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Osteoclast development and growth were studied by determining the number of labelled nuclei in osteoclasts of different sizes (based on the number of nuclei per osteoclast, N/O) and the number of osteoclasts with labelled nuclei at various intervals after tritiated thymidine ([3H]TdR) injection in young rats. The osteoclast smears were made from the cellular periosteum of the proximal tibia. The frequency distribution of the N/O osteoclasts types in the smears had profiles similar to that of in situ osteoclasts in whole mounts of proximal tibia, which indicates that the osteoclast population of the smears was representative of that on the bone surface. A vast majority of the osteoclasts had a 1–6 N/O, and a number of the cells had as many as 26 or more nuclei. Furthermore, profiles of N/O frequency distributions were similar over the course of the study. Nuclei with [3H]TdR label were initially observed in osteoclasts between 4 and 12 hr after isotope injection. However, fusion of labelled nuclei to osteoclasts continued for at least 150 hr. In general, the labelled osteoclasts exhibited a significantly larger number of nuclei than the unlabelled osteoclasts. The probability of an osteoclast incorporating one or more labelled nuclei increased with time after injection and with an increase in N/O. Labelling intensity decreased with time post injection and with an increase in N/O. The data suggest that turnover of nuclei is more rapid in osteoclasts with high N/O values.

Ninety-six percent of mice that were bled of 50 percent of their blood volume when they were 9 weeks old succumbed to myelogenous leukemia by 15 months after phlebotomy, the majority of them dying between 7 and 10 months after this treatment. These results suggest that (i) anemia is an effective stress for triggering myelogenous leukemia in animals that are predisposed to the disease, (ii) the RF mouse is "naturally" prone to the development of myelogenous leukemia, and (iii) the concept of two-step de novo induction of myelogenous leukemia appears to be applicable in this animal.

All of the 128 Sprague-Dawley female rats bled two-thirds of the blood volume at 1, 2, or 3 months after irradiation (50, 170, or 350 roentgens) succumbed to leukemia by 16 months after bleeding. Some nonbled irradiated rats developed leukemia possibly as a result of triggering by a radiation-induced anemia. The threshold leukemogenic x-ray dose is probably below 25 roentgens. Elevated levels of basophils, neutrophils containing the "pseudo-Pelger" type nuclear anomaly, and myeloblasts exhibiting Auer rods seen in these animals were generally not found in earlier models of rat myelogenous leukemia.