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Rats were irradiated with one tibia shielded (95% marrow exposure), total body exposed (TBI, 100%), and only one tibia exposed (5%), or they were sham irradiated (SI, 0%). Plasma Fe-59 clearance time (T 1/2) and Fe-59 content ratio in the right and left tibia (RT/LT) were assayed to determine the erythroid activity of the overall marrow of the animals and the relative marrow activity in the exposed and shielded tibias, respectively. When a major fraction of the overall marrow was shielded or irradiated, the overall erythroid activity levels were identical to those of the SI and TBI animals, respectively. Interestingly, enhanced normoblastosis was observed in the marrow of the exposed tibia of individual animals exhibiting normal erythroid activity in 95% of the marrow. Conversely, localized marrow with normal erythroid activity was found in a shielded tibia of individual rats, demonstrating an enhanced erythroid activity in a major fraction of the total body. It was concluded that 88 mrad can alter marrow functions in a small isolated skeletal region as effectively as in the whole body, and tandem assays of the Fe-59T 1/2 and Fe-59 RT/LT can facilitate ultra-low-dose X-ray studies involved with partial body exposures.

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.

Osteoblasts and osteoclasts are known to adhere tenaciously to bone surfaces even when the fibrous periosteal covering is removed. Brush-smear isolates and in situ, whole bone mounts of the osteogenic periosteum on the inner and outer calvarial tables of young adult rats were examined at the light microscopic level after exposing the cells by removing the fibrous periosteum. Based on staining, enzymatic activity, and overall morphology, most of the cells could be classified as osteogenic in nature. However, detailed analysis revealed that some cells exhibited certain unique morphologic appearances that suggested the existence of subpopulations or variant forms of the conventional or prototypic osteoblast. Three variant forms were the large nucleate, small nucleate, and multinucleate osteoblast-like cells, the latter with nuclear numbers ranging from 2-10. The finding of all such forms in smears, short-term cell cultures, and in situ specimens greatly lessened the probability that the unusual forms were artifactual. Phase contrast microscopic studies and sectioning of cellular isolates for study at both the light and electron microscopic levels provided additional support for the existence of the multinucleate osteoblast-like cell. The small size of these osteoblast-like subpopulations and the orientation of the cells to the commonly used plane of section of most histologic bone specimens, particularly with the multinucleate type, could explain why they had escaped earlier detection.

Osteoclasts residing on rims and walls of bony concavities on remodeling proximal tibia from growing rats were examined by light microscopy following stripping of the periosteal connective tissue layer. Comparison of these cells in situ and after transfer to glass slides revealed the presence of numerous mononucleate osteoclasts, as well as typical multinucleate forms, all exhibiting a ruffled border and acid phosphatase and succinic dehydrogenase activity. Macrophage-like cells were situated adjacent to osteoclasts in situ. Osteoblasts were relatively inconspicuous. The possibility that the basic functional osteoclast unit is a mononucleate cell is discussed.

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.

Skeletons of altitude-acclimatized, altitude-deacclimatized, and sea-level control dogs were completely analyzed for water, fat, and nonfatty organic and inorganic fractions. The total skeletal and marrow volumes as well as their contents were calculated and compared. Neither the marrow volume nor the bone (trabecular and cortical) volume was affected by the altitude changes. The relative distribution of marrow and bone in the various parts of the skeleton was also unchanged. Marrow fat in the whole skeleton as well as in the individual parts of the skeleton was lowered on acclimatization and increased on deacclimatization. Of the fat lost, one-third was from the flat bones while one-half of the fat deposited on deacclimatization occurred in flat bones. The marrow water as well as the functional marrow (fat-free, by definition) in either the whole or the various parts of the skeleton was increased on acclimatization and decreased on deacclimatization.

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.

Five adult monkeys (Macaca mulatta) having an average weight of 3.4 kg were analyzed for the overall skeletal contents of water, fat, nonfatty organic fraction, volatile inorganic fraction and ash. Volumes of bone and marrow for each skeletal region were calculated according to an earlier method used in the study of the dog and rabbit (Gong and Arnold, '65; Gong and Reis, 1970). A shortcut method for calculating marrow volume and fresh volumes of trabecu-lar and cortical bone using only two variables, the ash volume (or mass) and the fresh volume of the skeletal region, was also shown. It was found that the marrow constituted approximately 50.7% of the overall skeletal volume. Comparison of the composition of the marrow in the long bone and flat bone regions of the skeleton showed that the general distribution of the various components in these regions was similar to those found in the dog and rabbit; namely, a relatively high fat content in the long bone marrow and relatively high nonfatty organic and water contents in the flat bone marrow.

Four rabbit skeletons were completely analyzed for marrow contents by means of a volumetric technique developed previously in a study of the dog skeleton (Gong and Arnold, 1965). The distribution of the marrow in various skeletal areas as well as the relative contents of the three marrow components (water, fat and non-fatty organic fraction) were determined. It was found that more than two-thirds of the rabbit marrow was located in the various flat bones and one-third in the long bones. In spite of this, almost half of the skeletal marrow fat was located in the long bones. The nonfatty organic fraction of the marrow in the long bones, on the other hand, constituted only one-sixth of that found in the whole animal skeletal marrow. In terms of total marrow, about 51% of the rabbit skeletal volume was marrow. Based on the concept that fat-free marrow is a more adequate index of active marrow than the red marrow (Gong and Arnold, 1965), the distribution of the various centers of marrow activity was described and discussed.