Japanese scientists have transformed stem cells into healthy mouse eggs from start to finish in a lab dish, in a feat that opens a new avenue for basic research into human fertility problems.

Mastering the entire process by which fertile eggs are created is “a key achievement in reproductive biology and regenerative medicine,” the scientists wrote in a study published online in Nature Monday. The lab-made eggs, known technically as oocytes, produced healthy fertile offspring, they reported

“We are very excited about this,” said stem-cell biologist Katsuhiko Hayashi at Kyushu University in Fukuoka, Japan who led the research effort. “Now we can make any number of eggs in the culture. It has become much easier.”

Their new finding reflects a growing command of the basic biology of reproduction. Until now, stem cells could be transformed into eggs only by transplanting them at an early stage of development into the ovaries of a host mother where natural maternal chemistry could take over and complete the process.

In theory, the new technique one day could become a last resort for women who have had no success conceiving through conventional in vitro fertilization, or for people who had cancer in childhood and lost their ability to produce viable eggs. At present, though, it works only in mice.