Coming home to roost

Hoo boy.

Scientists who want to study human embryonic development have heretofore been self-limited by a 14-day rule:  embryos can only be experimented on up to 14 days of age, when they start to develop a nervous system.  This is an attempt to avoid censure for unethical experimentation on human subjects, and is seen as something of a concession, since it does not accept that human life begins at conception.

And, inevitably, they seek work-arounds.  One reported this week by Nature is the creation of human chicken hybrid embryos.  Why would someone want to do this?  (Jokes about the San Diego Chicken are NOT called for here.)

Well, apparently 14 days of embryo age is when critical organization takes place, directed by “organizer cells” that don’t appear before then.  So a group of researches did this:  they took embryonic stem cells (which itself might well require creating and destroying an embryo), and made “embryo-like structures” that had cells that either were, or were just like, these “organizer” cells.  (Apparently these structures were not capable of growing into babies, but even if not, ethical issues remain.)  Then they transplanted these cells into chicken embryos, and watched the resulting hybrid grow, and learned something about how human embryos develop.  They figure this is less of an ethical problem than trying to experiment on a fully human embryo older than 14 days, and that hybrids like this might be able to take the place of experimenting on human embryos to answer many of their questions.

Other scientists disagree with this last statement, and still think they must experiment on fully human embryos to get their answers.

Either way, at a minimum it seems that this work will require creating embryos solely for research, and there is in principle no limit on manipulating the human organism in the name of knowledge.  Work is common on some kinds of “hybrid” animals with human cells, such as immune-deficient mice who have human cells transplanted to reconstitute their immune systems.  But that work usually is done with human cells transplanted into fully-formed mice, which appears different from early, hybrid embryos.

The article describing this work says that the hybrid embryos “didn’t live long enough to hatch.”  Wonder what they would have been like if they had.

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Mark McQuain
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I’m rather surprised the14-day rule has lasted as long as it has, particularly after that conference in the UK last year where the majority of researchers attending urged breaking that barrier. Given what you describe here, there seem to be a growing number of new pathways to allow us to slip further down this slippery slope of experimentation on a few vulnerable members of our species for the benefit of the rest of us.