[Cancer stem cells and the niches]

The fate of stem cells is tightly controlled by specialized microenvironments (niches). Cell cycle quiescence is a key behavior of stem cells, which protects them from being exhausted by exogenous insults. Since the discovery of cancer stem cells, which are quiescent and thus resistant to anti-cancer therapy, there has been considerable interest regarding whether or not there are distinct niches for quiescent and expanding cancer cells, respectively. In our recent study using whole-mount immunofluorescence imaging techniques, we found that arteriolar niches promote hematopoietic stem cell (HSC) dormancy and that the NG2+ peri-arteriolar niche cells themselves are quiescent, suggesting that bone marrow arterioles comprise a specialized microenvironment that promotes quiescence of both HSCs and niche cells. In this review, we will argue about the advance of our knowledge on normal stem cell niches and the roles of microenvironments in cancer.