They isolated stem cells from very early mouse embryos, sorted those they had begun to develop into early-stage sperm cells and coaxed them through various stages until they resembled sperm cells known as gametes.

A microscopic view of male sperm in an undated photo. In a breakthrough that could help improve the treatment of male infertility, scientists have produced mice using sperm grown in the laboratory from embryonic stem cells. [Reuters]

After injecting the cells into mouse eggs, the embryos that formed were transplanted in female mice. Seven baby mice were born, six of which developed into adults.

"This is the first time it is shown that it is possible to make mature sperm from stem cells," said Professor Karim Nayernia who headed the team at the University of Gottingen in Germany.

"It is the scientific basic for understanding male infertility. Because before we treat any disease we should understand the disease on the molecular and genetic level," he added in an interview.

Stem cells are master cells in the body that can transform themselves into other cell types. Scientists believe they could act as a type of repair system to provide new treatments for illnesses ranging from diabetes to heart disease.

But their use is controversial because stem cells found in early embryos have the greatest therapeutic potential. Adult stem cells have a more limited range.

Professor John Burn, head of the Institute of Human Genetics at Newcastle University in England, described the research published in the journal Developmental Cell as a milestone.

"One could imagine in years to come the knowledge gained from this analysis of the pathway of development will lead us into ways of understanding why some men are infertile," he said.

About one in six couples worldwide experience some form of infertility. Roughly 40 percent of cases are linked to a problem in men. A low sperm count, no sperm at all or poor movement or shape are causes of male infertility.

Nayernia, now at Newcastle University, has been working on the research for three years. His team created 400 embryos but only a handful resulted in live mice because of technical and developmental problems.

"This is important work which builds on a number of discoveries showing that embryonic stem cells can generate sperm and eggs in the lab," said Harry Moore, a professor of reproductive biology at the University of Sheffield, in a statement.