Reorganization of Actin Cytoskeleton and Microtubule Array during the Chondrogenesis of Bovine MSCs

Mesenchymal stem cells (MSCs) are multipotent stem cells that are capable of self-renewal and can be committed into classical mesodermal tri-lineage differentiation (adipocytes, osteocytes and chondrocytes). During chondrogenic differentiation MSCs change their shape due to the reorganization of cytoskeletal components. This has been well documented for human and rodent models. Morphological changes of microtubule network and actin filaments that occur during the chondrogenic differentiation of MSCs from large animal models remain unknown. In this study we described the morphological changes of cell shape, area, actin structures and microtubule array that occur in bovine MSCs during the chondrogenic differentiation of bovine bone-marrow isolated MSCs. Chondrogenic differentiation of bMSCs occur more rapidly on glass substrate compared to the cells plated on vitronectin, and in 7 days after the commitment we observed clusters of small round-shaped cells that expressed glycosaminoglycans. During the differentiation microtubule (MT) array of MSCs became non-radial, and non-centrosomal MTs that grew transversely to the cell radius appeared in the inner cytoplasm and near the cell edges. At the end of differentiation process we observed the thick bundles of MTs that grew in parallel to the cell edge and basket-like structures of curved MTs around the nucleus. The main changes of actin structures in differentiating MSCs included the disappearance of thick transverse stress fibers and actin arches and reorganization of actin into chaotic network of thin cortical fibers. Our results imply the important role of both actin and MT cytoskeletal systems in chondrogenesis and reveals new perspectives for experimental regulation of these process in vitro systems.