Monkeying around with stem cells.

Capable of living almost indefinitely in test tubes and retaining their ability to differentiate into any kind of tissue, embryonic stem (ES) cells can be placed back into a developing embryo without harming it. By manipulating the genes in a rodent's ES cells and reintroducing those cells into embryos, investigators have created mice with traits that mimic human diseases or that reveal the functions of proteins. Researchers also use ES cells to study embryonic development in mice.

Now, investigators believe they have isolated ES cells from rhesus monkeys, the first time this feat has been accomplished in a primate. The cells, isolated from 6-day-old monkey embryos, have survived in test tubes for a year without differentiating; moreover, the cells' DNA still appears normal, report James A. Thomson and his coworkers at the University of Wisconsin Regional Primate Center in Madison.

To further establish that they had isolated ES cells, the investigators injected the cells into mice. Multiplying and reacting to cues from the surrounding, mature tissue, the ES cells eventually produced cells with the characteristics of bone, brain, stomach, and skin, the researchers report in the Aug. 15 Proceedings of the National Academy of Sciences.

"The reason we're interested in ES cells is as an in vitro [test-tube] model of human development. The monkey cells should replicate it almost exactly," says Thomson. He notes that the primate ES cells should be particularly useful for studying placental and neural development, areas in which primates differ significantly from mice.

Thomson's group also notes that working with monkey ES cells should provide valuable experience for researchers who believe that transplantation of human ES cells, once found, could help treat or cure a variety of diseases.