U.S. researchers say they have successfully produced the world's first genetically modified monkeys composed of cells taken from separate embryos.

Scientists at the Oregon National Primate Research Center announced Thursday that they have created the world's first monkey chimeras with the birth of three male rhesus monkeys named Roku, Hex and Chimero. Chimeras are animals created by mixing together genetically distinct materials from different cells. Previous examples of chimeras had been restricted to mice. This is the first time scientists tried to create a chimera using primate cells.

The rhesus monkeys are made up of a mixture of cells from different rhesus monkey embryos. The mixed embryos were implanted inside the womb of a surrogate mother. The team initially introduced embryonic stem cell cultures into a single monkey embryo, a technique that's been done with mice embryos. The attempt failed, so the team had to work with living embryos. The researchers succeeded after they mixed cells from very early stage embryos that contained totipotent stem cells. These stem cells were capable of giving rise to a whole animal as well as vital extra-embryonic tissues like placenta.

"The cells never fuse, but they stay together and work together to form tissues and organs," said co-author Shoukhrat Mitalipov of the Oregon Health and Science University in Portland.

The breakthrough may have major consequences in the field of stem cell research. Chimeras are used to understand the role of genes in embryo development and in studying the overall mechanisms of development.

"We need to go back to basics. We need to study not just cultured embryonic stem cells but also stem cells in embryos. It's too soon to close the chapter on these cells. We cannot model everything in the mouse," Mitalipov said. "If we want to move stem cell therapies from the lab to clinics and from the mouse to humans, we need to understand what these primate cells can and can't do."

Mitalipov stressed that there are no plans and indeed no practical use for creating human chimeras. The research is published online in the January issue of the scientific journal Cell.

While scientists have hailed the research as very important, other groups have called the research "deeply disturbing". The British Union for the Abolition of Vivisection said the research raises enormous ethical concerns.

"Using such highly sentient animals in this research raises enormous ethical concerns and imposes a heavy welfare burden, resulting in severe suffering to many animals," said Dr. Jarrod Bailey, the organization's scientific consultant.

"As few genetically modified animals show the 'desired' characteristics, many will be killed even before any research can take place, while others will die of severe and unrelated malformations caused by the genetic modifications. The monkeys who do exhibit characteristics of 'interest' are destined to suffer greatly by their very nature, and via the experiments to which they will be subjected."