British scientists have made a major breakthrough with the discovery that both sperm and eggs can be grown from stem cells.
They say the discovery may in future solve the shortage of donated eggs and sperm needed for fertility treatment.
Some experts are claiming the technique comes too close to human cloning and are already cautioning that the discovery raises serious ethical questions.
They say that it could mean that a single man could provide both the sperm and egg for fertility treatment, making him genetically father and mother of his child.
Researchers from the Centre for Stem Cell Biology at the University of Sheffield deny these claims.
The team studied six human embryonic stem cell (HESCs) lines taken from very early embryos that had been donated by couples undergoing IVF treatment.
These cell lines are the building blocks of human development and turn into any type of cell, such as organs or tissue. But scientists have been unclear about when HESCs begin to differentiate into primordial germ cells (PGCs), which are the ancestral cells that eventually form sperm and eggs.
In their study the Sheffield researchers allowed the human stem cells to develop into collections of cells called embryoid bodies, and then tested them to see which genes were active in them.
They found that a very small proportion of cells in the embryoid bodies, had within two weeks of development, begun to "express" some of the genes found in PGCs, and some had also begun to express proteins that are only found in maturing sperm, which suggested that human stem cells can develop into PGCs, and eventually eggs and sperm.