Erythrocytic and granulocytic differentiation of transplanted bone marrow cells in mice treated with thyroid hormones

Feeding of C57BL mice with dried thyroid glands (added to their food) for 10 days activates 59Fe incorporation into splenic heme and decreases peripheral granulocyte levels. The importance of the differentiation of haemopoietic stem cells under these conditions was investigated by using the repopulation assays. Transplantation of bone marrow cells into recipients irradiated on day 10 of treatment with thyroid hormones, i.e., in the phase of activation of erythropoiesis, results in an increase in erythropoiesis and a decrease in granulopoiesis. However, following irradiation and transplantation of animals during the phase of rebound, i.e., approximately 5-8 days after cessation of thyroid hormone treatment, erythropoiesis is depressed and granulopoiesis is activated, the values exceeding markedly the control levels. The practical importance of changes in the host inductive milieu favouring granulopoiesis is documented by the results showing that hormonal marrow has an increased capacity to prolong the survival of animals lethally irradiated in the phase of rebound after pretreatment with thyroid hormones.