The research team led by Dr. Mitinori Saitou at Kyoto University began by coaxing mouse embryonic stem cells into primordial germ cells, which give rise to sperm or egg cells. The germs cells were then transplanted into the testes of 7-day-old mice, which had been bred to be sterile.
The result? The mice not only went on to produce normal-looking sperm, but the sperm also successfully fertilized mouse egg cells in Petri dishes. The researchers created 214 embryos, which were implanted into female mice, resulting in the birth of 65 healthy male and female mouse pups. Those pups grew up to have healthy, fertile offspring of their own.
Saitou and his colleagues (along with many other research groups) have been working on figuring out how to turn embryonic stem cells into healthy sperm or eggs for years. This latest achievement is being lauded by colleagues around the world. "It's a brilliant set of experiments," George Daley, director of the stem-cell transplantation program at Children's Hospital Boston, told the Wall Street Journal. "They restored fertility in the mice. It lays the groundwork for major insights into sperm development and fertility."
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In another recent experiment, a separate group of scientists at Japan's Yokohama City University Graduate School of Medicine successfully grew sperm in a lab dish and used it to give rise to healthy offspring. But in that case, rather than using embryonic stem cells, the researchers used testicular tissue from baby mice, bathed it in nutrients in a dish and were able to produce sperm in the lab for two months.
The discovery, reported in Nature in March, was preliminary, but holds great potential if replicated in humans. Healthland reported:
The findings suggest that testicular tissue can be frozen and saved for later use — a potential advance that could help preserve fertility in certain patients, such as young boys facing cancer treatment. Adult cancer patients can freeze sperm before starting chemotherapy or radiation, but that isn't an option for boys who haven't entered puberty yet.
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Saitou cautions that his work is also too early to translate to humans: "We have a long way to go before it can be applied in humans," he said. The Journal reports that he's already turned to his next scientific goal: to use embryonic stem cells to create healthy, functioning eggs.