Despite major refinements in cancer therapy drugs, our progress at increasing the cure rates of most cancers has been hampered by high relapse rates. A possible biological explanation of the high frequency of relapse and resistance to currently available drugs has been provided by the cancer stem cell (CSC) proposition. Basically, the CSC theory hypothesizes the presence of a hierarchically organized, relatively rare population of cells that is responsible for tumor initiation, self-renewal and maintenance, mutation accumulation and therapy resistance. Since first postulated by John Dick, multiple reports have provided support for this hypothesis by isolating (more or less) rare cell populations, where the ability to initiate tumors in vivo has been demonstrated. Most progress and stronger data supporting this theory are found predominantly in myelogenous leukemias, whose study has benefited from over half-a-century progress in our understanding of the normal hierarchical organization of hematopoiesis. This review, however, also analyzes the advancement in the quantitative and functional analysis of solid tumor stem cells and in the analysis of the tumor microenvironment as specialized, nurturing niches for CSCs. Overall, this review intends to briefly summarize most of the evidences that support the CSC theory and the apparent contradictions, if not skepticism from the scientific community, about its validity for all forms of cancer, or alternatively on just a few cancers initiated by a limited number of somatic or germinal mutations.
(c) 2010 Wiley-Liss, Inc.