Cancer Stem Cells – Basics, Progress and Future Potential

The primary characteristics of adult stem cells are maintaining prolonged quiescence, ability to self-renew and plasticity to differentiate into multiple cell types. These properties are evolutionarily conserved from fruit fly to humans. Similar to normal tissue repair in organs, the stem cell concept is inherently impregnated in the etiology of cancer. Tumors contain a minor population of tumor-initiating cells, called "cancer stem cells" that maintain some similarities in self-renewal and differentiation recognized as features of normal adult stem cells. Therefore, various methods developed originally for the analysis and characterization of adult stem cells are being extended to evaluate cancer stem cells. Relevant methods that are used generally across normal stem cells as well as cancer stem cells are summarized. The hypothesis that cancer could be a stem cell disease actually existed for several years before definitive ‘proof-of-concept’ experiments unequivocally established their involvement in initiating the tumorigenic state. Over the last decade, the combination of fluorescence-activated cell sorting with functional read-outs e.g. in vitro colony forming units in colony initiating assays, and competitive repopulating assays in animal models has enabled the prospective purification and enrichment of cancer stem cells (CSCs) in leukemia and also in certain solid tumors. Combination of two or more of these methods for validation of cancer stem cells appears to be a promising approach for the precise isolation and analysis of cancer stem cells. However, a major limitation for research in the field of stem cells and CSCs has been the lack of research resources in the form of in vitro models for these studies.