Decreased CD90 expression in human mesenchymal stem cells by applying mechanical stimulation

Human MSC were cultured in osteoinductive medium with or without cyclic uniaxial mechanical stimulation (2000 μstrain, 200 cycles per day, 1 Hz). Cultured cells were analysed for expression of collagen type I, osteocalcin, osteonectin, and CD90. To evaluate the biomineral formation the content of bound calcium in the cultures was determined.After 14 days in culture immunfluorescence staining revealed enhancement of collagen type I and osteonectin expression in response to mechanical stimulation. In contrast, mechanically stimulated cultures stained negative for CD90. In stimulated and unstimulated cultures an increase in the calcium content over time was observed. After 21 days in culture the calcium content in mechanical stimulated cultures was significantly higher compared to unstimulated control cultures.These results demonstrate the influence of mechanical force on the differentiation of human MSC into osteoblast-like cells in vitro. While significant enhancement of the biomineral formation by mechanical stimulation is not detected before 21 days, effects on the extracellular matrix became already obvious after 14 days. The decrease of CD90 expression in mechanically stimulated cultures compared to unstimulated control cultures suggests that CD90 is only transiently expressed expression during the differentiation of MSC to osteoblast-like cells in culture.Mesenchymal stem cells (MSC) are pluripotent cells with the ability to differentiate along osteogenic, chondrogenic, and adipogenic lineages [1]. MSC, first described by Friedenstein [2], have also been denoted as mesenchymal progenitor cells, fibroblast colony-forming units, colony-forming unit-fibroblasts and marrow stromal cells. The best studied and accessible source of MSC is the adult bone marrow. In contrast to hematopoietic stem cells (HSC), MSC lack an unique surface antigen for positive selection. Identification of MSC is based on differentiation properties and an extensive panel of monoclonal antibod