THE TRAGIC BOMBING of civilian populations in Hiroshima and Nagasaki, the first use of nuclear weapons in which humans were the target, signaled the search for stem cells of the blood-forming system. Those people who died with the lowest dose of lethal radiation had destruction of sufficient amounts of their blood-forming (hematopoietic) systems that they could not regenerate enough white cells to fend off disease-causing infections, or did not have enough platelets to clot their blood. Mice given doses of whole body X-irradiation died at about two weeks post radiation of hematopoietic failure (1). Shielding even a single bone or the spleen from radiation prevents this irradiation syndrome. Soon thereafter, using inbred strains of mice, whole-body radiated mice could be recovered by injection of suspensions of cells from the blood-forming organs, for example, the bone marrow (2). The injected bone marrow cells regenerated the blood-forming system (3-5). Then, and now, the only treatment for hematopoietic failure following whole body radiation is transplantation of bone marrow cells or, in fact, the hematopoietic stem cells (HSC) that are responsible entirely for rapid and sustained regeneration of the blood-forming system in these hosts (for reviews, see 6,7).
The hematopoietic system is destroyed not only by the minimal doses of lethal X-radiation or nuclear radiation, but also by chemotherapeutic agents, which, like radiation, largely kill dividing cells. By the 1960s, physicians pushing to treat cancer that had spread (metastasized) beyond the primary cancer site to lymphatics and draining lymph nodes, or via the bloodstream to the rest of the body, were attempting to take advantage of the fact that cancer cells, like stem cells, are cells that have a high fraction of their cells undergoing cell division. They began using agents derived from nitrogen mustards that kill dividing cells and other empirically discovered agents, as well as broad fields of radiation, to attempt to kill back cancer cells from the approximately 10^sup 11^ to 2 × 10^sup 12^ cancer cells that exist at the time of diagnosis to no cancer cells at all. Till and McCulloch and colleagues did several quantitative experiments to test how many and what kinds of injected bone marrow cells would be required to regenerate the blood-forming system of mice treated like cancer patients with doses of chemotherapy combinations that could kill all or most cancer cells, but could also kill blood formation. They used clever genetic marking experiments in their bone marrow transplants to follow the regeneration in the host mice, and found, remarkably, that only rare cells in the marrow were responsible for most of the regeneration of the blood-forming system (8-11). These genetic marking experiments established that there must exist in bone marrow single cells that can both self-renew and generate all of the populations of cells in the blood; these cells were called at that time pluripotent hematopoietic stem cells, a term that has later been modified to multipotent hematopoietic stem cells (HSCs) (12,13).
THE ISOLATION OF HSC IN MOUSE AND MAN
To search in bone marrow for those cells that contain the activity of HSCs we developed assays for the clonal precursors of T lymphocytes (14) and of B lymphocytes (15,16) in addition to the clonal precursors of myeloerythroid cells found in spleen colonies (8-11,17). We developed monoclonal antibodies and fluorescence-activated cell sorting (18,19) to isolate prospectively cells from the bone marrow to put into these clonal assays. In 1986 we reported a high degree of enrichment of multipotent HSC, and in 1988 their full isolation (13,16). At that point mouse hematopoietic multipotent marrow cells represented 1 in 2,000 cells in the young adult mouse marrow bones, and were 2,000-fold enriched for the ability to radioprotect lethally irradiated hosts by donor-derived reconstitution of all blood cell types for life (13). …