Stem cell migration between BM and PB. (A) Stem cells are in dynamic equilibrium between PB and extravascular BM sites. Stem cell migration occurs via circulating blood. (B) Stem cell migration from nonirradiated BM via circulating blood to irradiated BM sites. (C) Stem cell migration through cross-circulation from a normal BM donor to a total body irradiated recipient. (D) Stem cell harvesting by apheresis. The apheresis-induced stem cell efflux from PB is compensated for by continuous stem cell influx from BM sites into circulating blood, thus preventing the circulating stem cell pool from becoming exhausted during apheresis.