Abstract
Syngeneic bone marrow was transplanted into recipients either immediately or 2 days after lethal doses of irradiation. In the preirradiated animals, the plating efficiency of 2 per cent was significantly less than the 13-15 per cent observed in animals receiving the bone marrow inoculum immediately after irradiation. It is suggested that the lower plating efficiency reflects a less favorable microenvironment with fewer ecologic niches. Once growth of colony-forming units (CFU) commenced, the growth rate in the preirradiated group was more rapid so that by the eighth day the splenic content of CFU was similar in both groups. Possible mechanisms for the latter are considered and it is suggested that the differentiation in growth rate may be ascribed to regulation of stem cell growth through a short range cell-cell interaction.
