The first step in stem cell testing is to actually cultivate a mature stem cell in a lab dish. Once the researcher has the cell, he can determine whether the body can use it. Cells that produce insulin, for instance, are only useful to the body if they continue this function once they have been implanted. Thus, much time is spent figuring out whether cells will actually integrate with the surrounding tissues in the body.
Researchers need a model to test this viability, so they rely on animals that have a similar genetic makeup to our own. Experimentation on this level requires a painstaking attention to detail, and plenty of trial and error. A stem cell that is meant to restore spinal function to an animal may not cure the issue, but may allow the animal some control over its bladder. These benefits aren’t exactly what the researchers hoped for, but they can still benefit many patients. So there is a tradeoff of possible outcomes that scientists must be aware of.
In most of these cases, the results of a single test do not match actual use cases. Most of the animal models are “best guesses,” and may not fit the human scenario perfectly. This is most obvious in mice: where the same disease can exist in both humans and mice, but have very different outcomes. Researchers require many test cases in different animals to try and approximate the goals of treatment, or else the drugs won’t function properly when used in the real world.