A unique cell population found in human bone marrow is described this week in Nature Immunology. This subset, that expresses the homing molecule L-selectin, is the earliest stage yet reported that gives rise to certain types of white blood cells. The identification of this population will aid our understanding of normal blood cell development and importantly might lend cues during irregular development that leads to various forms of leukemia.
Gay Crooks and colleagues scrutinized human bone marrow cells for their developmental potential in culture dishes and in immunodeficient mice that develop blood cells upon transplantation with human stem and progenitor cells. They identified the L-selectin-positive subset that is a developmental intermediate between quiescent hematopoietic stem cells - the source of all blood cells - and a previously identified subset that are developmentally skewed to form antibody-producing B cells. They show this subset lacks the ability to form red blood cells or platelets, but can make multiple types of white blood cells, including T cells and natural killer cells.
Importantly, the newly identified bone marrow cells differ from the early progenitors found in newborn umbilical cord blood. Although cord blood cells are more readily accessible and highly proliferative, they exhibit a more restricted developmental potential, suggesting that cord blood cells are more specialized than the newly identified bone marrow cells.