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 (> 1yr). 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.

This document briefly summarizes and highlights ongoing studies on the cellular and molecular processes involved in the induction and progression of myeloid leukemia in dogs chronically exposed to low daily doses of wholebody gamma irradiation. Under such conditions, select groups of dogs exhibit extremely high frequencies of myeloproliferative disease (MPD) (i.e., /congruent/50%) of which myeloid leukemia is most prominent. 2 figs.

Female beagle dogs were chronically exposed to low daily doses of 60Co gamma rays (7.5 cGy day−1) and responded in one of three distinct hemopathological patterns. These patterns, reflective of distinct subgroups, were characterized by (a) low radioresistance resulting in progressive hematopoietic suppression, terminal aplastic anemia (AA), and relatively short (<400 days) survival (−S-AA subgroup); (b) high radioresistance, initially coupled with strong but aberrant regenerative hematopoiesis, and later with the development of myeloproliferative disease (MPD) (+-R-MPD subgroup); and (c) high radioresistance, coupled with an early phase of strong regenerative hematopoiesis, but later with no myeloproliferative disease (+R-nonMPD subgroup). In this study, the changes in circulating blood cell levels (granulocytes, monocytes, erythrocytes, lymphocytes and platelets) were sequentially assessed in time and fitted to a flexible, quadratic-linear-type response model previously developed. These analyses provided definition to (a) an initial suppressive, radiotoxic phase and (b) the subsequent recovery phase for each of these subgroups. The magnitude and severity of blood cell loss during the initial suppressive phase was generally greatest in the −S-AA subgroup and lowest in the +R-nonMPD subgroup, with the +R-MPD subgroup showing intermediate level responses. A notable exception included a very high net loss of blood granulocytes by the +R-nonMPD subgroup relative to the other two subgroups. By contrast, the magnitude of blood cell restoration, as well as blood cell maintenance levels during the secondary, recovery phase, was generally highest in the +R-MPD subgroup, of intermediate strength in the +R-nonMPD subgroup, and extremely weak or absent in the −S-AA subgroup. Notable exceptions were in the +R-nonMPD subgroup's high recovery rates of monocyte and lymphocyte blood levels, as well as the high maintenance levels of blood granulocytes during recovery. These results are consistent with our earlier observations of blood responses of chronically irradiated male dogs, in that subgroups of female dogs prone to specific radiogenic hematopathologies (i.e. AA and MPD) can be readily identified and staged in specific preclinical periods by a series of marked differential blood responses.

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: (i) continuously irradiated (7.5 cGy/day) during both fetal life and after birth, (ii) continuously irradiated during fetal life only, and (iii) 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 (a) circulating blood levels of red cells, platelets, granulocytes, and monocytes, and (b) 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.

The presence of micromegakaryocytes in the blood and bone marrow of humans during the preleukemic phase of acute and chronic myelogenous leukemia and myelomonocytic leukemia is well documented [I, 4, 5, 7, 8, 10, 15), and it has been suggested that it is the single most typical abnormality suggesting a subsequent course towards overt leukemia (6). Three adult purebred beagles that died with myelogenous leukemia during continuous whole-body exposure to 2.5 roentgens/22-hour day of 6OCO gamma irradiation had micromegakaryocytes and megakaryoblasts in the peripheral blood three to ten weeks before leukemic myeloblasts were observed in buffy coat preparations. The cells of interest were 6 to 30 JLm in diameter, mono-, or binucleated, with round or oval nuclei, indistinct nucleoli, and they had a high nuclear/cytoplasmic ratio (fig. I). Many cells had cytoplasmic blebs (fig. 2). Cytochemical characterization revealed strong positive staining for both acetylcholinesterase, and a-naphthyl acetate esterase, as well as abundant glycogen with the periodic acid-Schiff stain. The cells were negative with Sudan black, and no detectable myeloperoxidase or naphthol AS-D chloroacetate esterase activity was seen. Other hematologic abnormalities observed during this preleukemic period included progressive refractory anemia, occasional nucleated red blood cells, anisopoikilocytosis, monocytosis, giant platelets, and a left shift to metamyelocytes. Over the same period, bone marrow preparations from iliac crest aspirates and rib biopsies revealed granulocytic hyperplasia with a moderate left shift but without an excess of myeloblasts. There was also erythroid depletion, and increased numbers of mono-, bi-, and multinucleated megakaryocytes (fig. 3). Ultrastructural characterization clearly indicated the cells to be micromegakaryocytes and megakaryoblasts. Even in the most immature cells, the cytoplasm contained both dense serotonin granules and the characteristic a (bull's-eye) granules, indicating their megakaryocytic lineage (fig. 4). Although the majority of micromegakaryocytes showed dysgenesis of the demarcation membrane system, the cytoplasm did contain smooth, membranous vesicular sacs of a rudimentary demarcation membrane system (fig. 4) and in a few cells there were flattened, laminar cisternae of the demarcation membrane system with peripheral formation of dystrophic (i.e., agranular, giant) platelets. Terminally, the dogs were severely anemic (0.94, 1.10, and 1.70 X 10 red blood cells/ul), and thrombocytopenic (5.0, 7.5, and 90.0 X 10 Total leukocyte counts were 11.5, 14.8, and 57.1 X 10/ JLI, respectively. The terminal peripheral blood differential leukocyte count showed a marked left shift (17%myeloblasts) in only one dog. However, bone marrow imprints obtained at necropsy from sternum, rib, and femur showed, in all dogs, a granulocytic hyperplasia with maturation defects and myeloblast counts of 21.0, 11.0, and 35.5% of the total granulocytic series. In all dogs there was a paucity of erythroid elements and in only one dog were there a few remaining megakaryocytes.

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.

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.