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In attempting to evaluate the possible health consequences of chronic ionizing radiation exposure during extended space travel (e.g., Mars Mission), ground-based experimental studies of the clinical and pathological responses of canines under low daily doses of 60Co gamma irradiation (0.3-26.3 cGy d-1) have been examined. Specific reference was given to responses of the blood forming system. Results suggest that the daily dose rate of 7.5 cGy d-1 represents a threshold below which the hematopoietic system can retain either partial or full trilineal cell-producing capacity (erythropoiesis, myelopoiesis, and megakaryopoiesis) for extended periods of exposure (>1 yr). Trilineal capacity was fully retained for several years of exposure at the lowest dose-rate tested (0.3 cGy d-1) but was completely lost within several hundred days at the highest dose-rate (26.3 cGy d-1). Retention of hematopoietic capacity under chronic exposure has been demonstrated to be mediated by hematopoietic progenitors with acquired radioresistance and repair functions, altered cytogenetics, and cell-cycle characteristics. Radiological, biological, and temporal parameters responsible for these vital acquisitions by hematopoietic progenitors have been partially characterized. These parameters, along with threshold responses, are described and discussed in relation to potential health risks of the space traveler under chronic stress of low-dose irradiation., (Published by Elsevier Science Ltd on behalf of COSPAR.)

Eleven cases of myeloproliferative disease occurred in a group of 24 beagle dogs placed in a 60Co gamma-ray field at about 13 months of age and irradiated at an exposure rate of 5 R/22-hour day for duratior of life. Of these 11 dogs, 5 (described in this paper) were diagnosed as having erythroleukemia. The bone marrow showed marked erythroblastic hyperplasia, with maturation arrest of the erythroid elements, and increased numbers of myeloblasts and promyelocytes. The terminal peripheral blood was characterized by marked anemia and thrombocytopenia, with circulating erythrocytic precursors and abnormal erythrocyte morphology. Splenomegaly and hepatomegaly occurred in 4 of the 5 animals. In the spleens and livers of all 5, there was extensive leukemic infiltration and proliferation. The extent of leukemic involvement in other tissues and organs varied in individual dogs.

Beagles continuously exposed to low daily doses (10 R) of whole-body 60Co gamma-radiation are prone to develop either early occurring aplastic anemia or late occurring myeloproliferative disorders (Seed et al., 1977). In this study, we have examined by a combination of light microscopy and scanning and transmission electron microscopy the sequential changes in the morphology of biopsied rib bone marrow of continuously irradiated dogs that developed either aplastic anemia, myelofibrosis, or myelogenous leukemia. Characteristic modification of key elements of marrow architecture have been observed during preclinical and clinical phases of these hemopathological conditions. The more prominent of these changes include the following. (i) In developing aplastic anemia: severe vascular sinus and parenchymal cord compression, and focally degenerate endosteal surfaces. (ii) In developing myelofibrosis: hyperplasia of endosteal and reticular stomal elements. (iii) In developing leukemia: hypertrophy of reticular and endothelial elements in the initial restructuring of the stromal matrix and the subsequent aberrant hemopoietic repopulation of the initially depleted stromal matrix. These architectural changes during preclinical phases appear to be related to the pathological progression to each of the radiation-induced hemopathological end points.

A rapidly developing, progressive form of endosteal myelofibrosis (MF) (with myeloid metaplasia) has been shown to occur at low frequency (approximately 4%) in dogs exposed continuously to low daily doses (10 R/day) of whole-body gamma irradiation. We report in this study the morphological details of the endosteal surface during both preclinical and clinical phases of developing MF by combination light microscopy and scanning/transmission electron microscopy. Pronounced alterations of the endosteum were observed and included: (1) during the early preclinical phases, a progressive time-dependent transition of the endosteal surface from predominantly resting to actively formative and resorptive states; and (2) during the late preclinical phase, aberrant autonomous osteogenic process(es) characterized by a marked reduction in the resorptive, osteoclast-associated endosteal areas occurring concomitantly with further increases in formative areas of the endosteum. Localized patches of overlapping, morphologically transformed endosteal cells (i.e., round-osteoblastic to branched-reticular shaped) were observed within the morphologically reactive, formative endosteum. Osteogenic-endosteal changes coincided with major restructuring of the hematopoietic parenchyma and supporting stromal network. We discuss the possibility that the early occurring endosteal changes are causally linked to normal reparative functions that operate during regenerative hematopoiesis following local and systemic injury. Based on morphological data collected during the late preclinical phase, we speculate that the mechanism of myelofibrosis induction involves the failure to terminate early osteogenic-dependent repair sequences.

