ABSTRACT
Stem cells are defined by their ability to give rise to various cell lineages while maintaining the capacity to
self-renew. During embryogenesis, organ development depends on these cells, and in adults, cell and tissue
losses are compensated by the activity of stem cells. Stem cells are therefore indispensable for the integrity
of complex and long-lived organisms [1]. Adult human stem cells have been isolated from a wide variety of
tissues, and, in general, their differentiation potential may reflect the tissue of their origin. These cells lack
tissue-specific characteristics but, in response to appropriate signals, can differentiate into specialized cells
with a phenotype distinct from that of the precursor. It may be that stem cells in adult tissues are reservoirs of
reparative cells, ready to mobilize and differentiate in response to wound signals or disease conditions. The
presence of multipotent stem cells in adults may open up new therapeutic opportunities based on tissue and
organ replacements, and thus these cells play a crucial role in tissue-engineering approaches. Mesenchymal
stem cells (MSCs) were first identified in the pioneering studies of Friedenstein and Petrakova [2], who
isolated bone-forming progenitor cells from rat marrow. These cells have the capacity to differentiate into
cells of connective tissue lineages, including bone, fat, cartilage, and muscle (Figure 36.1). In addition, they
play a role in providing the stromal support system for hematopoietic stem cells in the bone marrow.
