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Some hope that sperm cells could one day be derived from the skin cells of a man who is otherwise sterile and that a similar process cold produce viable egg cells from a sterile woman's body.

Israeli and UK researchers have created human sperm and egg precursor cells in a dish, starting from a person's skin cells. The achievement is a small step towards a treatment for infertility, although one that could face significant controversy and regulatory hurdles.

The experiment, reported online in Cell on 24 December1, recreates in humans parts of a procedure first developed in mice, in which cells called induced pluripotent stem (iPS) cells reprogrammed cells that can differentiate into almost any cell type are used to create sperm or eggs that are subsequently manipulated to produce live births by in vitro fertilization.

In 2012, stem-cell biologist Mitinori Saitou of Kyoto University in Japan and his collaborators created the first artificial primordial germ cells (PGCs)2. These are specialized cells that emerge during embryonic development and later give rise to sperm or eggs. Saitou made them in a dish, starting with skin cells reprogrammed to an embryonic-like state through iPS-cell technology (see 'Stem cells: Egg engineers'). They also were able to achieve the same result starting with embryonic stem cells.

Although his cells could not develop beyond this precursor stage in the dish, Saito found that if he placed them in mouse testes, they would mature into sperm, and if he placed them in ovaries, they would mature into functional eggs. Both sperm and eggs could be used for in vitro fertilization.

Efforts to engineer similarly functional gametes in humans have produced PGC-like cells, but with such a low efficiency success rate of turning stem cells into gametes that it was difficult for others to expand on the work.. Previous efforts also required the introduction of genes that would render the cells unusable in the clinic.

Ewen Callaway reports on the ethical challenges of using lab-made sperm and egg cells in fertility treatments.

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Now a team led by Azim Surani of the University of Cambridge, UK, and Jacob Hanna of the Weizmann Institute of Science in Rehovot, Israel, has replicated the in vitro portion the first half, says Hanna of Saitous efforts in humans.

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Rudimentary egg and sperm cells made from stem cells