Scientists have reached a major milestone in the field of stem cell research. A team at the Oregon National Primate Research Center (ONPRC) say their work has led to the first successful birth of three chimeric monkeys - monkeys developed from stem cells taken from two separate embryos.

The process that led to the monkeys' birth involves taking a stem cell from one embryo and transplanting it into another host embryo. The cells grow together in the host embryo to form a larger organism. These embryos are inserted into a host female who becomes pregnant. The animal that is eventually born from this procedure is called a chimera. The technique has been used successfully in the past using cells from rodents, but never before using cells from primates.

The central purpose of the research was to examine the differences between natural stem cells in early embryos - known as totipotent cells - and later stage or pluripotent cells which can be grown in a culture. Totipotent cells are the precursor to other stem cells that have the ability to divide and produce all of the differentiated cells in the placenta and the body. Pluripotent cells, however, can only develop into the parts of the body, not the placenta, and are therefore more limited.

A release from ONPRC summarized the findings this way:

In mice, either totipotent or pluripotent cells from two different animals can be combined to transform into an embryo that later becomes a chimeric animal. However, the current research demonstrated that for reasons yet unknown, chimeric animals can only develop from totipotent cells in a higher animal model: the rhesus macaque (monkey). (ONPRC) showed this to be the case by successfully producing the world's first primate chimeric offspring, three baby rhesus macaques named Roku, Hex and Chimero.

Earlier experiments that used pluripotent cells from rodents produced chimeric mice, but 20 percent of the mice later developed cancer. Japanese researchers say they have developed a process for producing chimeras that doesn't lead to cancer, but it takes longer and is less efficient.

The use of stem cells in research has been controversial over the past 15 years, particularly in the United States, where the issue has even entered into Presidential politics. Former President George W. Bush infamously - and bizarrely - raised the specter of human-animal hybrids created from embryonic stem cells in a 2006 state of the union address. The President was roundly chastised for the remark, which came five years after his administration essentially halted the progress of stem cell research by cutting off funding to research that didn't rely on a limited amount of existing stem cell lines. Within weeks of taking office in 2009, President Barack Obama signed an executive order reversing the Bush policy.

Shoukhrat Mitalipov, Ph.D., an associate scientist in the Division of Reproductive and Developmental Sciences at ONPRC, says the research into stem cells and primates is important because of their potential uses in regenerative medicine.

"This is an important development - not because anyone would develop human chimeras - but because it points out a key distinction between species and between different kind of stem cells," Dr. Mitalipov explains. "Stem cell therapies hold great promise for replacing damaged nerve cells in those who have been paralyzed due to a spinal cord injury or for example, in replacing dopamine-producing cells in Parkinson's patients who lose these brain cells resulting in disease."

The ONPRC research will be published this week in the online edition of the journal Cell and will be published in a future printed copy of the journal.

You can see the chimeric monkeys Roku and Hex in the flesh in the video below: