PerspectiveSTEM CELLS

What is an adult stem cell?

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Science  11 Dec 2015:
Vol. 350, Issue 6266, pp. 1319-1320
DOI: 10.1126/science.aad7016

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The paradigm of the hematopoietic stem cell (HSC) has become deeply engrained in our minds, ever since bone marrow transplantation technology narrowed in on this almost magical multipotent entity that gives rise to all blood cells. The HSC's ability to “self-renew” as well as to proceed down hierarchical differentiation pathways involves a rigidly choreographed flow of events. The HSC paradigm currently serves as a template to interpret experimental observations on any other mammalian tissue. Yet, it is not obvious why evolution would have come up with the very same solution for the renewal of all tissues. Attempts to fit observations on solid tissues into the HSC hierarchy mold have led to confusing theories, terminologies, experimental approaches, and heated debates, many of which remain unresolved. Organs differ in size, architecture, and function, and are subject to markedly different biological and physical challenges. It therefore appears plausible that tissues, with their different regenerative demands, have evolved different ways to restore cell numbers.