Billions of new blood cells are produced every day from the hematopoietic stem cells (HSCs) in our bone marrow. Although scientists have determined how to make stem cells for other specific types of tissues in the lab, efforts to create these blood-forming stem cells in the lab have so far not been successful. One of the challenges is that the environment necessary for the stem cells' growth is poorly understood.

Stem cells are maintained in specialized microenvironments called niches, in which other cells secrete proteins, such as stem cell factor (SCF), that support stem cell maintenance. Most HSCs localize along the blood vessels in bone marrow. A new study reveals that endothelial cells and perivascular stromal cells in bone marrow are responsible for secreting the SCF that maintains HSCs in the body.

By replacing the gene Scf with one that encodes green fluorescent protein (GFP) in mice, Sean J. Morrison of the Howard Hughes Medical Institute at University of Texas Southwestern Medical Center (Dallas, TX) and collaborators at University of Michigan (Ann Arbor) and Cold Spring Harbor Laboratory (Cold Spring Harbor, NY) could systematically identify any cells that glowed green as ones involved in producing the crucial SCF. In this way, endothelial cells and stromal cells were identified as the major sources of SCF in bone marrow (Nature 481, 457–462; 2012). Furthermore, genetically preventing production of SCF from these cells in mice caused a severe reduction in blood cell number. Interestingly, the researchers also found that Scf expression by hematopoietic cells was not required for HSC maintenance in adult bone marrow.

Although endothelial cells and perivascular stromal cells were identified as functionally important components of the HSC niche, further research is needed to identify other signals from these cells that may promote the growth of blood-producing stem cells. This will allow scientists to replicate the necessary conditions to grow HSCs in the laboratory. The research also has implications for transplanting bone marrow and umbilical cord blood in humans: if blood-forming stem cells can be grown in the lab, they could then be transplanted prior to surgery to make the procedures safer and more effective.