Evidence for stem cells in the peripheral blood of mice.

By Jo WRIGHT GOODMAN AND GEORGE S. HoixSoN T HE QUESTION of stem cells in peripheral blood is as old as histology itself, for the description of the hemocytoblast is equivalant in most accounts to that of the large lymphocyte or lymphoblast. Despite the fact that large lymphocytes and, at least in some species, occasional lymphoblasts are found normally in the peripheral circulation, the presence of stem cells is not generally accepted. Evidence has been accumulating that cells capable of division and differentiation do circulate normally in peripheral blood. Specifically, donor-type hemoglobin has been identified in irradiated mice given homologous leukocytes as their only postirradiation treatment,’ and a significant decrease in female polymorphonuclear leukocytes has been found in an irradiated female dog kept alive by supportive therapy including transfusions of fresh whole blood from a male donor.2 Blood leukocytes from normal mice have been shown to repopulate lymphoid tissues of radiation-bone marrow chimeras,3 a finding implied by several earlier observations.4-8 Transplantation of stem cells from the blood can be inferred also from experiments in which lethally irradiated rats were observed to survive and rapidly to regain normal peripheral blood counts and bone marrow cellularity when in parabiosis with unirradiated rats.#{176}’ In man, peripheral leukocytes have been shown to be capable of dividing in vitro’2 and of synthesizing DNA.’3 Experience with blood transfusions in leukemic patients suggests that immunologically competent cells circulate normally. The present data show clearly that normal peripheral leukocytes from F1 hybrid mice can be transplanted and proliferate in lethally irradiated parent or closely related homologous strain animals to promote survival and to give rise to lymphopoiesis, granulopoiesis, and erythropoiesis of donor type. Iron59uptake studies in conjunction with hemolytic tests in the presence of specific antisera are used here to assess erythropoiesis by the transplanted stem cells.