A special class of adult stem cells, known as human induced pluripotent stem (iPS) cells, has for the first time been reprogrammed into cells that develop into human eggs and sperm. The research, carried out by members of the University of California, Los Angeles (UCLA)'s Broad Stem Cell Research Center, was published in the January 27 online edition of the journal Stem Cells.

Derived from adult body cells that have been engineered to return to an embryonic state, iPS cells have the ability to become every cell type in the human body - a characteristic they share with embryonic stem (ES) cells. In this study the iPS cells were coaxed into forming the germ line precursor cells that are capable of giving rise to sperm and eggs.

'This finding could be important for people who are rendered infertile through disease or injury', said Amander Clark, the senior author of the study. 'We may, one day, be able to replace the germ cells that are lost, and these germ cells would be specific and genetically related to that patient'. Many infertile couples would see this process as preferable to using eggs or sperm from a donor who would then become one of the child's genetic parents.

However, Clark cautioned that scientists are still many years from offering treatments involving iPS cells to infertile patients. There are many uncertainties and dangers that need to be resolved. For example, the process of reprogramming involves using viruses to deliver genes to the cells, potentially increasing the likelihood of genetic abnormalities and cancers.

Crucially, Clark's team found that the germ line cells derived from iPS cells did not perform certain key regulatory processes as well as those generated from ES cells. The associated increased risk of chromosomal errors, or abnormal growth, could have serious health consequences for any child conceived using egg or sperm obtained in this way.

Therefore Clark believes that it is vital that research using human ES cells continues. These cells can be derived from left over embryos used during in vitro fertilisation, and would otherwise be destroyed, yet their use is controversial and the topic remains fiercely debated. Despite this President Obama is expected to reverse President Bush's restrictive policies on ES cell research in the next few weeks.