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There are two caging systems in use in animal facilities. In the static system, the cages are stacked on racks whereby ambient air filters through the lids of the cages. In contrast, the cages in the other system are placed in special ventilated racks. These cages have an air-flow grommet; therefore, they are exposed to a constant flow of air supplied by these racks. The static cages are changed on a semiweekly basis, while the ventilated cages are changed on a biweekly basis, since the circulating air removes a buildup of metabolic products such as ammonia and carbon dioxide. We are studying the effects of static versus ventilated caging systems on mice. Based upon empirical evidence, the lethal dose of irradiation varies between these two caging systems. Therefore, the goal of this experiment is to test the hypothesis that the type of caging system has an effect on irradiation survival and on the subsequent engraftment of hematopoietic cells.
A total of 120 female C57Bl/6 mice were housed in groups of five in plastic microisolator cages containing corncob bedding. Sixty of the mice were housed in cages containing an air-flow grommet, and they were placed in a ventilated rack containing hepa filters. The water bottles used for the ventilated cages were standard for this system, plastic bottles containing a small sip hole. The remaining sixty mice were housed in cages that were not fitted with an air-flow grommet. These cages were placed on a static rack, and they were supplied with plastic water bottles each containing an inserted sipper, the standard for this system. All of the mice were provided with a laboratory light mouse food diet. For ten days prior to irradiation, the mice were given acidified water made from HCl with a pH between 2.2-2.4. On the day that the mice were irradiated, they were started on medicated water consisting of polymixin B, neomycin sulfate and acidified water for a pH between 2.3-2.5. Irradiation was administered in three separate doses and was given to twelve different sets of mice containing ten animals each. Each dose was given to four sets of mice, two from each caging type. The doses were 854, 941 and 1026 rads, and they were administered at a rate of 106 rads/minute as split doses given four hours apart during the light cycle. Sixty mice were monitored for survival over a 25 day period. The other sixty mice were used as recipients for bone marrow transplants. Both procedures were used to determine the LD 90-100, or the lethal dose in the range of 90-100%. Four weeks following transplantation, flow cytometry was conducted in order to determine the rate of engraftment/self-reconstitution.
By day 25, there was 100 percent survival for the mice in both caging systems irradiated at a dose of 854 rads. By day 25, there was 100 percent survival for the static mice irradiated at a dose of 941 rads. By day 24, the death rate was 100 percent for the vented mice irradiated at a dose of 941 rads. The survival curve for the static and the vented mice irradiated at a dose of 1026 rads was very similar. By day 22, the death rate was 100 percent for both groups of mice.
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