Abstract
Inbred Lewis rats were labelled with (3H‐thymidine (3H‐TdR) in utero and for 6 weeks after birth to obtain 100 % labelling of all the cells in the animals. 6 weeks after the last 3H‐TdR injection only cytokinetically resting cells were still labelled. These cells comprised of two types of reticular cells, endothelial cells and a small fraction of bone marrow ‘lymphocytes’. At this time the animals were treated with 4 × 500 mg/kg body weight hydroxyurea at 6 hrs intervals. A profound marrow hypoplasia was obvious 6 hrs after the last HU injection. All proliferating precursor cells of the bone marrow were destroyed, whereas the cytokinetically resting cells were not changed in their absolute number. Successful transplantation of the marrow treated with HU to 1200 R X‐irradiated recipients provided evidence for the presence of ‘stem cells’ in the bone marrow of the donors and indicated that in rats bone marrow regeneration can be achieved in the absence of cells morphologically defined as haemocytoblasts.
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