Mesenchymal stem cells: from biology to clinical use.

Stem cells are immature progenitor cells capable of self-renewal and multilineage differentiation through a process of asymmetric mitosis that leads to two daughter cells, one identical to the stem cell and one capable of differentiation into more mature cells. Stem cells may be: 1) totipotent, i.e. early embryonic cells (1-3 days from oocyte fertilization), which can give rise to all the embryonic tissues and placenta; 2) pluripotent, i.e. embryonic cells from blastocystis (days 4-14 after oocyte fertilization), which can differentiate only into embryonic tissues belonging to the inner cell mass (ectoderm, mesoderm, and endoderm); or 3) multipotent, i.e. embryonic cells from the 14th day onwards, fetal stem cells, cord blood stem cells, and adult stem cells, which can give rise only to tissues belonging to one embryonic germ layer (ectoderm or mesoderm or endoderm). Mesenchymal stem cells (MSC) are non-haematopoietic cell precursors initially found in the bone marrow, but actually present in many other tissues. MSC in culture are adherent, proliferating, and capable of multilineage differentiation into several tissues of mesenchymal origin, such as bone marrow stroma, adipose tissue, bone, cartilage, tendon, skeletal muscle, visceral mesoderm, and endothelial cells1-5. Well known and used for bone regeneration for many years, MSC came in the limelight at the end of the 1990s thanks to the evidence that, despite their adult stem cell nature, these cells are capable of pluripotent differentiation, which may be useful for regenerative medicine. In addition, since the beginning of 2000 it has become clear that MSC possess immune regulatory properties that may make them useful in autoimmune diseases.