Scientists have created early stage sperm from the skin of men who cannot make their own, raising hopes of new treatments for currently incurable male infertility.
A British expert said the stem cell research challenged the widely held view that the "door is closed" on men with the problem who want to have children.
Although still at a very early stage, the study raises the future prospect of lab-grown sperm that could allow them to become fathers.
More immediately, it offers researchers a new way to study sperm development and assisted conception treatments.
Lead scientist Dr Reijo Pera, of Stanford University, America, said: "Our results are the first to offer an experimental model to study sperm development. Therefore, there is potential for applications to cell-based therapies in the clinic, for example, for the generation of higher quality and numbers of sperm in a dish.
"It might even be possible to transplant stem-cell-derived germ cells directly into the testes of men with problems producing sperm."
Infertility affects 10% to 15% of couples and in men often has a genetic origin, most commonly the loss of key genes on the male Y chromosome.
All three infertile men taking part in the study had missing regions of Y chromosome DNA associated with the production of few or no sperm.
Fibroblast connective tissue cells from skin samples taken from the men were first genetically engineered to transform them into induced pluripotent stem cells.
These are adult cells whose developmental clock has been turned back so they assume the properties of embryonic stem cells.