Using human embryonic stem cells, U.S. researchers have successfully created the types of cells that make eggs and sperm, which is expected to help us understand some of the more mysterious stages of early human development. “We are really trying to look at the origins of normal and abnormal human development by going to the source,” Dr. Renee Riejo Pera of Stanford University in California told Reuters, which reports that these stages of the human reproductive cycle can’t be studied in animals because the genes involved are found in humans only. Researchers will now be able to study these development phases to learn more about inherited diseases and possibly develop new treatments for infertility. In the study, published in the journal Nature, researchers created germ cells (which give rise to eggs and sperm), then turned on and off genes they thought were crucial to turning stem cells to immature germ cells. Producing too few germ cells, or poor quality ones, is a major cause of infertility in humans. “We think if there’s immature germ cells that are available in a person, we might be able to use this system to mature them and push them forward into development,” said Dr. Kehkooi Kee, one of the researchers involved.