Remember stem cells? They were one of the biggest scientific controversies during the early years of George W. Bushs presidency.

At the time, scientists had realized thatembryonic stem cells had the incredible capacity to transform into virtually any cell in the human body and so could potentially lead to new treatments or cures for a multitude of illnesses. On the other hand, extracting these stem cells required destroying human embryos, an action opposed by some pro-life individuals.

EMBRYONIC stem-cell THERAPIES ARE GETTING TESTED IN ACTUAL PATIENTS

The stem-cell debate got really heated. But then ... it just sort of fizzled out from public view. So whatever happened to stem cells?

A couple of things helped lessen the controversy. By the late 2000s, researchers discovered other ways to createcells similar to embryonic stem cells without destroying human embryos, a promising advance that helped defuse the culture-war aspect. Then, in 2009, Obama somewhat loosened the Bush-era restrictions on federal funding for stem-cell research and thecompromise seemed to quiet both sides down a fair amount.

So, lately, scientists have been patiently continuing their stem-cell research in a less noisy atmosphere. And that work has actually led to a few advances like restoring some sight in 10 patients with vision diseases. But the stem-cell controversy is far from dead. Researchers still might need cells from embryos to create certain treatments. If it turns out that non-embryonic stem cells aren't good enough, that could re-ignite the culture wars. So here's a guide to the debate:

Shinya Yamanaka (right) receiving flowers from Sweden's ambassador to Japan in 2012, after it was announced that Yamanaka won a Nobel Prize in medicine. (Jiji Press/AFP/Getty Images)

Embryonic stem cells attracted scientific attention because they have the potential to grow into virtually any cell in the human body say, insulin-producing cells for people with diabetes, brain cells for people with Parkinsons, or even wholenew organs to replace faulty ones.

But for many people, there was one huge ethical problem: creating them required destroying an embryo. That's why, in 2001,George W. Bush decided to limit federal funding of research to a list of 60 pre-existing embryonic stem-cell lines (so as to discourage the destruction of any more embryos). Many scientists viewed the rules as too strict. Hence the controversy.

Obama SOMEWHAT relaxed Bushs restrictions on embryonic stem cells

But then in 2007, Japanese scientistShinya Yamanaka and his colleagues managed to coax cells from adult humans into embryo-like flexibility. In other words, they were able to create cells that seemed to resemble embryonic stem cells but that didn't require destroying an embryo. (These new cells were named induced pluripotent stem cells, IPSCs.) Other researchers began finding that adult stem cells have similar, but more limited, properties, too.

Meanwhile, the politics shifted. In 2009, Barack Obama came into office and signed anexecutive order that somewhat relaxed Bushs restrictions on embryonic stem cells. Under the new rules, the federal government would fund work on new stem-cell lines, but only if they had been made from leftover embryos from fertility clinics andwith non-federal money. That compromise seemed tohelp thecontroversy settledown.

A figure of visual ability after an embryonic-stem-cell-derived treatment (red line) in patients with macular degeneration over the course of 360 days. (Schwartz et al., The Lancet, October 15, 2014)

While the controversy has calmed down, stem-cell research is taking off and scientists are making advances with both embryonic and non-embryonic cells.

Much of the initial research on stem-cell therapies has focused on eye treatments. (That's because stem-cell therapies can be unpredictable and have sometimes lead to tumors in previous experiments. A tumor in an eye would be relatively easier to deal with and remove than tumors hidden deeper inside the body.)

In October 2014, researchers from the company Advanced Cell Technology (now called Ocata Therapeutics)showed that they had created new retina cells from embryonic stem cells for 18 patients who were going blind. Afterward, 10 of them had improved eyesight. Another group of researchers in Japan is trying to do the same thing with non-embryonic cells (those aforementioned IPSCs).

10 PEOPLE WHO WERE GOING BLIND HAD Improved eyesight AFTER EMBRYONIC STEM-CELL THERAPY

Other embryonic stem-cell research has focused on developing cells that can help treat spinal-cord injuries. A company called Geron startedsafety tests in such patients in 2010.

Although a few groups are continuing to work on embryonic stem cells, many are now focusing on non-embryonic stem cells like IPSCs because they're less contentious. "Everyone jumped very, very quickly on the IPS[C] bandwagon because it was eligible for federal funding, and then also any of the controversy [regarding embryos] was dropped," says Susan Solomon, CEO of the nonprofit New York Stem Cell Foundation.

But Solomon also thinks researchers have moved away from embryonic stem cells too quickly. "We felt that it was way too early to do that," she adds. Her organization still studies embryonic stem cells, among others in part because they may be able to do things that non-embryonic stem cells can't. It's just too early to tell.

It's important to note that despite all the overhype over the years, stem-cell science has been moving at the same slow pace as most scientific fields. There are still no FDA-approved treatments that use either embryonic stem cells or IPSCs. And that means that controversy over whether embryonic stem cells are needed for science and medicine is still unresolved.

(Shutterstock)

That said, the fight over stem cells hasn't gone away forever. And there's likely to be more conflict in the future.

Even after the Obama administration relaxed the rules on funding stem-cell research, there are still plenty of hurdles. For example, federal funding is currently prohibited for research on embryonic stem-cell lines made through a technique calledSCNT or cloning, which requires creating embryos in the lab.

This technique could one day prove useful because it can turn a person's own cells into a customized embryonic stem-cell line and would therefore stop people's immune systems from rejecting stem-cell treatments.

In 2013 and 2014, two groups published the firstdemonstrations of this technique with human cells. But all such research in the US must be done with private funds.

On top of all of this, some states directly ban some or all stem-cell research within their borders no matter who's paying for it:

Note: Minnesota has a vague law on the books that's currently interpreted to mean that embryonic stem-cell research is ok. Missouri's law is a bit self-conflicting. For more details, check out The Hinxton Group's site, which includes quotations from the relevant regulations themselves.

"We went from more of a legislative vacuum to our current patchwork quilt, with legislation enacted in all of the jurisdictions where interest groups had enough clout to get the job done," Alan Regenberg, Director of Outreach and Research Support at the Johns Hopkins Berman Institute of Bioethics, told me in an email.

Several things could bring the stem-cell fight back. For example, a clinical trial could come out with some really impressive results on some sort of stem-cell treatment renewing the debate over whether regulations should be loosened. Conversely, a social conservative could run for president and bring up the ethical issues on the campaign trail. And no matter who lands in the White House in 2016, its reasonable to expect some major changes in federal policy and fast. Both George W. Bush and Barack Obama implemented their rules within the first year in office.

In 2013, Obama's stem-cell policy survived Supreme Court case Sherley v. Sebelius.

A piece on the first embryonic stem-cellmedical trials in people, by Sarah Boseley at the Guardian

Update: Clarified the current interpretation of Minnesota's stem cell laws and changed the map to match.

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Stem cells were one of the biggest controversies of 2001. Where are they now? - Vox