Cancer stem cells: Here, there, everywhere?

Can every tumour cell propagate human cancers or is this property exclusive to an elite subset? Findings are divided. The latest set shows that — depending on circumstances — both perspectives can be correct. Cancer stem cells in human tumours have been defined in functional experiments as cells that are tumorigenic and self-renew when transplanted into immunocompromised mice. It has been shown for a number of tumour types that such cells are relatively rare. This has informed some approaches to therapy based on a 'cancer stem cell model', which targets these stem cells, rather than a whole tumour or cell population. New work suggests that for human melanomas at least, the cancer stem cell model may not apply. Rather, tumorigenic potential is a common attribute of melanoma cells. The experiments took melanoma cells from twelve patients, and using a xenotransplantation assay, found that about a quarter of the melanoma cells were tumour producing in mice. This suggests that a broad spectrum of cancer cells has the potential to contribute to tumour progression, and raises doubts over therapies specifically directed against small 'cancer stem cell' populations. The cover image depicts melanoma cells and tumours formed from such cells.