To put simply (possibly too simply) DNA contains more information than will ever be “expressed” in an individual human being. Our DNA contains coding for things that will never develop, because it dates back to before there was even a hominid species. It contains endogenous retroviruses that have embedded themselves into human DNA millennia ago, mitochondria that hide themselves for generations. It’s all there, waiting to be interpreted by RNA, like a slab of clay awaiting a sculptor.
Using the DNA, the RNA sends signals out to proteins that will build cells. It always tells the proteins things like “You over there, you’re going to make skin. You on the left: fingers. Way in the back, corneas, and you on the bottom step there, get to work on bile ducts.” As the cells multiply, their differentiation increases, turning single cells into fully formed organs and extremities and bones and muscles and so on. What the RNA also does as part of this process is decide whether, say, the proteins making the heart will grow one with mitral valve prolapse like what came from Mom’s side of the family, or maybe a mild atrial afibrillation that came from Dad’s – or nothing wrong at all, that’s been known to happen time and again in the ancestral line.
During fetal development, it’s all about cell differentiation and the formation of brand-new body structures. After birth, it’s “make more of these,” which is what triggers growth. Around adolescence, the message begins to change to “replace the cell that just died.” Epigenetics, the process by which the message from DNA to RNA to proteins becomes cells, is influenced at every stage by interruptions from various sources, sometimes leading to beneficial results, sometimes to disasters when replacing cells lost to apoptosis.
Adult stem cells have reached the final stage of the epigenetic process, having been given signals to differentiate and being exposed to all manner of environmental factors that interfere with the process of replacing expired cells with new ones. The copies, because of this, are not always identical, which is why we develop diseases from exposure and why we deteriorate with age. These stem cells have a lot of history to be erased, confounding their usefulness for research, and making them pretty much unusable for anyone except the individual from whom they were taken.
Fetal stem cells are more likely to be usable for research because they are still at the differentiation stage, but the messages being interpreted by the proteins are already being affected by external factors. Because differentiation has occurred, we need blood cells for blood, bone cells for bone, etc. – you can’t make a fetal stem cell change its differentiation (yet). Exposure to chemicals in the mother’s body or the environment around her, or to hormonal changes from any part of the endocrine system, can change the message being sent to the proteins. This message will remain in the harvested cells, making them somewhat useful for research, but not helpful for any clinical application of stem cell technology.
Embryonic stem cells, on the other hand, are about as close to a tabula rasa as we can get. The RNA has not yet begun to trigger the proteins into cell development, so without reverse engineering, we can program these cells to become anything. We’ve shown this in insects, by altering the histones and by transplanting differentiated cells to other parts of the body, and ending up with eyes and legs coming out of parts of the body that don’t normally sport eyes or legs. It’s the embryonic cells that allow us to observe the process as it happens and find out how and why it works.
Going back to the clay metaphor, adult stem cells are like a partially dried-out bust, and you have to figure out how to make it look like the nose didn’t fall off. Fetal stem cells are like an armature and a foundation, and you have to figure out which parts were supposed to be each facial feature. Embryonic stem cells are the raw materials that will allow you to make a finished product the right way the first time.