Sperm have been created from stem cells and injected into eggs before, but the resultant embryos stopped growing after a couple of days. 鈥淲e think it鈥檚 a big step forward,鈥� says Karim Nayernia at the University of Newcastle upon Tyne, UK, a member of the team that carried out the research.

Stem cells were extracted from early mouse embryos, and coaxed into becoming sperm by manipulating the environment in which they were grown in the lab. These sperm were then injected into normal mouse egg cells to fertilise them (Developmental Cell, DOI: 10.1016/j.devcel.2006.05.010). Of 65 embryos implanted, seven mouse pups were born, six of which survived to adulthood.

Problems with the procedure remain, however. The six mice that survived were all abnormally large or abnormally small, and were infertile. Many developed lung tumours and none lived for more than five months 鈥� well short of the typical three-year mouse lifespan. Also, the sperm themselves had shortened tails and were unable to penetrate eggs on their own.

鈥淭he [decision is] a big deal, because the Supreme Court rarely ever grants a review,鈥� says Richard Lazarus, professor of law at Georgetown University in Washington DC and co-counsel for the plaintiffs in the case.

The Supreme Court only agrees to hear about 1% of roughly 8000 petitions it receives each year. 鈥淚t means that they think it鈥檚 a very important issue and they need to review it now,鈥� he adds.

On trial

The case will put the evidence for climate change 鈥渙n trial鈥� (see Climate change: Let the Supreme Court decide) to determine whether the available data are enough to say that CO₂ emissions pose a reasonable threat to the public鈥檚 wellbeing.

The Clean Air Act mandates the EPA to regulate emissions from mobile sources that 鈥渃ause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare鈥�.

A later section of the law defines welfare to include weather and climate effects. Last year, a group of 12 US states and several non-profit environmental groups led by Massachusetts sued the EPA, claiming that this law obligates the EPA to regulate cars鈥� CO₂ emissions.

In July, the US Court of Appeals ruled against Massachusetts, which then appealed to the Supreme Court.

Regulate pollutants

The EPA and the US Department of Justice, which is representing the EPA in the case, could not be reached for comment because severe flooding in Washington DC forced their offices to close on Monday.

Gloria Bergquist, spokesperson for the Alliance of Automobile Manufacturers, a co-defendant in the case, notes that 鈥渘obody knows why the Supreme Court has chosen this case. It could be that the Supreme Court just wanted to clarify the issue or uphold the lower court鈥檚 decision on different grounds.鈥�

If the court rules in favour of the plaintiffs, it could either force the EPA to regulate CO₂ as a pollutant or merely compel it to reconsider the issue.

A ruling in the case is not expected until spring or summer 2007.