Stem cells have made what seemed like impossible biological feats attainable: treating diabetes, helping paralyzed patients to walk again, repairing heart tissue damaged by a heart attack. But even for stem cells, regenerating the most fundamental elements of life — egg and sperm — from scratch seemed too great a challenge. Or maybe not. Scientists in Japan used two kinds of stem cells from mice — those taken from days-old embryos and those reprogrammed from adult mouse skin cells — and created viable egg cells that they then successfully fertilized to generate the first ever pups born from stem-cell-derived eggs. The apparently healthy mouse pups are the ultimate test of the regenerative capabilities of stem cells and represent a breakthrough that could potentially lead to new treatments for infertility in human couples.