The first person to have tissue frozen was a teenager suffering from lymphoma.

Her case was a one-off in 1993. But over the last two and a half years, the service has been offered on a routine basis to girls and young women whose fertility may be endangered by chemotherapy.

Most of the people who have had their ovary tissue frozen were girls, but half are now thought to be over 17.

Doctors say they could be asked to reimplant tissue at any moment.

Around the world several hundred women have had their tissue frozen, but only one woman has had it reimplanted.

Doctors are waiting to see how the American woman, Margaret Lloyd-Hart, responds to the reimplantation operation which was performed last week.

Ethical issues

In advance of any UK cases, doctors are considering the ethical and technical implications of reimplanting frozen testicular and ovarian tissue.

The Royal College of Obstetricians and Gynaecologists has set up a working party which held its first meeting last week.

It helped define the remit of the working party: to develop guidelines for doctors as to what sort of men and women would benefit from freezing tissue, how freezing should be done, in what conditions it should be done and how tissue should be stored.

Professor David Baird, the party's chairman, says it is not so much concerned with the actual use of the tissue since the issue of freezing tissue is only in its early stages.

This means the party is not looking at the controversial issue of designer babies - women storing their tissue so they can delay having a baby until their career has taken off.

"People do not realise just how experimental all this is," said Professor Baird.

Other issues which will come up when tissue begins to be reimplanted is safety and when is the best time to reimplant.

If a woman has had tissue removed because she is having chemotherapy for cancer, there could be a risk that the tissue might include cancerous cells.

If these were reimplanted, it could endanger the patient's recovery.

However, Dr John Toy of Cancer Research Campaign said, unless the woman had ovarian cancer, it was unlikely there would be any cancer present.

This was because cancer rarely spread to the ovaries.

Storage

The working party is looking at how tissue is stored on a long-term basis. Professor Baird says tissue has to be stored in proper conditions so that it does not deteriorate or be at risk of cross-contamination from other stored tissue.

Laboratory guidelines on storing tissue need to be set down to cover all the disciplines connected to ovarian and testicular tissue storage, including gynaecology and paediatric oncology.

"A lot of these things are common sense," said Professor Baird. "Reimplantation is a new technique and we do not know if it will work yet."

On long-term ethical issues, Professor Baird says there is general agreement that tissue would be destroyed if the person who stored it died.

"The tissue belongs to the individual. It is not part of their estate to be handed on and it would not be much use to anyone else anyway because it would be rejected very quickly if implanted into someone else," he said.