ananyo writes "Japanese researchers have coaxed mouse stem cells into becoming viable eggs that produce healthy offspring. Last year, the same team successfully used mouse stem cells to make functional sperm (other groups have produced sperm cells in vitro). The researchers used a cocktail of growth factors to transform stem cells into egg precursors. When they added these egg precursor cells to embryonic ovary tissue that did not contain sex cells, the mixture spontaneously formed ovary-like structures, which they then grafted onto natural ovaries in female mice. After four weeks, the stem-cell-derived cells had matured into oocytes. The team removed the oocytes from the ovaries, fertilized them and transplanted the embryos into foster mothers. The offspring that were produced grew up to be fertile themselves."