Not all stem cells are created equal. But just how close adult and reprogrammed stem cells can come to matching the capabilities of embryonic stem cells has become a contentious question in the debate over whether the federal government should continue funding research on embryonic lines. And many researchers maintain that a diverse stem cell portfolio will increase medical discovery dividends.

Human embryonic stem cell (hESC) research had been backed by federal funds for more than a decade, but a surprise August injunction by a federal judge threw the field's future into question. The judge made his ruling based on a strict interpretation of a 1996 budget amendment (the Dickey–Wicker Amendment) that prohibited federal funds from being used for research that destroyed or endangered an embryo.

"We thought the fight was over," Tom Harkin (D–Iowa) and chairman of the Senate Appropriations Subcommittee on Labor, Health and Human Services, Education, and Related Agencies said at a hearing on embryonic stem cell research on Thursday. Under the Obama administration, the number of embryonic stem cell lines available for federally funded research had more than tripled, but no money was going toward the creation of any cell lines (a process that destroys the embryo).

Despite an appeals court's temporary lift this month of the funding ban, the injunction "has placed a cloud of uncertainty over this entire field," Harkin noted.

Other lawmakers are keen to see embryonic stem cell research funding curtailed more permanently. Sen. Roger Wicker (R–Miss.), of the Dickey–Wicker Amendment, explained that the advent of hESC research in the years since the amendment was first written has only underscored what he described to be the intentions of the legislation: to avoid tax dollars from being spent on any research that has endangered an embryo.

As rationale for moving federal funds away from hESC research toward that involving adult stem cells, some researchers, lawmakers and advocates argue that adult stem cells have already been used to develop proved cures, such as those for some blood diseases. Others in that camp suggest that reprogrammed adult cells, (induced pluripotent, or iPSCs) can effectively replace the need for pluripotent embryonic stem cells.

"If we can use adult stem cells [and] reprogram them to act like embryonic stem cells, why would we not take that approach?" Wicker asked.

Adult stem cells, reprogrammed or not, however, have not been shown to have the same level of flexibility in becoming any cell in the body.

"Human embryonic stem cells remain the gold standard for pluripotency," Francis Collins, director of the National Institutes of Health, said at the hearing.

Other scientists argue that the discussion should not be an either–or debate. "I think it's a mistake to cast the different types of stem cells as competing priorities," George Daley, associate director of the Stem Cell Program at Children's Hospital Boston, said at the hearing. "ES cells are not contestants of Survivor that should be voted off the island," he said.

Sean Morrison, director of the Center for Stem Cell Biology at the University of Michigan at Ann Arbor, pointed out that pitting different kinds of stem cell research against each other makes little sense from a research perspective. "It's scientifically meaningless to frame this as a debate between embryonic stem cells and adult stem cells," he said at the hearing. Embryonic stem cells helped researchers develop other research tools, such as iPSCs, he pointed out.

The first hESC was isolated in 1998, and researchers are still working to find the strengths and nuances of each type of stem cell. "All of them are interesting; all of them are important," Collins said about the three types of stem cells. The science is still too new to know for sure which types of stem cells will be best suited to which kinds of research and treatments. "Shouldn't we be pursuing the most exciting options in parallel?" he asked.

A growing body of research is pointing to crucial differences in how each of these cell types behave. "Despite our best efforts" to make iPS cells behave like hES cells, "there are still some differences," Daley said. Recent findings have shown that iPSCs tend to retain a cellular memory of the type of cell from which they have been derived. Such a recollection can be an advantage if scientists are looking to turn a reprogrammed skin cell back into skin cells, he noted. But "the fact that that memory exists, that might actually thwart your research" if it treats iPSCs as purely pluripotent as hESCs.

Collins and others argue not just for a permanent removal of the injunction to resume research with confidence, but also for an extension of the number of embryonic stem cell lines available to federally funded researchers. He noted that even the Obama administration's 75 approved stem cell lines (increased from the Bush administration's 21) are not enough for robust research in the field. In order to thoroughly investigate cures for a diverse population, cell lines from a large sampling of individuals are needed. And to study many developmental disorders, such as fragile X syndrome, researchers would be well served to be able to study a stem cell line that contained the relevant mutations. "We're just scratching the surface of the potential," Collins said of embryonic stem cells.