Ernest McCulloch

It was half a century ago when Ernest McCulloch told his research partner, James Till, the good news. This moment of scientific triumph marked the official beginning of stem cell research. It also was a personal moment that became Till’s favourite memory of McCulloch.

“He was coming toward me, waving a piece of graph paper,” recalled Till, now professor emeritus of medical biophysics at the University of Toronto. “This was on a Monday morning after the Sunday when he had first observed the presence of bumps on the spleens of irradiated mice that had received transplants of normal bone marrow cells. On the graph paper he had plotted the number of bumps as a function of the number of nucleated marrow cells transplanted. The result was a straight line. Twice as many marrow cells transplanted: twice as many bumps.”

Both scientists later admitted that their observation was accidental. Nonetheless, Till said, “We knew from the beginning that we were involved in research that would be interesting.” In 1961 they published their findings in Radiation Research (1961;14:213-22, doi:10.2307/3570892). In a 50th anniversary tribute to the paper, Irving Weissman, director of the institute of stem cell biology …