Old disease, new culprit: tumor stem cells in cancer.

Eloquent studies from hematopoietic systems have provided proof that cancer arises from a tumor stem cell that possesses self-renewing properties. Until recently, it was believed that this tumor stem cell was unique to leukemic disorders; evidence now suggests that solid tumors also harbor cancer stem cells that are capable of initiating tumor growth in immunodeficient animals with as few as 10 cells. Consequently, the term "tumor-initiating cell" is now gaining favor within the field. Here, we conceptually discuss the current theories regarding tumor-initiating cells and their involvement in the development and progression of human malignancies. Special attention is given to laboratory techniques and strategies currently exploited to isolate tumor-initiating cells from larger populations, including their inherent strengths and weaknesses. The biological relevance of a tumor-initiating subpopulation is also pondered and arguments regarding their origin are presented. The therapeutic promise of targeting tumor-initiating cells is certainly eminent and we weigh the advantages of targeting this subpopulation. Lastly, the field of cancer stem cells appears to be well-placed to make significant strides over the next decade and we discuss potential obstacles that must be negotiated to achieve those objectives. The realization of these goals will undoubtedly further our understanding of this complex disease and should eventually lead to improved therapies in the not-so-distant future. 2007 Wiley-Liss, Inc.