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Hematopoietic stem cells: concepts, definitions and the new reality

Connie J. Eaves

Abstract

Hematopoietic stem cell (HSC) research took hold in the 1950's with the demonstration that intravenously injected bone marrow cells can rescue irradiated mice from lethality by re-establishing blood cell production. Attempts to quantify the cells responsible led to the discovery of serially transplantable, donor-derived, macroscopic, multi-lineage colonies detectable on the spleen surface 1-2 weeks post-transplant. The concept of self-renewing multi-potent HSCs was born, but accompanied by perplexing evidence of great variability in the outcomes of HSC self-renewal divisions. The next 60 years saw an explosion in the development and use of more refined tools for assessing the behavior of prospectively purified subsets of hematopoietic cells with blood cell-producing capacity. These developments have led to the formulation of increasingly complex hierarchical models of hematopoiesis and a growing list of intrinsic and extrinsic elements that regulate HSC cycling status, viability, self-renewal and lineage outputs. More recent examination of these properties in individual, highly purified HSCs and analyses of their perpetuation in clonally-generated progeny HSCs have now provided definitive evidence of linearly transmitted heterogeneity in HSC states. These results anticipate the need and use of emerging new technologies to establish models that will accommodate such pluralistic features of HSCs and their control mechanisms.

  • Submitted December 15, 2014.
  • Accepted February 3, 2015.