On the development of slowly-turning-over cell types in neonatal rat bone marrow (studies utilizing the complete tritiated thymidine labelling method complemented by C-14 thymidine administration).

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.