Professor John Burn is Medical Director of the Institute of Human Genetics at Newcastle University, where some of the most controversial stem cell research in the UK takes place.

He's taking on a formidable task as the UK parliament debates controversial amendments to the 1990 Human Fertilisation and Embryology Act - namely helping determine at what point a cell becomes a human.

Concerns about the misuse of funds, threats to the structure of the family, and the dangers of admixed (hybrid) embryos can all be adequately addressed without an act of parliament. Stem cell research is done in a highly regulated environment, with statutory bodies such as the Human Fertilisation and Embryology Authority (HFEA) having access to the requisite expertise. The authority has already proved its ability to reach reasoned conclusions on similarly touchy subjects.

But there is one argument against stem cell research that cannot be addressed by a committee — the question of when human life begins.

The Catholic church’s position is clear: from the moment of conception an embryo is a human being, entitled to full human status. It equates the deliberate generation of embryonic stem cells to murder. Like some of his predecessors, Pope Benedict XVI has declared that “ensoulment” might occur at conception. But, writes Burn, if souls are delivered, it is difficult to see how this could occur before 14 days. It is only then that the primitive streak forms, and a single embryo could be said to exist. Before this, the cells that make up the embryo could result in up to five identical embryos.

The Catholic church is more supportive of research on adult stem cells, but Burn says that recent research with induced pluripotent stem cells —adult stem cells which are made to act like embryonic ones—could be regarded as resulting in “instant ensoulment.”

The fact that stem cells have a potential role in the treatment of incurable diseases such as paraplegia and Parkinson's Disease means that we should avoid erecting blanket legal barriers, says Burn.

He also claims that the tabloid horror of admixed embryos (the cow-human hybrids) is misplaced. Adult stem cells are used, and "admixed embryos use tissue from the abattoir to preserve precious human eggs and advance laboratory research that offers real hope."

“Just as protests about cadaver organ donation were addressed rationally and led to the widespread acceptance that the definition of death could no longer depend on biblical interpretation, so medical need dictates that the origin of human individuality must be defined with similar pragmatic precision. A cell cannot have a soul,” concludes Burn.

Nature 2008; 451: 141–6