Where does stem cell as a concept lie along its evolutionary
trajectory? The phrase seems to have come to us via
histologists in the nineteenth century, who introduced it as
a general, abstract term for cells specifically involved in
repair or regeneration. With the discovery in the 1950s that
bone marrow cells could reconstitute the hematopoietic
systems of irradiated individuals, the modern stem cell
concept began to crystallize around the experimental
procedures of transplantation and reconstitution. These
and other studies gave us a good and enduring operational
definition for stem cells: those cells that when introduced