Circulating Stem Cells — Waxing Hematopoietic

During the development of the human embryo, hematopoietic cells first appear in the yolk sac, then migrate to the liver and from there to the bone marrow through the bloodstream. Until the 1970s, it was assumed that these cells resided exclusively in the bone marrow, but then hematopoietic cells capable of mitosis were identified in the blood of laboratory animals and humans.1,2 Before that, all normal blood cells were considered to be terminally differentiated, moving to the circulation only after they had completed a series of maturational mitoses in the marrow. Investigators wondered whether these newly found mitotic blood . . .