Starting with the skin cells of mice in vitro Hayashi , created a primordial germ cells (PGCs), which can develop into both sperm and eggs. To prove that these laboratory-grown versions were truly similar to naturally occurring PGCs, he used them to create eggs, then used those eggs to create live mice. The results were published in the Scientific American Magazine.

He calls the live births a mere ´side effect´ of the research, but that bench experiment became much more, because it raised the prospect of creating fertilizable eggs from the skin cells of infertile women. And it also suggested that men´s skin cells could be used to create eggs, and that sperm could be generated from women´s cells.

Despite the innovative nature of the research, the public attention surprised Hayashi and his senior professor, Mitinori Saitou. They have spent more than a decade piecing together the subtle details of mammalian gamete production and then recreating that process in vitro.

Their method now allows researchers to create unlimited primordial germ cells (PGCs), which were previously difficult to obtain, and this regular supply of treasured cells has helped to drive the study of mammalian reproduction. But as they push forward with the scientifically challenging transition from mice to monkeys and humans, they are setting the course for the future of infertility treatments — and perhaps even bolder experiments in reproduction. Scientists and the public are just starting to grapple with the associated ethical issues.

When news of the research broke it brought hope for infertile couples. The results also raised speculation between gay couples that understood that they could have children that are genetically related to two men, or two women.