The Regulation of Differentiation in Mesenchymal Stem Cells

Mesenchymal stromal/stem cells (MSCs) are a population of stromal cells present in the bone marrow and most connective tissues, capable of differentiation into mesenchymal tissues such as bone and cartilage. MSCs are attractive candidates for biological cell-based tissue repair approaches because of their extensive proliferative ability in culture while retaining their mesenchymal multilineage differentiation potential. In addition to its undoubted scientific interest, the prospect of monitoring and controlling MSC differentiation is a crucial regulatory and clinical requirement. Hence, the molecular regulation of MSC differentiation has been extensively studied. Most of the studies are in vitro, because the identity of MSCs in their tissues of origin in vivo remains undefined. This review addresses the current knowledge of the molecular basis of differentiation of cultured MSCs, with a particular focus on chondrogenesis and osteogenesis. Building on the information coming from developmental biology studies of embryonic skeletogenesis, several signaling pathways and transcription factors have been investigated and shown to play critical roles in MSC differentiation. In particular, the Wnt and transforming growth factor-b/bone morphogenetic protein signaling pathways are well known to modulate in MSCs the molecular differentiation into cartilage and bone. Relevant to the emerging concept of stem cell niches is the demonstration that physical factors can also participate in the regulation of MSC differentiation. Knowledge of the regulation of MSC differentiation will be critical in the design of three-dimensional culture systems and bioreactors for automated bioprocessing through mathematical models applied to systems biology and network science. Mesenchymal Stem Cells: A Brief Overview Mesenchymal stromal/stem cells (MSCs), a subset of stromal cells present at low frequency in most adult connective tissues, have been extensively studied for their multiple differentiation capabilities. Friedenstein and colleagues (1970) demonstrated that the bone marrow contains a rare population of plastic-adherent cells (approximately 1 in 10,000 nucleated cells) that were able to form single-cellderived colonies. The adherent cell clones expanded into round-shaped colonies composed of fibroblastoid cells, thus leading to the term colony-forming unit—fibroblasts (CFU-f); after proliferation some of the colonies could differentiate into aggregates resembling small areas of bone or cartilage (Friedenstein et al., 1970, 1976). These initial observations were then extended by the study of CFU-f proliferative abilities and phenotypic characteristics (Castro-Malaspina et al., 1980; Prockop, 1997; Caplan and Bruder, 2001). These cells were eventually defined as multipotent and able to differentiate into osteoblasts, chondrocytes, adipocytes, and even myoblasts. MSC Differentiation: The Caveat of In Vitro Studies MSCs have been studied in several species, especially humans (Castro-Malaspina et al., 1980; Haynesworth et al., 1992; Bruno et al., 2009; Yoo et al., 2009) and mice (Gindraux et al., 2007; Sung et al., 2008). MSCs are classically derived from bone marrow (Pittenger et al., 1999), but they have been isolated as well from most connective tissues (da Silva Meirelles et al., 2006), including, to mention a few, adipose tissue (Zuk et al., 2002), periosteum (Nakahara et al., 1991; De Bari et al., 2001a, 2006), and synovial membrane (De Bari et al., 2001b, 2003). From the accumulating studies in the literature, it is now apparent that MSCs, whichever their tissue source, have ability to respond to a variety of physiological or pathogenic stimuli, producing responses that in each specific context require thorough investigation in order to assess their biological significance and exploit any potential clinical impact. The differentiation of MSCs has been extensively studied, using mainly well-established in vitro assays with cultureexpanded MSCs. Findings, therefore, have the caveat that Regenerative Medicine Unit, Bone and Musculoskeletal Research Programme, Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, United Kingdom. HUMAN GENE THERAPY 21:1–13 (October 2010) a Mary Ann Liebert, Inc. DOI: 10.1089/hum.2010. 173