The team of Japanese scientists that successfully produced normal baby mice using sperm made from stem cells has now successfully produced baby mice from eggs made from egg stem cells in a breakthrough that, if successfully replicated in humans, could someday relieve worries women have about their biological clocks.

First author Katasuhiko Hayashi of Kyoto University in Japan, and his colleagues have successfully produced three fertile baby mice by fertilizing viable egg cells created from skin tissue taken from laboratory mice in IVF experiments.

The findings, published in the journal Science, demonstrated that the egg cells created in the experiment produced healthy mouse offspring, and that the offspring went on to produce their own healthy offspring. The mice also grew normally in terms of body weight and size, had a normal pattern of genomic imprinting, or the way genes are inherited, and were fertile and able to reproduce pups with comparable size of the litter.

"This is a significant achievement that I believe will have a sustained and long-lasting impact on the field of reproductive cell biology and genetics," Amander Clark, a stem cell biologist at University of California, Los Angeles, said according to Science Magazine.

The finding is seen as an important breakthrough in research to find ways of producing egg cells from infertile women with defective ovaries. The latest discovery could also one day allow women to have babies after menopause.

Researchers first took stem cells from female mouse embryos and stem cells reprogrammed from fetal cells called induced pluripotent stem cells. They then manipulated the activity of a few genes in the stem cells to transform them into cells that resemble precursors of female sex cells.

These early-stage version of eggs were then mixed with mouse ovarian cells and then implanted into adult mice. After four weeks and four days, Hayashi and his team collected the immature eggs, matured and fertilized them in the laboratory and transplanted the mature eggs into a surrogate mother mouse which later grew into fertile adults.

"It is remarkable that one can produce oocytes capable of sustaining complete development starting with embryonic stem cells," said Davor Solter, a developmental biologist at Singapore's Institute of Medical Biology, according to Science Magazine.

Evelyn Telfer of Edinburgh University, who was not part in the study, said the latest research was an incredible follow-up from the team's earlier work on producing viable sperm cells from skin cells. "It is a quite brilliant study. To be able to make an oocyte from scratch as it were really is incredible," Telfer said, according to The Independent.

However, Telfer added that researchers need to replicate the findings using skin cells taken from adult mice rather than mouse embryos to make the discovery more applicable to humans.