Six of nine decedent beagles exposed continuously to 2.5 R*/22 hour day of whole-body 60Co gamma-radiation died with myeloproliferative diseases: three cases of myelogenous leukemia and one each of monocytic leukemia, erythroleukemia, and erythremic myelosis. The three dogs that died with myelogenous leukemia had micromegakaryocytes and megakaryoblasts in the peripheral blood during the preleukemic phase when myeloblasts were not observed in the peripheral blood or in increased numbers in the bone marrow. In this study we have examined the megakaryocytes during the preleukemic period by a combination of light, transmission, and scanning electron microscopy. Morphologic abnormalities seen by light microscopy included mononucleated and binucleated forms, many with cytoplasmic blebs. The small mononuclear forms in the bone marrow tended to form clusters. Ultrastructural features included a paucity of both specific alpha granules and dense granules. The micromegakaryocytes showed dysgenesis of the demarcation membrane system. This membrane system appeared disorganized with a few dilated round, oval, or rarely, elongated vesicles and showed no evidence of platelet formation. The cells also had a paucity of endoplasmic reticulum, few mitochrondria, and sparse glycogen accumulations. The scarcity of cytoplasmic organelles gave a pale immature appearance to the cytoplasm. By scanning electron microscopy, the sponge-like surface of large mature megakaryocytes from unirradiated marrow contrasted with the characteristically smooth, topographically featureless surfaces of the micromegakaryocytes from preleukemic dogs.

A purebred female Beagle dog that had received 2,000 R of protracted wholebody gamma-irradiation from 60Co when 14 months old had hematologic changes consistent with a myeloproliferative disorder 3 years after the termination of radiation exposure. Peripheral blood and bone marrow findings during the 7-month period before death showed progressive anemia with increased numbers of platelets; immature granulocytes, monocytes and promonocytes. A period of partial remission occurred during which time the peripheral blood was aleukemic, although there was marked thrombocytosis and abnormal erythropoiesis which was evidenced by bizarre circulating nucleated red cells, anisocytosis, poikilocytosis and Howell-Jolly bodies. The dog had a terminal crisis with marked leukocytosis, most cells in the peripheral blood being bizarre monocytes and promonocytes. Tissues obtained at necropsy showed diffuse as well as focal infiltration of the spleen, liver, lymph nodes, heart, kidney and gastrointestinal wall with immature neoplastic cells resembling monocytes and monocytic precursors. The monocytic differentiation of the invasive cell population was confirmed by morphological, cytochemical, histological, ultrastructural and in vitro cell culture studies.

Male beagles chronically exposed to low daily doses of 60Co gamma rays (7.5 cGy/22h/day) show one of three hematopoietic patterns, which reflect three different distinctly responding subgroups: (1) low radioresistance with progressing aplastic anemia and shortened survival (-S-AA subgroup); (2) high radioresistance with a complex of progressing myeloproliferative disorders (+R-MPD group); or (3) high radioresistance with other nonMPD syndromes (+R-nonMPD group). Blood cell levels (granulocytes, monocytes, erythrocytes, lymphocytes, and platelets) were assessed and fitted to a flexible polynomial spline model, thus defining the (a) initial suppressive and (b) subsequent recovery phases for the subgroups. Results showed that relative to the overall magnitude of blood cell loss as well as to the maximum rate of suppression during the initial phase, the subgroups were generally ranked -S-AA much greater than +R-MPD greater than +R-nonMPD. Relative to the overall strength of the recovery response, the subgroups were generally ranked +R-MPD greater than +R-nonMPD much much greater than -S-AA. In terms of overall maintenance levels of circulating blood cells during the recovery phase, however, the +R-nonMPD subgroup consistently exhibited stronger responses than the +R-MPD subgroup. These results tend to support our contention that selected subgroups of dogs have strong propensities to specific hematopathologies (i.e. aplastic anemia and myeloid leukemia) under chronic irradiation and that these pathology-prone animals exhibit a series of marked differential hematopoietic responses during early preclinical phases, which serve effectively to prognosticate subsequent pathological progression.

Previous studies have shown that continuous whole-body exposure to low daily doses of gamma radiation is highly leukemogenic for beagles initially exposed during either young adulthood or fetal development. In contrast, terminated radiation-exposure regimens (continuous exposure terminated after accumulation of preset total radiation doses) markedly reduce leukemogenic potential. In this study, we examined leukemic incidences and postnatal hematopoietic function in three groups of dogs; continuously irradiated (7.5 cGy/day) during both fetal life and after birth, continuously irradiated during fetal life only, and nonirradiated. Results were compared to results from studies with similarly irradiated and nonirradiated groups of young adult dogs initially tested at 400 days of age. Hematopoietic function was assessed in terms of both circulating blood levels of red cells, platelets, granulocytes, and monocytes, and marrow concentrations and radiosensitivities of hematopoietic progenitors. Results indicated that under continuous fetal/postnatal irradiation, i.e. the high leukemogenic exposure regimen, a marked, progressive suppression in hematopoietic function occurred following birth. This suppression continued to 100-150 days of age and was followed by partial hematopoietic recovery that was associated with an acquired radioresistance by hematopoietic progenitors. In contrast, neonates that had been continuously irradiated during fetal life, but not postnatally, i.e. the low leukemogenic regimen, exhibited a similar initial suppression of hematopoietic function followed by partial recovery. However, no temporally linked acquisition of radioresistance by hematopoietic progenitors was demonstrated. These results support the hypothesis, developed from earlier studies with adult dogs, that the processes of acquired radioresistance and recovery in numbers of transformable hematopoietic progenitors are causally linked to early stages of the leukemogenic process under continuous ionizing irradiation.