Experiments were carried out to investigate the effect of thymectomy on the migration and differentiation of hemopoietic stem cells released from shielded parts of bone marrow after irradiation of mice with lethal doses of x-rays. In 2–3 wk after thymectomy, the rate of migration declined and the differentiation of stem cells into granulocytic colonies was inhibited. Transplantation of syngeneic thymus or lymph node cells into thymectomized mice enhanced the migration of stem cells from the bone marrow and restored the usual pathways of their differentiation.

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