CELPIP Reading Part 3 Practice Test: Till and McCulloch: The Discovery of Stem Cells

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In the early 1960s, two researchers at the Ontario Cancer Institute in Toronto made a discovery that would reshape modern medicine. James Till, a biophysicist who had grown up in Saskatchewan, and Ernest McCulloch, a physician who specialized in blood diseases, made an unlikely pair. Till brought a physicist’s habit of precise measurement and careful statistics, while McCulloch contributed a doctor’s understanding of how the human body works and what can go wrong with it. Their partnership is often held up as an example of what can be achieved when scientists from different fields work side by side rather than in separate departments.

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Their research grew out of a pressing question of the time: how does radiation damage living things? After the atomic bombs of 1945, and amid widespread fears of nuclear war, scientists were eager to understand why large doses of radiation were so deadly. One known cause was damage to bone marrow, the soft tissue inside bones where blood cells are made, yet little was known about which cells in the marrow actually produced new blood. To study this, Till and McCulloch exposed laboratory mice to radiation and then injected them with marrow cells taken from healthy mice. Their aim was to measure how sensitive these cells were to radiation and how well they could restore the animals’ blood supply.

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The breakthrough came from a detail that could easily have been dismissed. About ten days after the injections, small lumps appeared on the spleens of the treated mice. Instead of treating them as a curiosity, the researchers counted them and found that the number of lumps rose in step with the number of marrow cells injected. For the first time, scientists had a way to count these elusive cells directly. Further experiments showed that each lump had grown from a single cell, and that the lumps contained several different types of blood cells. Some of the cells inside them could even produce new lumps of their own. In other words, one cell could both copy itself and give rise to many kinds of specialized cells, the defining feature of what is now called a stem cell.

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The impact of the discovery is still felt today. By showing that blood-forming stem cells exist and can be measured, Till and McCulloch helped explain why bone marrow transplants work, and such transplants are now widely used to treat leukemia and other blood cancers. Their work also opened an entire field of research into how stem cells might one day be used to repair damaged tissue, and stem cells from marrow, blood and other sources are now studied in laboratories around the world. Some recognition came early, including a Gairdner award in 1969, but the highest honours arrived only decades later: both men were named to the Canadian Medical Hall of Fame, and in 2005 they shared the Lasker Award, one of the most prestigious prizes in medical research.