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Unformatted text preview: he nervous system,
where they function to replace damaged tissue.
Stem cells were first identified in the hematopoietic (blood-forming) system by Ernest McCulloch and James Till in 1961 in experiments showing
that single cells derived from mouse bone marrow could proliferate and
give rise to multiple differentiated types of blood cells. Hematopoietic stem
cells are well-characterized and the production of blood cells provides a
good example of the role of stem cells in maintaining differentiated cell
populations. There are several distinct types of blood cells with specialized
functions: erythrocytes (red blood cells) that transport O2 and CO2; granulocytes and macrophages, which are phagocytic cells; platelets (which are
fragments of megakaryocytes) that function in blood coagulation; and lymphocytes that are responsible for the immune response. All these cells have
limited life spans ranging from less than a day to a few months, and all are
derived from the same population of hematopoietic stem cells. More than
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