The life span of adult cells is restricted, becoming senescent and eventually undergoing apoptosis. Cells are replaced by new ones which originate from a population of stem cells. An example is provided by the hemopoietic system in the bone marrow. In an adult body, stem cells may be identified by markers present in both embryonic and tissue-specific stem cells. Oct3/4, CXCR4, Nanog, CD133 and CD34 are examples of such markers. The stem cells share several properties, such as they may: 1) replicate and give rise to new stem cells, 2) differentiate into heterogenous tissue and organ specific cells and, finally, 3) stem cells are self-protected against various toxic agents and radiation. In adult tissues, stem cells reside in specialized sites (niches), for example in the bone marrow, skin, digestive tract epithelium, and skeletal muscles. Stem cells residing in these stem cell niches are anchored there by chemokines and adhesion molecules. Stroma-Derived Factor-1 (SDF-1), α-chemokine and stem-cell expressed receptor CXCR4 play an important role in the retention of stem cells in such niches. Several stress factors may attenuate the SDF-1-CXCR4 axis in the bone marrow, which leads to the release of stem cells from their niches into circulation. Another important stem cell niche-anchoring mechanism is the interaction of Wnt (a ligand) with Frizzled LRP (receptor). The recently accumulated evidence suggests that malignancy arises from an arrest in maturation and mutation of stem cells. Cancer stem cells are responsible for tumor growth, its relapse and metastasis. The first direct evidence for the existence of cancer stem cells came from observations of acute myeloblastic leukemia. Currently, cell populations enriched for cancer stem cells have been isolated from several tumors. The cancer stem cells represent less than 1% of tumor cells in the mouse models.
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