Scientists have raised the possibility that fertilised human embryos might be created in a laboratory dish from artificial sperm and eggs before being implanted in the womb.
Sheffield University researchers took a step towards developing the vital ingredients for human reproduction from embryonic stem cells, sparking ethical questions of how far experts should go in developing artificial reproduction.
The consequences of such work might even mean gay couples or single men could produce children while women's fertility would no longer be ended by the menopause.
It is a far cry from present artificial fertility techniques, where embryos are made in the lab from human eggs and sperm collected from donors.
Although the successful mimicking of biological functions still might be some way off, it joins other research, including a project on an "artificial womb", in raising profound issues about the nature of human life and parenthood.
A researcher from the Sheffield team will today tell the annual conference of the European Society of Human Reproduction and Embryology in Copenhagen that human embryonic stem cells might be developed in the lab into early forms of cells that become eggs and sperm.
They hope their work could remedy severe shortages of donated human eggs and sperm, and improve assisted reproduction, for instance, by implanting synthetic cells in men to correct fertility problems.
Behrouz Aflatoonian, a PhD student at Sheffield, said studies with embryonic, or master, cells from mice had shown them capable of developing into a further stage, primordial germ cells, and subsequently eggs and sperm.
Cells from embryos donated by couples undergoing fertility treatment were used to see if the same was true in humans.
"The human embryonic stem cells were allowed to develop into collections of cells called embryoid bodies. The embryoid bodies were tested to see which genes were active, or 'expressed', in them and it was found that within two weeks a very tiny proportion of cells in the embryoid bodies began to express some of the genes that are found in human primordial germ cells."
Mr Aflatoonian added: "Embryoid bodies can differentiate into all sorts of tissue, so we need to choose the cells that are going to develop into primordial germ cells and then work out how far we can encourage them to grow into gametes [sperm or eggs].
"Producing functional gametes is far more difficult, because we have to recreate for the cultured cells the environment of the developing follicle for egg development or the tissue of the testis for sperm. We want to test whether the human embryonic stem cells can differentiate to cells that produce the hormones for sperm and egg development and isolate these as well."
Professor Harry Moore, head of the Sheffield laboratory, said the work would enable scientists to study the earliest processes of how the ovary and testis developed.
"Many scientists believe that environmental chemical pollutants that mimic the action of hormones might interfere with human development at this stage and cause congenital abnormalities, infertility and possibly cancer, in particular testicular cancer."
Anna Smajdor, a researcher in medical ethics at Imperial College, London, said that in the future, "this technology would offer an obvious solution to infertile couples".
"Gay couples could have children genetically related to both. Single men could even produce a child using their own sperm and an engineered egg, opening the way to a new form of cloning. Women's fertility would no longer need to be curtailed at the menopause.
"These possibilities raise new questions about how we define parenthood and about how we decide who has access to these new technologies."
