Developing a way to reliably produce haematopoietic stem cells is a bloody tough problem. Unlike most tissues, cells of the haematopoietic system emerge from several embryonic sites and then circulate through the body. This mobility has perplexed researchers, who hope that mimicking the in vivo environment will help them culture these stem cells. Now, two British research teams report in Cell Stem Cell their complementary techniques for isolating these cells. These methods could form the lifeblood of creating easier alternatives to bone marrow transplantation.

Alexander Medvinsky and his colleagues at the University of Edinburgh, UK, went straight to the heart of haematopoietic stem (HS) cell development — the aorta-gonad-mesonephros (AGM) region, where the first multipotent HS cells are thought to arise in the embryo. By dissociating and reconstructing the mouse AGM region's three-dimensional structure, the researchers developed a new method to expand and track the development of HS cells. They eventually tracked down a population of cells containing markers and generally found on separate types of cells1.

“Medvinsky characterized a cell that's wearing two hats: it's an endothelial cell and a blood cell,” says M. William Lensch, of Children's Hospital Boston and the Harvard Medical School, both in Massachusetts. “When you see a cell like this, it lends credibility [to the idea] that they develop directly from the vasculature” rather than from elsewhere in the developing embryo.

Medvinsky's population of cells contained one marker associated with the inner lining of blood vessels (vascular endothelial cadherin (VE-cadherin)) and another one (CD45) specific to blood cells themselves; he thought that these cells might constitute pre-HS cell progenitors capable of acquiring stem cell function. Indeed, when he injected the VE-cadherin+/CD45+ cells into irradiated mice, the cells rapidly developed into a large pool of definitive HS cells that restored haematopoiesis. Because these cells mature within the AGM microenvironment, this constitutes an active niche that drives the specification of fully mature HS cells, says Medvinsky.

Rather than trying to isolate HS cells directly, a team led by Majlinda Lako of Newcastle University, in Newcastle upon Tyne, UK, developed a relatively efficient way of coaxing haematopoietic differentiation from human embryonic stem (hES) cells. They cocultured hES cells with AGM-derived stromal cell lines and found that haematopoietic activity increased at least 31-fold compared with that seen using previous coculturing methods2. They then injected the induced hES cells into the femurs of immunocompromised mice and found substantially greater engraftment efficiencies than those previously reported for cells cocultured with other cell lines. Finally, Lako's group screened around 40 signaling molecules for positive enhancers of haematopoietic differentiation and flagged the transcription factors TGF-β1 and TGF-β3 as the most efficient inducers of haematopoiesis.

Together, the studies show that nascent cells must mature within the proper context to become definitive HS cells regardless of whether you start with pre-HS cells or hES cells, says Hanna Mikkola, of the University of California, Los Angeles. “The message from both papers is you really need to have the correct embryonic environment for functional maturation in culture.”

The question of how that maturation occurs remains unanswered. The TGF family members identified by Lako are probably involved, Mikkola says, but she doubts these factors tell the whole story. Lako agrees. As a follow-up, Lako's group is currently sifting through a library of other candidate factors, including calcium signaling molecules and insulin-like growth factors, for other key regulators of HS cell development.

Lako's results are impressive, says Medvinsky. But he thinks that coculturing hES cells with his VE-cadherin+/CD45+ cells could be even more successful. “With our system we might be able to produce a better outcome.” Lako, however, suspects her stromal cell lines may already contain some of Medvinsky's 'niche factors'. “It's very likely that we're using the same signals to induce our human ES cells,” she says.

In either case, both authors recognize that more work will be needed to nail down the molecular cues before fully transplantable HS cells can be cultured. “Because you can do this all in a lab dish now, you have the ability to really focus on what the molecules are,” notes Lensch. These two studies now inject new blood into achieving that goal.