The main purpose of this article was to describe the morphology of mesenchymal stem cells (MSCs) differentiated in vitro towards osteogenic and chondrogenic lineages and to focus on the ultrastructural features associated with these processes. Human mononuclear cells (hMNC) were isolated, expanded, and analyzed for the expression of specific cell surface markers to demonstrate their stem cell characteristics. Human mononuclear cells were differentiated in vitro in an osteogenic and in a chondrogenic sense for 7, 14, 21, and 28 days. Subsequently, they were processed using electron microscopic analysis (FEISEM). Alizarin red and alcian blue staining were carried out to demonstrate the deposition of mineral salts and proteoglycans in the extracellular matrix. Undifferentiated MSCs showed a cell surface covered by filopodia and ondulopodia. During differentiation, the MSCs changed their shape from a round to a fibroblastic-like shape. At the end of the differentiation, several filaments with a parallel orientation in the osteogenic samples as well as a network organization in the chondrogenic samples were detected in the extracellular spaces. This study demonstrated that there are morphological features associated with the undifferentiated and differentiated states of the MSCs, which could be utilized as new parameters for identifying and classifying these cells.
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