Adult stem cells: generation of self-organizing mini-organs in a dish

When pluripotent stem cells were first identified, their ability to divide indefinitely and form all cell types of the body made them in some ways more interesting for both basic research and future therapeutic applications. As far as we knew, adult stem cells had only a limited ability to divide in culture, were difficult to find in the body, and could only form relatively few cell types. This view changed dramatically during the first decade of the twenty-first century. Firstly new markers of the cells were discovered, which allowed them to be identified in many tissues of the body. Secondly, culture conditions and proteins were found that supported the growth of adult stem cells without compromising their ability to differentiate. Adult stem cells have thus returned to center stage in the search for therapies that can lead to tissue and organ regeneration. It is clear that their applications will develop far beyond the present use in hematopoietic stem cell therapies to reconstitute bone marrow after chemotherapy. For this reason, we devote this chapter to more detailed information of their biology and discovery, with a view to making it clear why they have become particularly interesting both for clinical use and perhaps drug discovery.