First chimeric monkeys born in US

Three chimeric rhesus monkeys born in the USA have been described as the world's first primate chimeras. Although the term 'chimera' is derived from Greek mythology, in genetics it is an animal made from cells from different fertilised embryos. This creates an organism that is a mixture of different sets of cells with different genetic material.

The team, from the Oregon National Primate Research Centre, created the three monkeys from cells from between three and six embryos. They took cells that can become any tissue in the body, known as totipotent cells, from early embryos and mixed them together.

They then allowed the 'mixed' embryos to develop and implanted them into female rhesus monkeys. The resulting offspring, one called Chimero, and twins called Roku and Hex (six in Japanese and Greek), are healthy and developing normally.

As co-author Dr Shoukhrat Mitalipov explains: 'The cells never fuse, but they stay together and work together to form tissues and organs'. For example, all three monkeys are biologically male, but Roku's body includes some patches of female cells.

Initially, the researchers tried to create chimeric monkeys using a technique similar to that previously used in mice — injecting embryonic stem cells into embryos. However these attempts were unsuccessful, which the researchers suggest may be because the cells used were pluripotent not totipotent. This means the cells can form any tissue except cells used for placental development, and are not able to form entire organisms.

Until now, researchers have carried out much stem cell research in mice, but this failure implies there are key differences between mouse and primate embryonic development.

Professor Robin Lovell-Badge, from the UK National Institute for Medical Research, who was not involved in the study, told BBC News: 'Assumptions about the way human embryos develop have always been based on the mouse', adding that this could be a 'dangerous assumption'.

'We cannot model everything in the mouse', said Dr Mitalipov. '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'.

The team also suggested that the supposedly pluripotent embryonic stem cells cultured for long periods in a laboratory may not be as 'potent' as those freshly isolated from an embryo, potentially losing the ability to develop into any tissue or organ.

The researchers have stressed that producing human chimeras is not necessary, and in the UK any chimeras containing human stem cells must be destroyed before they reach 14 days old.