Mesenchymal stem cells (MSCs) are adult stem cells that were initially isolated from bone marrow. However, subsequent research has shown that other adult tissues also contain MSCs. MSCs originate from mesenchyme, which is embryonic tissue derived from the mesoderm. These cells actively proliferate, giving rise to new cells in some tissues, but remain quiescent in others. MSCs are capable of differentiating into multiple cell types including adipocytes, chondrocytes, osteocytes, and cardiomyocytes. Isolation and induction of these cells could provide a new therapeutic tool for replacing damaged or lost adult tissues. However, the biological properties and use of stem cells in a clinical setting must be well established before significant clinical benefits are obtained. This paper summarizes data on the biological properties of MSCs and discusses current and potential clinical applications. 1. Introduction A stem cell is an undifferentiated cell with the capacity for multilineage differentiation and self-renewal without senescence. Totipotent stem cells (zygotes) can give rise to a full viable organism and pluripotent stem cells (embryonic stem (ES) cells) can differentiate into any cell type within in the human body. By contrast, trophoblasts are multipotent stem cells that can differentiate into some (e.g., mesenchymal stem cells (MSCs), hematopoietic stem cells (HSCs)), but not all, cell types. Adult tissues have specific stem cell niches, which supply replacement cells during normal cell turnover and tissue regeneration following injury [1–3]. The epidermis, hair, HSCs, and the gastrointestinal tract all present good examples of tissues with niches that contribute stem cells during normal cellular turnover . The exact locations of these stem cell niches are poorly understood, but there is growing evidence suggesting a close relationship with pericytes [1, 4, 5] (Figure 1). MSCs have been isolated from adipose tissue , tendon , periodontal ligament , synovial membranes , trabecular bone , bone marrow , embryonic tissues , the nervous system , skin , periosteum , and muscle . These adult stem cells were once thought to be committed cell lines that could give rise to only one type of cell, but are now known to have a much greater level of plasticity [16, 17]. Despite the vast variety of source tissues, MSCs show some common characteristics that support the hypothesis of a common origin [1, 18]. These characteristics are: fibroblast like shape in culture, multipotent differentiation, extensive proliferation
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