It isn't often that a scientific publication get its author on the cover of Time, but that's what happened to Jamie Thomson when his laboratory at the University of Wisconsin derived the first human embryonic stem (hES) cell lines1. Two years later, Nature Biotechnology published the second paper describing the production of hES cell lines by Benjamin Reubinoff, Martin Pera and their colleagues at Monash University in Melbourne2. This paper confirmed Thomson's results, and went further, showing that hES cells could be differentiated in vitro.

The first cells were derived with a medium containing mouse fibroblasts, “which is kind of a standard tissue culture that you use when you don't know what you're doing,” explains Thomson, “because they secrete a lot of stuff. Then over the years, we've found that the things that do support mouse ES cells don't support hES cells.” Pera says hES cell culture has progressed on many fronts, particularly “refining the culture system to make it easier to expand the cells and defining the medium to eliminate animal products.” The remaining challenges, he says, are scaling up the culture and creating reliable techniques to grow up an entire culture from a single ES cell. Already, Pera says, many promising culture methodologies have been described; now they need to be assessed by multiple research groups. In January of this year, Thomson's group described the first fully defined xeno-free medium for culturing human stem cells and isolated two cell lines derived in fully defined medium, though both lines had chromosomal abnormalities3.

But Thomson thinks that the major barriers for culturing hES cells have already been overcome. “There will be improvements,” he says, “but it will be kind of diminishing returns from now on.” In particular, the twin specters of genetic instability and tumorigenicity will be mastered, though not exorcised, through better technique, says Thomson, who was part of the team that first described the emergence of chromosoma abnormalities in cultured hES cells4. “If you're really careful with the culture conditions, the cells are quite stable. It's a concern that has to be managed,” he adds, “but it is ultimately manageable.”

“The odd abnormal cell may not be that much of a problem if it doesn't have an advantage” that lets it outcompete healthy cells, agrees Pera. Nonetheless, abnormal cells will arise even in ideal culture conditions; the key will be identifying and removing potentially dangerous cells before they are used in patients. In this issue, Pera and colleagues5 show that five hES cell sublines with chromosomal abnormalities all express the CD30 receptor and that the protein's expression in normal hES cells prevents apoptosis, but Pera says additional biomarkers will be essential to weed out cells likely to run amok. Understanding, and preventing, the process that sets them down that path is more important.

Martin Pera: “Stem cell culture isn't an absolute reflection of embryonic development; it's sort of a caricature.”

Of course, deriving and culturing hES cells is really a means to an end. The ultimate goal is understanding and controlling differentiation well enough that the cells can be used to generate tissue for human therapies. “We just basically need more people beavering away at it,” says Thomson, who believes that developmental biologists will be able to make most clinically relevant cell types within a decade. “Ultimately, we're going to arrive at a molecular blueprint of the pluripotent stage and then we'll know what switches we need to tweak,” agrees Pera, though he is quick to point out that the ability to create differentiated cells is a very far cry from the ability to use the cells for therapy. But he warns that there will be many bumps in the road towards creating differentiated tissues, stemming largely from our ignorance of basic biology. “Stem cell culture isn't an absolute reflection of embryonic development: it's sort of a caricature.”