Some people think that destroying an unimplanted blastocyst to make embryonic stem cells is wrong. Some equate it with killing a human being. To overcome these objections, some scientists are working out alternative techniques, like using just one cell from an embryo with 8–10 cells, or removing just a few cells rather than all of them from a blastocyst that is older than typically used. So far, these techniques tend to have lower success rates and have not been proven in humans. Perhaps more importantly, these techniques could unintentionally destroy or damage the embryo, leaving the ethical issues unresolved.

Still other alternatives include making embryonic stem cells that stop developing on their own (called “dead” or “arrested” embryos) or genetically engineering sperm, egg or donor nucleus so that embryos would be unable to move past a certain point in development. These solutions are also problematic: they would require gauging whether development has stopped, or they would require fertilizing an egg, just to destroy it. Advocates of these techniques also assume that a flawed embryo is morally inferior to one that has the potential to continue development. Additionally, whatever flaw stops the embryos from developing could mean that the embryonic stem cells are also flawed. That could make the cells less useful for study.

Another approach is to try to prompt nonembryonic stem cells to behave like embryonic cells, a feat that has been accomplished in mice and humans by fusing adult cells with embryonic stem cells. (These cells have chromosomes from both cells, although there might be ways to remove the extras.)

Credit: Jenny Nichols (c) Institute for Stem Cell Research

In reports released in 2007, rapidly dividing adult mouse cells behave shockingly like embryonic stem cells if they are genetically engineered to produce a set of just four proteins. These cells differentiate into many types of tissues and divide indefinitely. If mixed in with an early mouse embryo that then grows into a mouse pup, the engineered cells differentiate into multiple organ types. Whether or not these manipulated cells are as versatile as embryonic stem cells is not yet known, though results look promising.