Short and long-term repopulating hematopoietic stem cells in the mouse

The formation and development of blood cells, or hematopoiesis, normally takes place in the bone marrow, which serves as the major hematopoietic organ during adult life. A small population of bone marrow cells (BMC), designated as hematopoietic stem cells, underlies the process of blood cell formation. This population is de tined by its extensive self-renewal capacity and ability to contribute to blood cell formation for extended periods of time. It has been shown by retransplantation studies that single and uniquely marked murine hematopoietic stem cells are able to provide blood cells for a period of time that even approximates the lifespan of a mouse. Most of the stem cells in the bone marrow do not actively participate in blood cell lormation but remain in a quiescent state, which makes stem cells relatively resistant to the effects of irradiation or chemotherapy. The frequency of stem cells in the bone marrow is very low and has been estimated in the mouse at 1-2 per 100,000 BMC. Therelore, study of stem cell biology ill vitro and ill vivo requires the puritication of this extremely rare cell. Over the years, the mcthods used for puritication have gradually changed, influenced by the changing functional detinition of the stem cell and understanding of its biology. The development of assays to identify primitive hematopoietic cells have been an intcgral part of thc search for the stem cell. In this chapter some of these assays will be introduced and the methods used for the puritication of murine hematopoietic stem cells will be discussed.

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