The longer that researchers work with embryonic stem cells the more issues seem to crop up. The idea that ESCs can survive indefinitely in culture, thereby providing an inexhaustible source of cellular treatments, is only partially true. Recent studies have shown that while ESCs will reproduce quite happily again and again in suitable growth medium, over time they develop chromosomal abnormalities similar to those found in some cancers. Stem cell pioneer James Thomson agrees that embryonic stem cell lines have a limited "shelf life." He notes that "over time, you accumulate mutations. It’s a fact of life. It’s just a question of differences in the rates. If you accumulate enough of those mutations, you could actually create a cancer." (Humber 2004) In fact, the dual threat of mutations and the introduction of mouse viruses in Thomson’s original stem cell lines is one of the reasons cited by researchers for lilting the Bush administration’s restrictions on funding for new lines of embryonic stem cells. (Humber 2004)
Moreover, while it is true that embryonic stem cells can be used to create "any kind of cell in the body," that same developmental elasticity works against ESCs as well as in their favor. In fact, only a few researchers have been able to differentiate an embryonic stem cell culture into a pure cell culture of the exact kind of cell they were seeking. It is even more questionable whether the researchers who have been able to differentiate the ESCs into targeted cell populations have been able to consistently repeat the task. In the vast majority of "successful" attempts to change embryonic stern cells into specific cell types, the result was instead a petri dish that contained an unhealthy melange of unwanted cells along with the target strain. (Holland et al 2006)
But for now, only embryonic stem cells can he considered truly pluripotcnt, their inner essence still molten enough to shape. "They (ESCs) are a blank slate," stated Dr. Theo Palmer, neuroscientist at the Stanford University School of -Medicine. "They do not know what their role is. An adult stem cell has enormous potential that’s already been realized." (Humber 2004) Palmer asserted that embryonic stem cells should he easier to work with than the adult version for this very reason.
While scientists, admittedly, are still groping for ways to reliably "retrograde" the blank slate of the embryonic stem cell, the possibilities just seem too great to ignore. And embryonic stem cells, since they come from a point where the organism has yet to mature, simply provide much more insight into the complexities of stem cell function and development. Ironically, years of research spent on embryonic stem cells are very likely to teach scientists how to best reprogram an adult stem cell to make tissues as easily as an embryonic stem cell can.