McCulloch and Till discovered the blood-forming stem cell, the hematopoietic stem cell (HSC), through their pioneering work in mice. McCulloch and Till began a series of experiments in which bone marrow cells were injected into irradiated mice. They observed lumps in the spleens of the mice that were linearly proportional to the number of bone marrow cells injected. They hypothesized that each lump (colony) was a clone arising from a single marrow cell (stem cell). With Andrew Becker they demonstrated that each nodule arose from a single cell.

Evidence that stem cells are capable of self-renewal. (Order of authorship in the original publication: Siminovitch, McCullch, Till). Digital facsimile from tspace.library.utoronto.ca at this link.