A continuous administration of tritiated thymidine (3H‐TdR) into pregnant rats results in a complete labelling of the offspring. Further injections with 3H‐TdR for 26 days maintained the 100% labelling. Four weeks after discontinuation of 3H‐TdR application, distinct classes of cytokinetically resting labelled bone marrow cells were detected. These animals received X‐irradiation of the body with 750 R with one leg shielded, or X‐irradiation of one leg with 750 R, or X‐irradiation of one leg with 1500 R. In none of the experiments did the reticular and endothelial cells show indications of destruction or stimulation. In all X‐irradiated parts the small bone marrow lymphocytes decreased their labelling intensity. the same behaviour was seen in the marrow of the shielded leg. After 1500 R to one leg, a subsequent rise of mean grain count was detected after 3 days. No change of the labelling pattern of lymphocytes of other organs could be observed. The behaviour of labelled bone marrow lymphocytes during recovery from radiation injury suggests that these cells play a role in the regenerative process of bone marrow. There may be a migration from shielded areas into X‐irradiated parts if the dose is high enough to prevent efficient endogenous regeneration.
Summary. 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. In bone marrow these were cells of the bone marrow matrix and a fraction of small bone marrow lymphocytes. At this time the effect of hydroxyurea (HU) on bone marrow and especially on the resting cells of this organ was studied. Animals which had received 4x 500 mg HU/kg at 6 hr intervals showed a destruction of all proliferating cells of the bone marrow. Regeneration started 4 days after HU‐treatment and was not completed before 10 days afterwards. The resting matrix cells did not change in number or labelling intensity which shows that they were neither destroyed nor stimulated to proliferate during regeneration. In contrast, the resting small bone marrow lymphocytes showed a dilution of labelling intensity, and after an initial increase, a decrease of their number during recovery of bone marrow. At the same time a new class of labelled cells which was identified as undifferentiated blast cells appeared, and later on labelled erythroid and myeloid precursor cells could be observed. The labelling of these cells disappeared c 6 days after HU‐treatment. Animals which received a single injection of the total dose of 2000 mg/HU/kg showed essentially the same effects but a markedly smaller response to the drug.
1. The continuous H3-thymidine (H3-TdR) intravenous infusion of rats from day 9 of pregnancy until term results in the labeling of every single cell nucleus in the body of the offspring. Completely labeled newborn rats were sacrificed at serial intervals after birth. This provided the opportunity to follow the labeling pattern of all bone marrow cells during the development of this organ. Cytokinetically three types of cell renewal systems could be found: one type showed a rapid turnover losing all H3-TdR within 6 days, e.g., erythropoietic and myelopoietic cells. Another lost most of its labeling throughout the same time except a small but significant fraction (bone marrow lymphocytes). The third retained a high fraction of labeled cells with a high labeling intensity for weeks and months (reticular cells and endothelial cells). 2. A continuation of H3-TdR administration of 100 per cent labeled newborn rats throughout the first three weeks of neonatal life resulted cytokinetically in the same three different cell types. However, a larger fraction of reticular and endothelial cells as well as bone marrow lymphocytes remained labeled weeks and months after the last injection as compared with the first group of 100 per cent labeled animals. 3. An administration of C14-TdR to 100 per cent labeled newborn animals throughout the first 6 days after birth Showed that 30 to 40 per cent of the "resting" bone marrow cells have turned into a resting state before or at least at the time of birth. Forty per cent of these cells still had some proliferative capacity during the first 6 days of life. Twenty per cent showed in the first days after birth a rapid turnover whereas later on they acquired their final cytokinetic state. The possible influence of radiation toxicity on the time of establishment of resting cells is discussed. This approach appears of use to study the functional properties of resting bone marrow cell types in regenerative processes and the cellular origin in the bone marrow.
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