Santa Monica, CA -- (SBWire) -- 03/05/2010 -- The National Institutes of Health is proposing a small change to the definition of "human embryonic stem cell" that could have a big effect on their long-term ability to lead to cures for a variety of diseases.

President Obama asked the NIH to come up with new guidelines for funding human embryonic stem cell (hESC) research, and those guidelines define the cells as ones that

"are derived from the inner cell mass of blastocyst stage human embryos, are capable of dividing without differentiating for a prolonged period in culture, and are known to develop into cells and tissues of the three primary germ layers."

In plain English, that means that the cells come from embryos that are 4 to 6 days old, when they have grown into a ball of roughly 100 cells. Scientists in the lab can convert these cells into stem cells that retain the ability to grow into any type of tissue in the body, dismantling the embryo in the process.

But some researchers have complained that the definition was too restrictive. For instance, it would appear to exclude stem cells derived from younger embryos -- called blastomeres -- with only eight cells. It would even exclude embryonic stem cells derived from a single blastomere cell using the same biopsy technique employed for pre-implantation genetic diagnosis. This method has the advantage of producing human embryonic stem cells without destroying the human embryos they come from.

In a filing made Friday afternoon, the NIH acknowledged the problem with its original definition:

"This definition had the unintended consequence of excluding certain hESCs which may otherwise be appropriate for Federal funding. For example, the current definition excludes hESCs from an embryo which fails to develop to the blastocyst stage."

So the agency proposes changing the definition of human embryonic stem cells to include

"pluripotent cells that are derived from early stage human embryos, up to and including the blastocyst stage, are capable of dividing without differentiating for a prolonged period in culture, and are known to develop into cells and tissues of the three primary germ layers."

The proposed change was published in the Federal Register. Members of the public can comment on the change at http://hescregapp.od.nih.gov/comments/add.htm before it takes final effect.

One stem cell researcher, Dr. Robert Lanza, has already voiced enthusiastic support for the change. His company, Advanced Cell Technology, has developed several cell lines from single blastomeres. One of those cell lines was used to grow retinal pigment epithelial cells that could treat patients with a rare eye disease called Stargardt's macular dystrophy. The company asked the FDA for permission to conduct a Phase I/II clinical trial last year.

"It would have been a disaster to exclude these valuable hESC lines from consideration for federal funding, especially since the leftover embryos used to generate them meet all the NIH requirements," he said in an e-mail. "It shouldn’t matter what cells they were derived from. In fact, it could be strongly argued that these hESC lines are more ethical since they can be derived without embryo destruction."