It has been reported previously (1) that plasma of rats made leukopenic by irradiation could, upon injection into a normal rat, elicit a peripheral leukocytosis which is due, at least in part, to the release of large numbers of neutrophils from the bone marrow. In the present experiment, utilizing the radiation protective technique of postirradiation parabiosis, it was investigated whether the unirradiated partner of an irradiated parabiont would manifest any changes in the stem cell compartment of its bone marrow, i.e., colony-forming units (CFU) as a result of its union to an irradiated partner. The pattern of hematopoietic recovery in the irradiated parabiont as measured by total femoral cellularity and CFU content was also investigated. Results indicated that in the unirradiated partner of an irradiated (leukopenic) parabiont the CFU increase to 3.5 times their original number; only part of this increase is attributed to the phenomenon of parabiosis itself. After an early fall in bone marrow cellularity and CFU concentration, the irradiated parabiont begins to recover both cellularity and CFU content, the latter, however, shows fluctuations which are discussed. A general scheme is proposed for the mechanism of postirradiation protection against the hematopoietic form of radiation death.

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