Chimeras Raise Questions About When Life Begins

An 8-cell human embryo

Developmental Biology is a fascinating field. It’s truly amazing to learn how complex life forms grow and develop starting from a single cell, and to watch parts of the process happen. Besides being interesting, Dev Bio is also extremely useful in healthcare. By learning how something develops, you can sometimes figure out what went wrong when a birth defect happens, and hopefully learn how to avoid or repair the problem.

A sub-discipline of Dev Bio, embryology, also has importance as a major battleground for Pro-Life advocates for multiple reasons. A big complaint is the use of embryos, particularly human embryos, in various types of research. Laws between countries differ, but here in Canada there are pretty strict rules governing the ethics of research with human embryos. For example: the embryos cannot be created specifically for the research (they usually are left over from things like in-vitro fertilization), and the embryos cannot be allowed to develop past 14 days [1]. Nonetheless, according to mainline Pro-Life doctrine, those embryos are fully human as much as you or me, so their destruction amounts to murder.

Also according to mainline Pro-Life doctrine, embryology itself supports this conclusion. There are long lists out there with quotes from embryology literature that Pro-Life people have collected to try to demonstrate this point. Given those long lists, one would think that all people who study embryology would be either Pro-Life or some sort of callous evil monster!

And certainly they would be, if it actually were so simple.

In reality, embryology has revealed some strange properties of embryos that make it difficult to think of them as “individuals” in the same way we would think about an adult human. One of those properties is the ability to form chimeras from multiple embryos.

Early embryos can be split to form twins, or pushed together to form chimeras. (adaptation of Fig. 3 from Arias et al. 2013[2])
Experiments have shown that if an early mouse embryo is split into two bunches of cells, the two bunches will develop into twin adult mice. Also, if two embryos or bunches of cells are pushed together into one big bunch, only one adult mouse will form. If the two bunches were from genetically different mice, the adult mouse will have a patchwork of tissue with different DNA. Things like monozygotic twins in humans suggests that human embryos can do the same sorts of things.

So here’s where the problem shows up. You can’t do that sort of thing with individuals as we think of them. If someone gets cut in half, that doesn’t make two people. If two people get smushed together, or maybe are born physically joined with a twin, that’s still two distinct people. There’s something unique happening with early embryos that has implications for how we think about when life begins in the “individual” sense.

Think for a moment about the mainline Pro-Life, and often Christian, stance, that a human life in the fullest sense begins at conception. If life begins at conception, does it also begin when an embryo splits? If humans have a soul that comes into being at conception, is an extra one made when the embryo splits? It’s probably fairly easy to reply, “sure, splitting an embryo is just like another conception event. A new individual and new soul are now there.”

A chimera mouse on the right with two of its offspring. The patchy-coloured fur on the chimera is caused by the patches of genetically different cells that make up the mouse.

But the problem gets worse when we start smushing embryos together. What happens to that other individual? Has one of them died, and if so, which one? If we were to, hypothetically, continually split and rejoin an embryo, are we creating and killing individuals as immorally as it would be to murder an adult? A really committed mainline Pro-Life person could still defiantly reply, “yes, that must be murder too.” That sounds like like a really bare assertion, though. I would suggest that these unusual properties of embryos should be a hint that we might need a more descriptive way of thinking about them than as equivalent to adults as individuals.

One idea, which is not new, is to think of these very early embryos as something along the lines of “potential life” rather than as individuals. Think about how dew drops form on a surface.

Dew DropsThere are many sizes of dew drop forming at any given time on a particular leaf, but they have to start somewhere small and grow before they are big enough to be formed enough to fall off the leaf as a drip. In the mean-time, the big drops can be split into  smaller ones that can grow into individual drips. And small drops can be pushed together to form a bigger drop. However, they are not drips falling away until they are big enough; they are only drops. Something different from a drip, but not entirely; only different because it is not formed fully enough to have the existential properties of a fully formed individual. A condensing drip.

Maybe we can think of very early embryos in a similar way. A condensing human; not a human yet in the fullest sense, but the elements are coming together and there is value already there. It’s not a perfect analogy, but it seems helpful. I’m sure there are better analogies, but it may be that there is no clear answer to be had. What is certain is that embryology seems to be providing more questions than answers on this point.

Maybe we can push together some ideas and form some more clear answers in the future.

Sources Cited
[1]Canadian Institutes of Health Research, Natural Sciences and Engineering Research
Council of Canada, and Social Sciences and Humanities Research Council of Canada,
Tri-Council Policy Statement: Ethical Conduct for Research Involving Humans,
December 2014.

[2]Arias AM, Nichols J, Schröter C. 2013. A molecular basis for developmental plasticity in
early mammalian embryos. Development 140: 3499-3510

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