The influence of continuous irradiation on the colony-forming activity of mouse bone-marrow.

Abstract

Injected stem cells of the bone-marrow of mice form colonies of haematopoietic tissue in the spleen of an irradiated host and this can be used to obtain stem-cell survival curves after irradiation in vivo or in vitro (Till and McCulloch 1961). The repopulation capacity of bone-marrow cells after fractionated irradiation has been studied, too (Porteous and Lajtha 1966). This paper reports changes in the count of stem cells in the bone-marrow of mice after continuous irradiation by gamma-rays at a low dose-rate. Both male and female C57B mice were used; at the start of irradiation they were 12 weeks old. The irradiation was applied continuously for 24 hours a day and was interrupted every other day for about 10 min to replenish food and clean the cages. The animals were placed into the irradiated space at various intervals so as to obtain groups given accumulated doses of from 40 to several hundred R. All animals used for a given series of experiments stemmed from the same group of the same age. For each series the stem-cell counts of irradiated mice were compared with non-irradiated controls. These controls were kept throughout the irradiation in the same space, but their exposure was prevented by concrete shields of proper dimensions. Thus the controls could not receive doses larger than 2 mR/day. So far, three different dose-rates have been tested-24-0 R, 8-0 R and 27 R per 24 hours. The mice were killed after the irradiation was completed, and a suspension of cells in Hanks' solution was prepared from the bone-marrow of tibiae and femurs. This suspension was injected into host animals of the same strain in doses of 105 nucleated elements per mouse. Host mice were irradiated by a dose of 650 R twenty hours before injection. By the 10th day after injection the mice were killed, and colonies of the haematopoietic tissue at the facies parietalis of the spleen were counted. The individual points in the plots are the mean values estimated for groups consisting of 12-15 mice. The background value, 0-3-0-6 colony per mouse, was subtracted. The results show that the count of stem cells in the bone-marrow responds to continuous irradiation by a decrease at all three dose-rates (figure). The initial decrease is the same at 24 R and 8 R per day, whereas a somewhat lower decrease was brought about by the same dose accumulated at a daily rate of 2-7 R, the difference, however, being not statistically significant. Accumulated doses larger than 200 R were tested only at daily rates of 24 R and 2-7 R. The difference between both plots is considerable. With several groups there has been observed at 27 R/day a recovery to normal values followed by another decrease, and this course recurred. As this experiment is rather timeconsuming at a rate of 27 R/day, it was carried out only twice. The same results could be obtained also for the second run, i.e. an alternating decrease to R.B.