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EFFECT OF RADIATION ON THE MOUSE STEM CELL COMPARTMENT IN VIVO CLIFFORD W. CURNEY* Recent investigations of die physiology of erythropoiesis [1-3] have uncovered facts, materials, and a model system which permit a detailed and systematic investigation into the behavior of the stem cell compartment . Less attention has been directed toward the morphologic characteristics of these primitive mesenchymal cells, and initial inquiries have been ofa functional nature, in which potentialities for production ofblood by this compartment ofcells in the mouse have been studied. One ofthe methods ofinvestigating the stem cell compartment has involved a quantitative estimation ofthe damage produced by radiation [4]. It has been established that there is a relationship between radiation dose and stem cell responsiveness; and the rate at which recovery takes place, both spontaneously and under experimental attempts to modify the radiation response, has been determined. The basic model employed is the hypertransfused mouse [2, 5]. When mice are transfused until their hematocrits exceed 75 per cent, there is virtually a suppression oferythropoiesis, and almost no evidence of red cell formation in the marrow and spleen can be observed. Consequently, the only cells in such animals capable ofresponding to anerythropoietic stimulus are those of the primitive undifferentiated mesenchyme, or stem cell compartment. Our investigations tend to substantiate the conclusions of Erslev [6] and Alpen and Cranmore [7] that the primary action of erythropoietin is upon the stem cell, inducing differentiation into die red cell series. A measured amount oferythropoietin (in unitsofbiological activity, *John and Mary R. Markle Scholar in the Medical Sciences; Department ofMedicine, University ofChicago, and Argonne Cancer Research Hospital (operated by the University ofChicago for the United States Atomic Energy Commission). 233 standardized by the method ofGoldwasser and White [8]) can be administered to hypertransfused animals, and a quantitative stimulus to the stem cell compartment is reflected by the height ofthe reticulocyte curve three days later, the area under the reticulocyte curve, or the incorporation ofa tracer dose ofFe59.1 Recently Cotes and Bangham [9] have used a hypoxic environment to produceplethora, and wehave found almost total suppression of erythropoiesis in such animals, accompanied by a sensitivity to erythropoietin comparable to that seen in transfused mice [10]. The plethoric mouse can be employed in radiobiology since it is possible to obtain a quantitative measure ofstem cell damage as a consequence ofirradiation ofthe stem cell compartment immediately prior to challenge with a stimulating dose oferythropoietin. The rate ofrecovery following a given dose ofradiation can also be determined by administering a challenging dose of erythropoietin to diffèrent groups of mice at different times after the same dose ofirradiation. Using this model, it is also possible to obtain a quantitative expression ofprotection associated widi "radioprotective agents," either by measuring decreased radiation damage when the stem cell compartmentis challenged immediately following irradiation, or by determining the pattern of recovery when the erythropoietin is administered at varying times after irradiation and after the process of recovery has been initiated. In die first experiment, different groups ofhypertransfused mice were given different doses oftotal-body irradiation as indicated in Figure 1. All animalsreceived a standardchallengedose of6 units oferythropoietinwithin 30 minutes after irradiation, and this initiated a wave oferythropoiesis, presumably by inducingdifferentiationofstemcells. Thiserythropoietic response was determined by measuring the Incorporationinto newly formed red cells ofa tracer dose ofradioiron given intravenously 48 hours after irradiation. Results are plotted as average values plus or minus one standard deviation. It will be seen diat the impaired erythropoietic response, as measured by the per cent ofthe tracer dose ofradioiron which ultimately appeared in newly formed red cells in the peripheral blood, decreases exponentially with increase in radiation dosage after an initial small shoulder at low doses ofirradiation. The similarity of this curve to that ofradiation 1 The erythropoietin used in these experiments was prepared by Armour and Company Research Division, AEC Subcontract fzi, under Contract flS AT-(n-i)-- ? o _? 3 ? DAY Fig. 6.—Erythropoietic response in spleen and peripheral blood following a single injection of erythropoietin in polycythemic mice. (Reproduced with permission ofJ. Lab. Clin. Med.) These experiments indicate possibilities forfuture investigation in radiobiology . Several words of caution must be interjected, however, if this model is to be employed. First, there is...


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