OBSERVATIONS ON THE EFFECT OF SPLEEN-SHIELDING 



Suspensions of marrow made from mice at 0, 1, 3, and 6 hours after 

 exposure to 600 r had no effect on survival when they were injected intra- 

 venously into mice exposed previously to 900 r even though the number of 

 cells obtained was comparable to that from normal non-irradiated mice. 



By 1 8 hours after total-body exposure to 600 r, the marrow of the donor 

 mice had become so aplastic that 24 mice were required to provide enough 

 cells to make up the desired concentration for injection into 6 irradiated 

 mice. An equivalent number of donor mice was required to provide 

 marrow 24 hours after 600 r. Not only had the factor in the marrow that is 

 effective in enhancing survival been destroyed or inhibited, but some altera- 

 tion in the marrow had also taken place that caused it to have a toxic effect 

 on the recipient mice. Ten of 18 mice died within a few minutes following 

 intravenous administration of this suspension, and none survived beyond 

 the 10th day. The manifestation of this 'toxic' substance was not apparent 

 during the first 18 hours after irradiation, and no studies have been made 

 to determine how long it persists. 



A striking contrast, however, exists between marrow cells taken from a 

 mouse 1 day after it has been irradiated and those taken 8 days after 

 irradiation. Although it was necessary to sacrifice 36 or more animals 

 to obtain enough cells to inject 4 animals with a suspension containing 

 9x10^ cells, all 4 survived the 28-day period of observation and no im- 

 mediate deaths occurred. Later experiments revealed that survival, follow- 

 ing the administration of 8-day (600 r) marrow, paralleled that of mice 

 injected with normal marrow. With a suspension containing about 1 -0 X 10^ 

 cells from either normal or 8-day marrow, 25 per cent of the irradiated 

 recipients survived. Only 10 per cent survived when less than 1-0 X 10^ 

 cells were given. 



Suspensions of bone marrow cells removed from mice 6 days after 600 r 

 were injected into mice that had been exposed previously to 900 r. Indica- 

 tions are that at 6 days some mechanism is already affecting survival. 

 However, the diflficulty encountered in obtaining a sufficient number of cells 

 from such aplastic marrow for a suspension containing about 8x10^ cells 

 has made the extension of this study impractical. 



Other studies were made in which the donor mice received smaller amounts 

 of radiation ( Table VIII, page 131). The marrow from donor mice exposed 

 to 100 or 200 r was as effective as normal marrow in enhancing the survival 

 of mice exposed to lethal amounts of radiation. Marrow removed as early 

 as I hour after irradiation was as effective as that removed 3 days later. 

 It appears that 300 r depresses but does not inhibit the activity of the 'factor' 

 when the marrow is given 1 day or less after total-body irradiation. 



Exposures of 400 to 500 r destroy or inhibit the factor when the marrow 

 cells are removed 1 day after X-radiation. However, these exposures, 

 like 600 r, require many donors and give a 'toxic' effect that is expressed in 

 a fairly high frequency of deaths immediately after the intravenous adminis- 

 tration of the suspension. 



It has been demonstrated recently that the 'toxic' effect is present in the 

 supernatant material * of the bone marrow suspensions. Centrifugation 



* Centrifuged at 2,500 rpm for 5 minutes. Supernatant was not completely cell-free. 



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