When the embryo is just a few days old and no more than a ball of cells, a small region, known as the inner cell mass (ICM) can be isolated and grown in culture. The ICM has the potential to give rise to all tissues of the adult organism (multipotency). Indeed, if left in place, it will develop into the foetus while the surrounding cells will give rise to the placenta and other tissues.

Under certain culture conditions you can generate embryonic stem (ES) cells from the ICM. ES cells can be kept in culture for a long time because they can self renew – continuously producing identical daughter cells. In addition to this self renewal capacity, scientists have discovered that by tweaking culture conditions, ES cells can be directed to develop into pretty much any cell type. These findings led to the idea that ES cells might provide a virtually limitless source of specialised cells for various replacement therapies – brain cells for neurodegenerative disorders, heart cells for heart disease and so on. Furthermore, creating embryos by therapeutic cloning would allow scientists to make patient-identical ES cells and thus circumvent problems of tissue rejection.

The future looks bright? Perhaps, but cloning is a difficult, lengthy procedure limited both by the availability of donor eggs and by low success rates. This means that cloned patient-identical ES cells are far from being a feasible therapeutic resource at present. Normal (not cloned) ES cells also face their fair share of technical issues, which presently precludes their clinical use. Aravinda Chakravarti (Baltimore, USA) and colleagues discovered that human ES cells grown in culture for a long time accumulate potentially cancerous mutations. Not the best news if they are to be injected into patients. And human ES cells are currently grown on non-human ‘feeder’ cells raising significant safety issues. Lastly, current protocols for making specific cell types from ES cells are not consistently efficient or robust and need improving; especially if they are to be generated in large numbers.

These are principally technical hindrances which, once fully understood, can hopefully be overcome. Perhaps less easy to overcome is the ethical objection to stem cell use. Some consider it unacceptable to destroy human embryos for ES cell production. Indeed the US has curbed federal funding for ES cell research under such Pro-life pressure. The debate comes down to when you consider life to begin, and at what point you consider that a human embryo has rights or needs protection. A photograph of Pro-life campaigners, appearing in a recent edition of New Scientist, showed one person carrying a placard which read “embryos are not lab rats”. In their view then a microscopically-small ball of human cells, grown in a culture dish, deserves more respect and privileges than a living, breathing rodent.

Therapeutic cloning, not surprisingly, meets with objection from the Pro-life camp. But also from those who fear the technology might be used for so-called reproductive cloning – generation of cloned individuals. Consequently some countries, including the US, would prefer to see the technology banned altogether. The possibility that any technology might be used for undesirable ends is an age-old dilemma. However, to cease inquiry for fear of misuse would defy scientific principles. Wouldn’t it instead be better to prevent undesirable ends by means of our legal systems?

The US objection to human embryonic stem cell research gives Europe a head start in this field. And, despite the technical hurdles, the self-renewing yet multipotent nature of ES cells means they remain an invaluable starting material offering real promise for future therapy.