On Nov. 10, 1960, James Edgar Till and Ernest Armstrong McCulloch submitted their seminal paper “A Direct Measurement of the Radiation Sensitivity of Normal Mouse Bone Marrow Cells” to the journal Radiation Research (Volume 14, Issue 2, published in February 1961), elucidating their foundational findings on bone marrow cells of irradiated mice and the resulting “spleen colonies” (colony-forming units).

The landmark studies of McCulloch and Till in the 1961 and 1963 defined the hallmark properties of stem cells: the ability to self-renew and differentiate. Working with mouse bone marrow cells, McCulloch and Till developed an assay to quantitate a class of early blood-forming progenitor cells and to define the potential of these cells both to self-renew and to undergo extensive differentiation into many different blood cell types. Stem cells are very difficult to study due to their rarity (that is, 1 in 10,000 bone marrow cells in the mouse). In addition, their quiescent nature diminishes the ability to obtain a complete stem cell phenotype or “footprint” to aid in their identification and purification.

It was nearly 30 years after the discovery of CFU-S by McCulloch and Till that bone marrow hematopoietic stem cells (HSCs) were isolated successfully, enabling HSCs to be studied directly rather than through colony-forming assays. Yet this delay did not deter McCulloch and Till, their colleagues, and their students from continuing to establish the fundamental physiological roles of HSCs and hematopoietic progenitor cells.

They realized that the clonogenic assays, although quantitative, did not directly measure the engraftment potential of the stem cell. The colony assay for CFU-S, although providing an observable window on the potential of mouse HSCs in the short term (that is, 1 to 2 weeks post-transplant), did not establish the potential of these cells for long-term engraftment. Other groups set out to identify the external factors that influenced the self-renewal and differentiation of HSCs. Identification of new growth factors suggested extrinsic regulation rather than intrinsic control of stem cell properties. As scientists discovered these growth factors, they began to argue against the stochastic hypothesis of clonal expansion of stem cells.

They developed an alternative deterministic hypothesis: that HSC renewal and differentiation were regulated by the external microenvironment. These two opposing arguments, heated at times, have been at least partially reconciled by recognition that the clonal expansion and regulated differentiation of HSCs both contribute to long-term engraftment of donor HSCs in recipient bone marrow. However, each hypothesis will be challenged over and over again, as they have been most recently with studies showing the conversion of HSCs into cells of other tissues.