also confirmed by histological staining. Time-lapse microscopy data were obtained and contained new evidence that two cell shape features, eccentricity and filopodia (= 'fingers') are highly informative to classify myogenic differentiation from all others. However, no robust classifiers could be identified for the other cell differentiation paths. The results suggest that non-invasive automated time-lapse microscopy could potentially be used to predict the stem cell fate of hMSCs for clinical application, based on morphology for earlier time-points. The classification is challenged by cell density, proliferation and possible unknown donor-specific factors, which affect the performance of morphology-based approaches.

Cell sources for bone tissue engineering: insights from basic science

One of the goals of bone tissue engineering is to design delivery methods for skeletal stem/progenitor cells to repair or replace bone. Although the materials used to retain cells play a central role in the quality of the constructs, the source of cells is key for bone regeneration. Bone marrow is the most common cell source, but other tissues are now being explored, such as the periosteum, fat, muscle, cord blood, and embryonic or induced pluripotent stem cells. The therapeutic effect of exogenous stem/progenitor cells is accepted, yet their contribution to bone repair is not well defined. The in vitro osteo- and/or chondrogenic potential of these skeletal progenitors do not necessarily predict their differentiation potential in vivo and their function may be affected by their ability to home correctly to bone. This review provides an overview of animal models used to test the efficacy of cell-based approaches. We examine the mechanisms of endogenous cell recruitment during bone repair and compare the role of local versus systemic cell recruitment. We discuss how the normal repair process can help define efficacious cell sources for bone tissue engineering and improve their methods of delivery.

In search of the skeletal stem cell: isolation and separation strategies at the macro/micro scale for skeletal regeneration

Skeletal stem cells (SSCs) show great capacity for bone and cartilage repair however, current in vitro cultures are heterogeneous displaying a hierarchy of differentiation potential. SSCs represent the diminutive true multipotent stem cell fraction of bone marrow mononuclear cell (BMMNC) populations. Endeavours to isolate SSCs have generated a multitude of separation methodologies. SSCs were first identified and isolated by their ability to adhere to culture plastic. Once isolated, further separation is achieved via culture in selective or conditioned media (CM). Indeed, preferential SSC growth has been demonstrated through selective in vitro culture conditions. Other approaches have utilised cell morphology (size and shape) as selection criteria. Studies have also targeted SSCs based on their preferential adhesion to specified compounds, individually or in combination, on both macro and microscale platforms. Nevertheless, most of these methods which represent macroscale function with relatively high throughput, yield insufficient purity. Consequently, research has sought to downsize isolation methodologies to the microscale for single cell analysis. The central approach is identification of the requisite cell populations of SSC-specific surface markers that can be targeted for isolation by either positive or negative selection. SELEX and phage display technology provide apt means to sift through substantial numbers of candidate markers. In contrast, single cell analysis is the paramount advantage of microfluidics, a relatively new field for cell biology. Here cells can be separated under continuous or discontinuous flow according to intrinsic phenotypic and physicochemical properties. The combination of macroscale quantity with microscale specificity to generate robust high-throughput (HT) technology for pure SSC sorting, isolation and enrichment offers significant implications therein for skeletal regenerative strategies as a consequence of lab on chip derived methodology.