Biology of stem cells: the role of microenvironments.

From the discovery of the first line of human embryonic stem cells, thousands of studies have been published concerning adult stem cells and their possible alleged therapeutic potential. However, very little real progress has been made in the application of cell therapy to patients. We can conclude that there remains a great deal for us to learn about the biology of stem cells, and especially, the mechanisms that regulate their differentiation and use under conditions of biosafety. In this chapter, we are going to review some of the mechanisms that seem to control the biology of stem cells, in particular the microenvironments, also called niches, where they house and which exert a strong influence over them. The regulation, survival, proliferation and differentiation of stem cells is ultimately determined by a combination of factors intrinsic to the stem cells themselves and extrinsic signals received from the microenvironment. A better understanding of the cellular components of microenvironments and their cellular and molecular interactions with the other components of the niche, including the stem cells themselves, will be key to make progress in this field.