Mesenchymal stem cells in cartilage repair: state of the art and methods to monitor cell growth, differentiation and cartilage regeneration.

Degenerative joint diseases caused by rheumatism, joint dysplasia or traumata are particularly widespread in countries with high life expectation. Although there is no absolutely convincing cure available so far, hyaline cartilage and bone defects resulting from joint destruction can be treated today by appropriate transplantations. Recently, procedures were developed based on autologous chondrocytes from intact joint areas. The chondrocytes are expanded in cell culture and subsequently transplanted into the defect areas of the affected joints. However, these autologous chondrocytes are characterized by low expansion capacity and the synthesis of extracellular matrix of poor functionality and quality. An alternative approach is the use of adult mesenchymal stem cells (MSCs). These cells effectively expand in 2D culture and have the potential to differentiate into various cell types, including chondrocytes. Furthermore, they have the ability to synthesize extracellular matrix with properties mimicking closely the healthy hyaline joint cartilage. Beside a more general survey of the architecture of hyaline cartilage, its composition and the pathological processes of joint diseases, we will describe here which advances were achieved recently regarding the development of closed, aseptic bioreactors for the production of autologous grafts for cartilage regeneration based on MSCs. Additionally, a novel mathematical model will be presented that supports the understanding of the growth and differentiation of MSCs. It will be particularly emphasized that such models are helpful to explain the well-known fact that MSCs exhibit improved growth properties under reduced oxygen pressure and limited supply with nutrients. Finally, it will be comprehensively shown how different analytical methods can be used to characterize MSCs on different levels. Besides discussing methods for non-invasive monitoring and tracking of the cells and the determination of their elastic properties, mass spectrometric methods to evaluate the lipid compositions of cells will be highlighted.

Impact of oxygen environment on mesenchymal stem cell expansion and chondrogenic differentiation.

INTRODUCTION: In vitro expansion and differentiation of mesenchymal stem cells (MSC) rely on specific environmental conditions, and investigations have demonstrated that one crucial factor is oxygen environment. OBJECTIVES: In order to understand the impact of oxygen tension on MSC culture and chondrogenic differentiation in vitro, we developed a mathematical model of these processes and applied it in predicting optimal assays. METHODS AND RESULTS: We compared ovine MSCs under physiologically low and atmospheric oxygen tension. Low oxygen tension improved their in vitro population growth as demonstrated by monoclonal expansion and colony forming assays. Moreover, it accelerated induction of the chondrogenic phenotype in subsequent three-dimensional differentiation cultures. We introduced a hybrid stochastic multiscale model of MSC organization in vitro. The model assumes that cell adaptation to non-physiological high oxygen tension reversibly changes the structure of MSC populations with respect to differentiation. In simulation series, we demonstrated that these changes profoundly affect chondrogenic potential of the populations. Our mathematical model provides a consistent explanation of our experimental findings. CONCLUSIONS: Our approach provides new insights into organization of MSC populations in vitro. The results suggest that MSC differentiation is largely reversible and that lineage plasticity is restricted to stem cells and early progenitors. The model predicts a significant impact of short-term low oxygen treatment on MSC differentiation and optimal chondrogenic differentiation at 10-11% pO(2).