Scientists have taken the first step towards creating human eggs and sperm in the laboratory using stem cells.
The breakthrough raises the possibility of one day growing sperm and eggs artificially for IVF treatment, therapeutic cloning and medical research.
Stem cells are "master" cells that under the right conditions can develop into different kinds of tissue.
Those extracted from early stage embryos can potentially become any of the 165 different kinds of cell in the body.
Researchers at the University of Sheffield have already shown that embryonic stem cells from mice can be coaxed into becoming eggs and sperm.
Now a first step towards the same goal has been achieved with humans.
Scientists identified developing stem cells containing the genetic signature of primordial germ cells (PGCs), the ancestors of eggs and sperm.
Some cells also produced proteins only found in maturing sperm.
However much work still needs to be done before human sperm and eggs can be successfully grown in the laboratory. Some time in the future they could be used in IVF treatments - but only if proved to be safe.