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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.

Marrow cellularity in adult beagles (1–2 yr old) is highest in centrally located bones, with values between 8000 and 12,000 nucleated cells per sq mm. It decreases gradually towards the peripheral parts of the body, reaching values below 1000 per sq mm in bones distal to the elbow and knee. The first tail segment always contains some active marrow. The fifth segment has only stromal elements. In spongy bones fat cells appear to be distributed at random among the blood-forming elements. In the middle part of the femur the fatty marrow predominates in the center of the cylinder, while the subendosteal area is very cellular. The proximal and distal ends of the femur are more cellular than the middle. The small standard deviation of the cellularity in the spongy bones of the trunk and in the proximal and middle part of the humerus makes these sites areas of choice for quantitative studies of marrow regeneration. The large variations in cellularity of the marrow in the radius and tibia of young adult dogs make these sites unsuitable for such studies. The distribution of active and fatty marrow in dogs is similar to that of humans. The differential count of active marrow is quite constant between different dogs and in the various sites of each animal.

Rats were labeled with 3H-thymidine (3H-TdR) in utero and for 6 wk after birth in order to obtain 100% labeling of all bone marrow cells. Six weeks after the last 3H-TdR injection only cytokinetically resting cells were still labeled. At this time, the regenerative capacity of fractions obtained after centrifugation on a discontinuous albumin gradient was tested in 1200-R x-irradiated recipients, and the results were compared with the effect observed after transplantation of the same number of unfractionated bone marrow cells. One of the fractions obtained had a regeneratory capacity tenfold that of unfractionated cells. In contrast, when the response to a PHA stimulation test was evaluated, this fraction showed a decreased incorporation of 14C-TdR as compared to other fractions. In a second test system, fractions of bone marrow from 3H-TdR-labeled donors made hypoplastic by repeated injections of hydroxyurea were transfused into 1200-R x-irradiated recipients. Regenerative capacity was similar to that seen in the first experiment. The findings indicate a correlation between cytokinetically resting, small mononuclear cells and the regeneratory process in lethally x-irradiated recipients.

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