Scientists have opened the door to fertility treatments by creating fledgling sperm cells from bone marrow.

The breakthrough, announced yesterday, raises the eventual possibility of sperm being manufactured for use in IVF treatment or restoring fertility to men rendered sterile by cancer therapy.

However, as well as the scientific hurdles, there may be legal barriers to cross before the theories can be put into practice. The government's recent fertility White Paper proposes a ban on using artificially created sperm or eggs in assisted reproduction.

The research, conducted in Germany, is published in the journal Reproduction: Gamete Biology.

A team led by Karim Nayernia, from Gottingen University - and now based in Newcastle-upon-Tyne - began by taking bone marrow from male volunteers.

From these samples they isolated mesenchymal stem cells, which in previous experiments had been grown into body tissues such as muscle.

This time, using a form of vitamin A, the scientists coaxed the mesenchymal cells along a different pathway to become primordial germ cells (PGCs) - the first stage in the genesis of sperm.

Specific genetic markers also showed that a small proportion - about 3% - had developed to a further stage, as spermatagonial stem cells.

In the natural environment these cells eventually turn into mature, functional sperm that can fertilise an egg.

Professor Nayernia, who has now moved to the North East England Stem Cell Institute (Nesci) in Newcastle-upon-Tyne, said: "Our next goal is to see if we can get the spermatagonial stem cells to progress to mature sperm in the laboratory, and this should take around three to five years of experiments."

Professor Nayernia hopes future research will lead to new male infertility treatments, in particular the feasibility of restoring fertility to young men who had undergone chemotherapy for cancer before they had had the chance to bank their sperm.

This could involve culturing early-stage sperm cells in the laboratory from samples of bone marrow, or testes tissue. They would then be implanted back in the testes.

He acknowledged that the law may be a stumbling block.

The White Paper states: "The government proposes a ban on the use of non- naturally occurring gametes (sperm or eggs) in assisted reproduction treatment. In practice, this will mean that only gametes originating in the testes or ovaries may be used in treatment."

Allan Pacey, senior lecturer in andrology at Sheffield University and secretary of the British Fertility Society, said a blanket ban was the wrong approach but acknowledged that proven safety would be crucial if such sperm were to be used to create embryos.

"Even minor differences in the imprinting of genes between these artificial sperm compared with normally produced sperm could have serious health consequences for any children that might be born," he said.

Harry Moore, from the Centre for Stem Cell Biology at Sheffield University, said: "This finding is of interest but we really need to be very cautious about the interpretation.

"Nearly all the investigations claiming that adult stem cells can change into another cell type (so called trans-differentiation) have not been substantiated when rigorously tested.

"We are still many years away from developing any therapies for infertility using such techniques. These stem cell manipulations can lead to permanent genetic changes which would make them unsafe to use especially as a potential sperm or egg."