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Black boxes can be dangerous when it comes to human embryonic stem (hES) cell research. Therefore James Thomson and his colleagues at the University of Madison have devoted considerable time and effort to defining all the components needed for culturing hES cells.

hES cells have the potential to differentiate into any cell type, but the task of propagating them in culture is fraught with challenges. Researchers used to grow hES cells on a layer of feeder cells in the presence of a commercially available serum substitute, which made culturing the cells less reproducible and introduced potentially harmful molecules. This was highlighted recently by the identification of contamination in all the hES cell lines currently admissible for federally funded research with a mouse sialic acid which they acquired from mouse feeder cells or serum components.

Previous research in the Thomson and other laboratories showed that they could get rid of the feeder cells by supplementing the medium with high concentrations of basal fibroblast growth factors. Now Thomson and colleagues took on the challenge of finding an alternative to the commercial serum replacement and the matrix on which the cells grow.

They developed a medium with clearly defined ingredients and showed that this medium supported growth of the federally approved hES cell lines. Notably, they found that these cell lines lost the mouse sialic acid after some time in culture. When asked whether this gave these cell lines a new chance in therapeutic application Thomson was hesitant: “You have to jump through a lot of hoops to show safety, and if you had new cell lines derived in clearly defined conditions, it would simply be safer.”

His lab successfully derived two new hES cell lines in the defined medium, but both new lines unexpectedly showed karyotypical abnormalities after prolonged periods in culture. Thomson's plan is to repeat the process to see whether the abnormal karyotypes are related to the new medium itself, or to how the cells were handled.

The other undefined component in the currently used culture system is the matrix, often derived from mouse tumors. Thomson's lab showed an alternative with exclusively human products. The downside of this fully defined matrix is its prohibitive cost. Thomson views this issue very practically: “We got defined; now we need cheap.” Coming up with a cost-effective way of producing this matrix will be the next challenge.

Despite these hurdles, Thomson is confident that the question of hES cell derivation and culture will soon be largely answered, leaving the much greater challenge of working out differentiation conditions. “Good developmental biologists are needed”, concludes Thomson, but he also points to another significant roadblock to progress: “I think currently the bottleneck is predominantly political not scientific.”