In ancient myth, a chimera was an animal with a human head and, say, the body of a horse or a lion. That’s not what’s going on here.
In biology today, a chimera is an animal that comes from two or more embryos. This happens naturally, when twins are conceived but the two fertilized eggs fuse into one embryo, eventually producing one individual.
In research, scientists create chimeras in order to study how cells function. Mice chimeras are now commonplace in stem cell labs around the world. Researchers add stem cells to an early-stage mouse embryo (a blastocyst). If the experiment goes well, the developing mouse will have cells from two sources: the “host” embryo and the implanted cells. The implanted cells often integrate into the body and brain of the mouse pup. By this test, researchers know that the implanted cells are truly stem cells—or, more precisely, that they are pluripotent, capable of becoming any type of cell in the mouse body.
Researchers also implant human stem cells into mice. If they multiply and are fully integrated into the body, it’s pretty clear that they are pluripotent and capable of functioning within a living biological system and not just in a dish in a lab. In that case, the mouse is an “inter-species” chimera. Two embryos, of course, but from two different species, human and mouse.
For all the ways in which mice resemble human beings, there are big differences, some of which are particularly noticeable at the earliest stages of life. So when researchers at the Oregon National Primate Research Center at Oregon Health & Science University tried to put pluripotent monkey stem cells into monkey blastocysts, they failed. At the blastocyst stage, Rhesus monkeys don’t behave like mice.
The Oregon team, led by Shoukhrat Mitalipov, kept trying other approaches, finally discovering a completely different technique. Instead of using embryonic or pluripotent stem cells and adding them to a blastocyst, they backed things up, at least in terms of embryonic development. How far back? All the way to the four-cell stage. When a Rhesus monkey egg is fertilized (in this case, in a lab dish), it divides into two cells, then four. What happens if two cells in one blastocyst were combined with two cells from another blastocyst? Success—but still only partly so.
So they tried another approach, one that seems complex and counterintuitive. Researchers “aggregated” three blastocysts—and “they” began to function as one embryo. Four blastocysts—same result. Five, even six blastocysts. They did this 29 times and produced 29 viable chimeric embryos. Or to quote the original paper: “Remarkably, all 29 aggregates developed to blastocysts…”
Just what will this mean for the field of stem cell research? At the very least, this research points to the complexity of living biological systems. It’s nice to think that researchers can extract pluripotent stem cells, keep them multiplying indefinitely, direct them to develop just the right way, and implant them into the human body to regenerate tissues. If only it were that simple. As the field advances, it is clear that what was once called “pluripotency”—the ability to become any cell type—is anything but clear or simple to define.
All the more reason, I believe, why the field needs to move forward as a whole. It’s morally and scientifically simplistic to say that the field can advance without cells from embryos.
But does the Oregon work suggest a step too far? For many, it may be morally permissible to work with cells derived from blastocysts, perhaps donated from IVF clinics and due to be discarded anyhow. But what the Oregon work seems to signal is that when it comes to primates—including human beings—the cells in the living blastocyst are significantly different from the cells derived from the blastocyst. The cells in the living blastocyst, though dynamic and changing, can be regarded as totipotent, capable of becoming any cell type including the placenta and umbilical cord. Cells derived from the blastocyst—human “embryonic” stem cells or pluripotent cells—have lost part of this potential.
Does this mean that research, in order to go forward, needs access to cells as they exist in living blastocysts? That would be a step clearly beyond federal funding guidelines (the “Dickey-Wicker Amendment”). Even with private funding, it would likely exceed what most Americans can support. In some states and many countries, it would be plainly illegal.
And yet this is exactly what lead Oregon researcher Shoukhrat Mitalipov seems to have in mind. "We need to study not just cultured embryonic stem cells but also stem cells in embryos,” Mitalipov said in a release from the journal Cell. “It's too soon to close the chapter on these cells." Is that OK as long as he sticks to non-human primates?
Mitalipov is clearly right: "We cannot model everything in the mouse." Rodents and primates are different in unexpected ways at the earliest stages. Stem cells inserted in mouse blastocysts form chimeras, but not in primate blastocysts.
Quoting Mitalipov once again: "The possibilities for science are enormous." All the more reason to think this through. As complex as the science might be, the moral and religious implications are even more complex.
I for one need time to think this through. I hope to be back here before long with some more thoughts. For now, let me recommend a statement that I helped prepare a few years ago on the question of chimeras.
The paper, "Generation of Chimeric Rhesus Monkeys," was released on January 5 and will appear in the January 20, 2012 issue of the journal, Cell.