Physical Separation of the Cycling and Noncycling Compartments of Murine Hemopoietic Stem Cells

Till and McCulloch (22) have demonstrated that mouse hemopoietic tissue contains a class of cells capable of giving rise to macroscopic colonies in the spleens of radiated mice. These colonies have been shown to be derived from single cells (3), the “colony-forming cells,” which are capable of extensive proliferation (3, 22), of differentiation (12, 30), and of self-renewal (23, 24). These findings support the hypothesis that blood-forming tissue contains a pluripotent, hemopoietic stem cell which has the capacity to give rise to all blood cells, including copies of itself. Pluripotent stem cells then give rise to colonies in the spleens of radiated mice. Since only a portion of the injected stem cells reach the spleen and form colonies there, the cells that form colonies have been termed colony-forming units in the spleen, or CFU-s. The number of CFU-s is then directly proportional to the number of stem cells. The possibility of quantifying the stem cell has led to the initiation of many experiments with the aim of further characterizing it.