A method was introduced to document the progressive changes in the ability of rat skeletal marrow to respond to bleeding over a span of 60 weeks following a single sublethal whole-body x-ray dose. Rats were exposed to a 170-R (200-kVp x-ray) whole-body dose. At intervals of 3, 6, 7, 10, 25, 33, 40, and 60 weeks postirradiation, half of each of the x-irradiated and control groups was bled one-third or two-thirds of the calculated blood volume (cbv) each, and their overall skeletal marrow content of nucleated RBC was determined. The remainder of the x-irradiated and control animals were measured for total marrow content of nucleated RBC without bleeding. The increase in RBC precursors on bleeding in the x-irradiated animal was compared with that found in the control animal on bleeding. This ratio (x-irradiated RBC precursor increase ÷ control RBC precursor increase) constitutes the "erythroid response index" (ERI) and is expressed as a percentage. An ERI value of 100% is taken to mean a lack of residual injury. Any value of ERI less than 100% would then be the percentage of residual injury (residual injury (%) = 100% - ERI). When the residual injury was calculated for the various times studied and plotted, a simple negative exponential function was found between 7 and 60 weeks postirradiation. The loss of residual injury measured in this manner was found to have a half-time of 29.2 weeks.

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Copyright 1969 Academic Press, Inc.
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