Mobilization of Haemopoietic Stem Cells (Cfu) Into the Peripheral Blood of the Mouse; Effects of Endotoxin and Other Compounds

Factors affecting the circulation of haemopoietic stem cells (CFU) in the peripheral blood of mice were investigated. I.v. injection of sublethal doses of endotoxin, trypsin and proteinase appeared to raise the number of CFU per ml blood from about 30–40 to about 300–400 or more within 10 min. The effect was smaller when smaller doses of the substances were injected. After this initial rise the number of circulating cells returned to normal in a few hours. Following endotoxin there was a second rise which started 2–3 days after injection and attained a peak on the 6th–7th day. The first rise is explained as a mobilization of stem cells from their normal microenvironments into the blood stream; the second rise is considered to reflect proliferation of CFUs in the haemopoietic tissues. The spleen seems to be acting as an organ capturing CFUs from the blood and not as a source adding stem cells to the blood. The early mobilization of CFU after endotoxin injection did not coincide with a mobilization of neutrophils. The number of circulating band cells was increased during the first hours. The importance of ‘open sites’in the haemopoietic tissue for capturing CFUs was studied by emptying these sites through a lethal X‐irradiation and injecting normal bone marrow cells. When a greater number of syngeneic bone marrow cells was injected intravenously, the level of circulating CFU in irradiated mice was slightly lower than the level in unirradiated mice during the first hours.