The discovery of the first stem cells came about indirectly from the atomic bombings of Hiroshima and Nagasaki in 1945. Medical workers observed that exposure to radiation caused a precipitous drop in the survivors’ white blood cell counts, and experiments in mice showed that bone marrow transplants could offset those losses. Work over the following decades gradually revealed why: A population of cells in the marrow could both self-renew and differentiate into various, more specialized blood cell lineages. These were the blood-making stem cells.
They departed from the behavior of more specialized cells in several key ways. When a differentiated cell divided, it produced two copies of itself — and depending on the cell type, the number of times it could do so was limited. That wasn’t the case with the stem cells isolated from the bone marrow. When they divided, they did so over extremely long periods of time, in a process known as proliferation. Moreover, those divisions were asymmetric: Each stem cell produced not only a copy of itself but also a daughter cell fated to become a specific type of blood cell. For those daughter cells that gained a differentiated identity, there was generally no going back.