Chondrogenic Priming of Human Bone Marrow Stromal Cells: A Better Route to Bone Repair?

Chondrogenic pre-induction of human mesenchymal stem cells on beta-TCP: enhanced bone quality by endochondral heterotopic bone formation

New techniques to heal bone defects include the combination of bone substitute materials with mesenchymal stem cells (MSC). To find solutions not hampered by low material resorbability or high donor variability of human MSC, the potency of such composites is usually evaluated by heterotopic bone formation assays in immunocompromised animals. The aim of this study was to investigate whether resorbable phase-pure beta-tricalcium-phosphate (beta-TCP) could support heterotopic bone formation by MSC comparable to partially resorbable hydroxyapatite/tricalcium-phosphate (HA/TCP). Furthermore, in light of disappointing results with osteogenic in vitro priming of MSC, we tested whether chondrogenic pre-induction of constructs may allow for enhanced bone formation by triggering the endochondral pathway. beta-TCP granules of three different sizes and HA/TCP were seeded with MSC and transplanted subcutaneously into immunocompromised mice either immediately or after a chondrogenic pre-induction for 6 weeks. After 8 weeks, explants were analysed by histology. beta-TCP seeded with unprimed MSC revealed intramembranous bone formation without haematopoietic marrow with 3.8-fold more bone formed with granules smaller than 0.7 mm than with 0.7-1.4mm particles (p< or =0.018). Chondrogenic pre-induction of beta-TCP/MSC composites resulted in collagen type II and proteoglycan-rich cartilage-like tissue which, after transplantation, underwent endochondral ossification, yielding ectopic bone produced by human cells while haematopoietic marrow was derived from the mouse. Transdifferentiation of MSC-derived chondrocytes to osteoblasts or direct osteogenesis of cartilage-resident MSC is postulated to explain the human origin of new bone. In conclusion, beta-TCP was significantly more osteo-permissive (p=0.004) than HA/TCP for human MSC, and chondrogenic priming of beta-TCP/MSC represented a superior approach capable of supporting full bone formation, including marrow organization.

The role of hypoxia in bone marrow-derived mesenchymal stem cells: considerations for regenerative medicine approaches

Bone marrow-derived mesenchymal stem cells (MSCs) have demonstrated potential for regenerative medicine strategies. Knowledge of the way these cells respond to their environment in in vitro culture and after implantation in vivo is crucial for successful therapy. Oxygen tension plays a pivotal role in both situations. In vivo, a hypoxic environment can lead to apoptosis, but hypoxic preconditioning of MSCs and overexpression of prosurvival genes like Akt can reduce hypoxia-induced cell death. In cell culture, hypoxia can increase proliferation rates and enhance differentiation along the different mesenchymal lineages. Hypoxia also modulates the paracrine activity of MSCs, causing upregulation of various secretable factors, among which are important angiogenic factors such as vascular endothelial growth factor and interleukin-6 (IL6). Finally, hypoxia plays an important role in mobilization and homing of MSCs, primarily by its ability to induce stromal cell-derived factor-1 expression along with its receptor CXCR4. This article reviews the current literature on the effects of hypoxia on MSCs and aims to elucidate its potential role in regenerative medicine strategies.

Bone marrow stem cells in clinical application: harnessing paracrine roles and niche mechanisms

The being of any individual throughout life is a dynamic process relying on the capacity to retain processes of self-renewal and differentiation, both of which are hallmarks of stem cells. Although limited in the adult human organism, regeneration and repair do take place in virtue of the presence of adult stem cells. In the bone marrow, two major populations of stem cells govern the dynamic equilibrium of both hemopoiesis and skeletal homeostasis; the hematopoietic and the mesenchymal stem cells. Recent cell based clinical trials utilizing bone marrow-derived stem cells as therapeutic agents have revealed promising results, while others have failed to display as such. It is therefore imperative to strive to understand the mechanisms by which these cells function in vivo, how their properties can be maintained ex-vivo, and to explore further their recently highlighted immunomodulatory and trophic effects.

Cartilage repair: past and future--lessons for regenerative medicine

Since the first cell therapeutic study to repair articular cartilage defects in the knee in 1994, several clinical studies have been reported. An overview of the results of clinical studies did not conclusively show improvement over conventional methods, mainly because few studies reach level I of evidence for effects on middle or long term. However, these explorative trials have provided valuable information about study design, mechanisms of repair and clinical outcome and have revealed that much is still unknown and further improvements are required. Furthermore, cellular and molecular studies using new technologies such as cell tracking, gene arrays and proteomics have provided more insight in the cell biology and mechanisms of joint surface regeneration. Besides articular cartilage, cartilage of other anatomical locations as well as progenitor cells are now considered as alternative cell sources. Growth Factor research has revealed some information on optimal conditions to support cartilage repair. Thus, there is hope for improvement. In order to obtain more robust and reproducible results, more detailed information is needed on many aspects including the fate of the cells, choice of cell type and culture parameters. As for the clinical aspects, it becomes clear that careful selection of patient groups is an important input parameter that should be optimized for each application. In addition, the study outcome parameters should be improved. Although reduced pain and improved function are, from the patient's perspective, the most important outcomes, there is a need for more structure/tissue-related outcome measures. Ideally, criteria and/or markers to identify patients at risk and responders to treatment are the ultimate goal for these more sophisticated regenerative approaches in joint surface repair in particular, and regenerative medicine in general.