When an MP who uses a wheelchair because she has an incurable bone disease expresses the hope that a new line of research could help her, another offers to sacrifice two of her embryos, and a third accuses supporters of such research of thinking like Nazis, it is clear that something more than usually fundamental is happening in the Commons.

The question, debated in parliament yesterday and up for a vote on Tuesday, is whether scientists should be able to carry out experiments on a particular kind of human cell in the hope of treating serious degenerative illnesses such as Parkinson's and Alzheimer's.

Given that there are trillions of cells in the human body, and we slough off millions of them every day as we shed dead skin, that does not seem so controversial. And it might not be, but for two words intimately linked to the debate: "embryo" and "clone".

The cells in question are human embryonic stem cells, between two and 100 of which make up the human embryo in its first days after conception. Scientists want to see if they can persuade them to grow into replacement components for people whose original cells have failed: to replace nerve cells, for instance, in the brains of Parkinson's sufferers.

The embryos in question are human in potential only. They cannot think or feel. They have no distinct organs. They are trillionths of the size of babies, and are far below the time threshold for abortion. They are surplus to requirements for couples having IVF treatment for infertility; they are already fertilised, and would be destroyed if couples did not give their consent for them to be used for research.

Despite all this, they are still human embryos, and for this reason the Catholic church and the broader anti-abortion lobby is opposed to the rules being changed next week to allow stem-cell research to be widened.

Anne Begg, the Aberdeen South Labour MP who has the bone-softening disease osteoporosis and gets around the Commons in a wheelchair, said it was too late for her to hope for a cure. But "perhaps there may be something in the stem cell research which could help".

Fiona MacTaggart, Labour MP for Slough, suffers from multiple sclerosis, and has been unable to have children. Her speech in the Commons could not have been more personal and poignant.

"At the moment there are two fertilised embryos of mine in a medical facility, which are available for research," she said.

"It is ironic that they could be used in research that could help to deal with my infertility, and not be used in research which could be of help to deal with the thing about which I am more anxious, my multiple sclerosis. Unless we pass these regulations that will be the case for everybody."

The Catholic and anti-abortion movement's insistence that not one human embryonic cell should be sacrificed was put by Edward Leigh, Tory MP for Gainsborough, who described the research as "the killing of innocents".

"The Nazis surely were saying that some humans are sub-human, therefore they are expendable ... it is the same argument. People are arguing that these embryos are sub-human. That is a point of view I reject."

The government is backing the change in the rules, and the human fertilisation and embryology authority, the HFEA, will still have the last word on whether scientists get a research licence.

The parties have given MPs a free vote on the issue, treating it as a matter of individual conscience. The government is likely to get its way, but an energetic constituency-based letter-writing campaign by the anti-abortion movement has alarmed its opponents, who find it harder to rally their foot soldiers.

"It's difficult for a family or an individual to say 'This is a matter of life or death for me,' because at the moment, it's not," said Alastair Kent, head of the Genetic Interest Group, an organisation for genetic disease charities.

"It's a promising avenue of research. It may well lead to treatments which become a matter of life and death, and at that point you'll watch people queuing up to get hold of them."

The "pro-lifers" have some curious allies in their campaign - an offshoot of the environmentalist movement, not normally standing shoulder to shoulder with Catholics and opponents of the right to abortion, is against the change because it sees it as a step towards human cloning.

Dave King, who runs a campaign against genetic engineering, does not object to experiment on stem cells but does not want to see the rules changed until a worldwide ban on human reproductive cloning is in place: something which will be extremely difficult to negotiate and may never happen.

But the rule change as drafted makes no mention of cloning. What's the connection?

Stem cells are so called because they have the potential to grow into any kind of specialised human tissue. As leaves grow off the stem of a plant, chemical messages inside stem cells instruct them to divide into ever more specialised cells, until they have formed cells for the liver, heart, skin, nerve, blood and scores of other tissue types.

If scientists succeed in unravelling the mysteries of those chemical messages they will be able to grow replacement cells for people whose own supply has been destroyed by degenerative disease.

That achievement is still a long way off. But even if researchers get that far, there is another barrier to successful treatment. Cellular spare parts grown from alien cells are likely to be rejected by the immune system.

One way to get around this would be the cloning technique used to create Dolly the sheep: take a human egg, remove its own DNA, insert the nucleus from an adult patient's cell, apply a jolt of electricity, and create a cloned human stem cell. This could then be used to grow replacement cells which would be 100% compatible with the patient's own.

The government points out that, surprisingly, doing this for humans is not actually illegal at the moment, subject to HFEA approval, although no-one has yet applied for a licence. In other words, they say, there is no need to vote on it. But campaigners for a no vote next week maintain that the change in the rules on embryo research would create a huge incentive for scientists to begin this kind of limited human cloning.