Scientists injected laboratory-created sperm into eggs, and the resulting embryos grew to the point where they would normally be implanted into a womb. The experiment was done with mouse stem cells, but mice, genetically speaking, are so close to men, few scientists doubt that the same experiment can be done in humans. The breakthrough, made by researchers at the Whitehead Institute for Biomedical Research and Harvard University, was reported in the Dec. 11, 2003 issue of the journal Nature. “This is the first time that sperm cells made in a petri dish from embryonic stem cells have resulted in the creation of an embryo,” says George Daley, a Harvard Medical School biologist who led the experiments. (Egg cells were developed from mouse embryonic stem cells earlier this year by another group of scientists.) “Such experiments are being done solely for the purpose of studying how genes regulate the transformation of stem cells into germ cells, one of the basic mysteries of life.” However, Daley quickly admits that this work has implications for genetically modifying animals, for treating human infertility, and, perhaps, for other medical purposes.