The key was mixing cells from very early-stage embryos, or blastocysts, that consisted of just two to four cells each one of the cells still totipotent, capable of transforming into a whole animal as well as the placenta and other life-sustaining tissues. (This is in contrast to pluripotent stem cells, which can differentiate into any tissue type in the body, but not certain embryonic tissues or entire organisms.)
"The cells never fuse, but they stay together and work together to form tissues and organs," said Shoukhrat Mitalipov of the Oregon National Primate Research Center at Oregon Health & Science University. "The possibilities for science are enormous." [Images of the Chimeric Monkeys]
Try, try again
The researchers first tried creating chimeric monkeys using the process for chimeric mice. In this procedure, embryonic stem cells are injected into a host embryo after they have been cultured for as long as decades. These stem cells will mix with the host embryo's cells to produce tissues and organs and ultimately offspring. When these offspring are mated, the resulting offspring have cells derived solely from the implanted stem cells. If you were to pluck two cells from a chimeric mouse's body, you could get two different genomes complete sets of chromosomes and genetic information.
But the methods that work to create chimeric mice failed in rhesus monkeys, leading to offspring with cells only from the host embryo.
"Unfortunately that didn't work," Mitalipov told LiveScience in a telephone interview. "We produced offspring that way and they didn't show any contribution of stem cells." The stem cells seemed to have gotten lost somewhere, he said.
The researchers guessed that the culturing somehow had changed these embryonic stem cells. So they recovered stem cells from an embryo's inner cell mass (rather than from the freezer after being cultured) and, without culturing them, injected the stem cells into a host embryo.
Rather than one chimeric monkey infant, the result was two separate fetuses twins.
Finally, the researchers hit on a successful method, using early blastocysts that had split into no more than four separate cells. They took individual cells out of these clumps and aggregated them back together, mixing and matching between three and six individuals to create 29 new blastocysts. The researchers picked the 14 strongest-looking of them and implanted them in five surrogate mother monkeys.
All five got pregnant. Researchers terminated the pregnancies of three of them to test the fetuses for chimerism, and they found it. Soon after, the remaining two monkeys delivered twins (named Roku and Hex for the Japanese and Greek words for "six"

and a singleton, Chimero. All appear male, though testing on their cells reveals that they also contain individual female genomes.