Effekte mechanischer Reize auf humane osteoblastäre Zellen in einer dreidimensionalen Kollagen-Typ-I-Matrix

Application of different strain regimes in two-dimensional and three-dimensional adipose tissue-derived stem cell cultures induces osteogenesis: implications for bone tissue engineering

Mechanical strain has become an important tool in tissue engineering for progenitor cell differentiation. Furthermore, it is used to enhance the mechanical properties of engineered tissue constructs. Although strain amplitude and frequency are well investigated and optimal values are known; application of various strain schemes regarding duration and repetition are not described in literature. In this study, we therefore applied singular and repetitive cyclic strain (1 Hz, 5%) of 15 min short-time strain and longer strain durations up to 8 h. Additionally, a gradually increasing strain scheme starting with short-time strain and consecutive elongated strain periods was applied. The cultivation surface was planar silicone on one hand and a three-dimensionally structured collagen I mesh on the other hand. Adipose tissue-derived mesenchymal stem cells and an osteogenic model cell line (MG-63) were exposed to these strain regimes and post-strain cell viability, osteogenic marker gene expression, and matrix mineralization were investigated. Upregulation of alkaline phosphatase, osteocalcin, osteopontin, and BMP-2/4 revealed that even short-time strain can enhance osteogenic differentiation. Elongation and repetition of strain, however, resulted in a decline of the observed short-time strain effects, which we interpret as positively induced cellular adaptation to the mechanically active surroundings. With regard to cellular adaptation, the gradually increasing strain scheme was especially advantageous.

A uniaxial bioMEMS device for imaging single cell response during quantitative force-displacement measurements

A microfabricated device has been developed for imaging of a single, adherent cell while quantifying force under an applied displacement. The device works in a fashion similar to that of a displacement-controlled uniaxial tensile machine. The device was calibrated using a tipless atomic force microscope (AFM) cantilever and shows excellent agreement with the calculated spring constant. A step input was applied to a single, adherent fibroblast cell and the viscoelastic response was characterized with a mechanical model. The adherent fibroblast was imaged by use of epifluorescence and phase contrast techniques.

Finite element modeling of 3D human mesenchymal stem cell-seeded collagen matrices exposed to tensile strain

The use of human mesenchymal stem cells (hMSCs) in tissue engineering is attractive due to their ability to extensively self-replicate and differentiate into a multitude of cell lineages. It has been experimentally established that hMSCs are influenced by chemical and mechanical signals. However, the combined chemical and mechanical in vitro culture conditions that lead to functional tissue require greater understanding. In this study, finite element models were created to evaluate the local loading conditions on bone marrow-derived hMSCs seeded in three-dimensional collagen matrices exposed to cyclic tensile strain. Mechanical property and geometry data used in the models were obtained experimentally from a previous study in our laboratory and from mechanical testing. Eight finite element models were created to simulate three-dimensional hMSC-seeded collagen matrices exposed to different levels of cyclic tensile strain (10% and 12%), culture media (complete growth and osteogenic differentiating), and durations of culture (7 and 14 days). Through finite element analysis, it was determined that globally applied uniaxial tensile strains of 10% and 12% resulted in local strains up to 18.3% and 21.8%, respectively. Model results were also compared to experimental studies in an attempt to explain observed differences between hMSC response to 10% and 12% cyclic tensile strain.

Mechanical strain enhances extracellular matrix-induced gene focusing and promotes osteogenic differentiation of human mesenchymal stem cells through an extracellular-related kinase-dependent pathway

Human mesenchymal stem cells (hMSCs) are a population of multipotent bone marrow cells capable of differentiating along multiple lineages, including bone. Our recently published proteomics studies suggest that focusing of gene expression is the basis of hMSC osteogenic transdifferentiation, and that extracellular matrix proteins play an important role in controlling this focusing. Here, we show that application of a 3-5% tensile strain to a collagen I substrate stimulates osteogenesis in the attached hMSCs through gene focusing via a MAP kinase signaling pathway. Mechanical strain increases expression levels of well-established osteogenic marker genes while simultaneously reducing expression levels of marker genes from three alternate lineages (chondrogenic, adipogenic, and neurogenic). Mechanical strain also increases matrix mineralization (a hallmark of osteogenic differentiation) and activation of extracellular signal-related kinase 1/2 (ERK). Addition of the MEK inhibitor PD98059 to reduce ERK activation decreases osteogenic gene expression and matrix mineralization while also blocking strain-induced down-regulation of nonosteogenic lineage marker genes. These results demonstrate that mechanical strain enhances collagen I-induced gene focusing and osteogenic differentiation in hMSCs through the ERK MAP kinase signal transduction pathway.

A uniaxial bioMEMS device for quantitative force-displacement measurements

There is a need for experimental techniques that allow the simultaneous imaging of cellular cystoskeletal components with quantitative force measurements on single cells. A bioMEMS device has been developed for the application of strain to a single cell while simultaneously quantifying its force response. The prototype device presented here allows the mechanical study of a single, adherent cell in vitro. The device works in a fashion similar to a displacement-controlled uniaxial tensile machine. The device is calibrated using an AFM cantilever and shows excellent agreement with the calculated spring constant. The device is demonstrated on a single fibroblast. The force response of the cell is seen to be linear until the onset of de-adhesion with the de-adhesion from the cell platform occurring at a force of approximately 1500 nN.