First 'mixed embryo' monkeys born

US researchers for the first time have created the world's first mixed-embryo monkeys by merging cells from up to six different embryos, in what could be a big advance for medical research. The animals were born after researchers combined cells from different embryos and implanted them into female monkeys.

Such animals, which contain genetically distinct groups of cells from more than one organism, are called "chimeras". A US team, which has reported its work in the journal Cell, says the advance could have "enormous" importance for medical research. Chimeras are important for studying embryonic development, but research has largely been restricted to mice.

The three rhesus monkeys, named Chimero, Roku and Hex, are said to be normal and healthy. 

Initial efforts to produce living monkey chimeras by introducing cultured embryonic stem cells into monkey embryos - a well-established means for generating chimeric mice - failed.

Stem cells are the "master cells" that can transform into a variety of more specialised cells required in the body. The embryonic stem cells the team tried and failed with were at a developmental stage known as "pluripotency". This means they can transform into any tissue type in the body, but cannot turn into the placenta or an entire animal.

The researchers were only able to make monkey chimeras when they mixed cells from very early stage embryos, in which each individual embryonic cell was "totipotent". These totipotent cells are capable of giving rise to a whole animal as well as the placenta and other life-sustaining tissues.

The study also suggests that cultured primate and human embryonic stem cells, some of which have been maintained in labs for as long as two decades, may not be as potent as those found inside a living embryo. Monkey embryo Primate embryos seem to resist the integration of cultured embryonic stem cells

Chimeras can be used to understand the role of specific genes in the development of embryos as well as for studying the overall mechanisms of development.

For example, if studying a genetic mutation that causes cells to die, it is much more useful if the embryo is rescued by normal cells that allow the scientists to continue their work. Chimeric monkeys Monkeys are expected to be a better model for humans than mice

The latest research suggests the biological pathways to restrict cells in their ability to form different tissue types are passed earlier in monkeys than they are in mice.

Stem cell therapies hold promise for replacing damaged nerve cells in those who have been paralysed due to a spinal cord injury and, for example, the brain cells lost in Parkinson's Disease.

First mixed-embryo monkeys are born in US