MOUSE embryos have been made by fertilising eggs with immature sperm cells derived from embryonic stem cells. 鈥淏ut we will be quite surprised if the embryos develop normally,鈥� warns George Daley of Harvard Medical School.
Earlier this year, a group in Japan showed that mouse embryonic stem cells (ESCs) could be turned into sperm-like cells, while a group in the US turned them into egg-like cells (New 杏吧原创, 10 May, p 4). This work might one day make it possible for infertile couples to have children, using sperm or eggs derived by therapeutic cloning. But until now, no team had reported creating embryos fertilised with lab-made germ cells.
By growing male mouse ESCs in culture, Daley鈥檚 team was able to obtain spermatids 鈥� sperm precursor cells. The Japanese team created fully formed sperm, but only by growing precursor cells in testicular tissue. When Daley鈥檚 team injected the spermatids into eggs, a fifth of the embryos grew to the blastocyst stage, which consists of hundreds of cells (Nature, DOI: 10.1038/nature 02247). The team even extracted cells from the blastocysts, creating a new line of ESCs.
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But such embryos could grow into healthy mice only if the spermatids have the right 鈥渋mprinting鈥�. As sperm and eggs form, their chromosomes first lose a series of sex-specific markings, then gain a new pattern. Although the initial set of imprints was properly erased in the ESCs as they became germ cells, the team has yet to test whether the spermatids reacquired the proper pattern.