How Stem Cells Were Discovered

Our medical history guide covers the 1921 discovery of insulin. Four decades later, another Toronto research team made a discovery just as consequential — and they weren't even looking for it.

An experiment about radiation, not stem cells

In 1961, James Till and Ernest McCulloch, working at the Ontario Cancer Institute in conjunction with Princess Margaret Hospital, were studying how radiation damaged bone marrow in mice — injecting irradiated mice with fresh bone marrow cells simply to figure out how much marrow was needed to keep them alive.

An accidental observation

Ten days after the injections, McCulloch noticed small nodules forming on the mice's spleens. Investigating further, the pair found that each nodule was a colony of cells capable of producing every type of cell found in blood — the first direct evidence of what we now call multipotent stem cells. Their findings were published in Radiation Research on February 4, 1961; a 1963 follow-up in Nature confirmed that each colony traced back to a single originating cell.

A discovery that reshaped medicine

Till and McCulloch's accidental finding underpins essentially all of modern stem cell science and regenerative medicine — bone marrow transplants, cancer treatment research, and decades of subsequent biology all trace back to two researchers who set out to answer a much narrower question about radiation, in a lab a short walk from where insulin had been discovered 40 years earlier.

Sources

• University of Toronto — Remembering James Till, a Pioneer in Stem Cell Research
• The Canadian Encyclopedia — James Till
• Wikipedia — Till & McCulloch