The elaboration of a critical framework for understanding cancer: the cancer stem cell hypothesis.

The cancer stem cell hypothesis suggests that malignant tumours may arise from a limited number of specialised cells possessing the key 'stem' properties of self-renewal and the ability to produce differentiated progeny. Such cells purportedly constitute a small fraction of most tumours but have greater potential to produce new tumours than their 'non-stem' counterparts. However, they have proven difficult to identify and characterise in most malignancies. Cancer stem cells are liable to be resistant to most forms of conventional chemotherapy and radiation and so may help to explain tumour recurrence after a seemingly good response to initial therapy. This review examines the evidence for the existence of such cells, the therapeutic implications of this hypothesis, and problems posed by it, as well as outlining the concept of the stem cell niche and its possible role in tumour development and progression.