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Organoids, Chimeras, Ex Vivo Brains – Oh My!

By Henry T. Greely

Image courtesy of Wikimedia Commons

At about the time of the birth of modern neuroethics, Adina Roskies usefully divided the field into two parts: the neuroscience of ethics, what neuroscience can tell us about ethics, and the ethics of neuroscience, what ethical issues neuroscience will bring us (1). At some point, in my own work, I broke her second point into the ethics of neuroscience research and the ethical (and social and legal) implications of neuroscience for the non-research world. (I have no clue now whether that was original with me.)

The second part of Roskies’ division of neuroethics, the ethics of neuroscience research, has always had a special place in my heart because early work in it really helped mold the field we have today. In the early ‘00s, groups that mixed scientists, physicians, and ethicists, largely through the efforts of Judy Illes, explored what to do about abnormal brain scans taken from otherwise healthy volunteers. (See, e.g., 2, 3) It had become clear that, in the computer-generated imagery of a brain MRI, more than 20 percent of “the usual subjects” (college undergraduates, usually psychology majors) and about half of “mature” subjects had something “odd” in their brains. These could be variations of no clinical significance, such as “silent” blockages or benign tumor to potentially very serious problems, such as malignant tumors or large “unpopped” aneurysms. Happily, only small fractions of those oddities held clinical significance, but this still posed hard questions for researchers, many of whom were not themselves clinicians. What, if anything, should they tell, and to whom? And so, working together, scientists, clinicians, and ethicists talked with each other, learned from each other, and came up with useful answers, usually involving both changes to the consent process and a procedure for expert review of some worrisome scans.

That model of close and fruitful interaction between the neuroscience researchers and people with ethics or legal expertise has, I think, largely persisted in neuroethics, at least in North America. The early conversations about the disclosure of results deserve significant credit for that. 

Ethical issues in neuroscience research have continued to appear, on questions from confidentiality to consent. But largely without me. I’ve thought of my neuroethics work as almost entirely on the ethical, legal, and social effects of neuroscience outside the research setting.  Until now.* 

Last April, Nature published a comment written by Duke University’s Nita Farahany, me, and 15 others, including 11 scientists (4).  The piece, The Ethics of Experimenting with Human Brain Tissue, grew out of a May 2017 workshop held at Duke by its Science and Society Initiative with help from the NIH BRAIN Initiative.  It dealt with…wait for it…organoids, chimeras, and ex vivo brains, things I call “human brain surrogates”, and pointed out the dilemma they embody. Ethical constraints limit what we can do to the brains of living people, leading us to create surrogates for people’s brains – but the closer the surrogate comes to the living brain, the more we back into those same ethical constraints.

Image courtesy of Flickr

The best way to study how a human brain works is by studying a living human brain in a living human being. But this has problems. Humans are terrible lab animals – we disobey, we lie, and we can call lawyers. Researchers can’t “sacrifice” us at the right moment in the research and then carefully examine slices of our brain.  We hold rights that make much intrusive and risky, but potentially illuminating, research ethically (and legally) impossible. So, researchers look for surrogates for living human brains in living human beings, surrogates that do not have such an inconvenient moral status. Mouse brains in mice, monkey brains in monkeys, thin slices of human brains in Petrie dishes – all these have research value. But we know, from decades of disappointment in moving those findings to humans, that, although similar in some ways, none of them is the same as, or perfectly predicts, a living human brain in a living human.

Enter three new or improved technologies. (For more information on any of these technologies, see the Nature Comment; for much more information, read its references.)

1) “Organoid” is the term generally used to refer to a small ball of human cells grown in cell culture from stem cells (human stem cells for human organoids). The stem cells may be embryonic stem cells, induced pluripotent stem cells, or other types of stem cells, but the effort has been to get cells that will all become one or more cell types found in an organ. Thus, there are human liver organoids, kidney organoids, gut organoids…and yes, brain organoids. The human neural organoids have been grown for over three years – and some of them have survived for over two years (4). They have diameters of about 4 millimeters (or a sixth of an inch), about the size of a very small pea (4). They have no vasculature and so the cells need to be in contact with the oxygen and nutrient bearing (and waste bearing-away) culture media. Currently human neural organoids have about two to six million neurons (no other brain cells so far, just neurons). They self-organize, grow synapses, fire, and continue to get more and more complex as time goes on. Still, by comparison, the human brain is estimated to contain approximately 86 billion neurons (5). 

2) Chimeras – in this case, human/non-human brain chimeras – are creatures with some human brain cells and some non-human brain cells. (Thus far, in brains at least, they are always non-human animals with some human cells, not humans with some non-human cells.)  Chimeras have been used in research for many years, though organoids are opening new possibilities: such as transplanting human organoids into rodent brains – which turn out to grow blood vessels for them (6). 

3) Researchers have also long used human brain tissue kept alive outside the body – ex vivo tissue – but what is used and how is, like chimeras, becoming “new and improved.” Instead of keeping flat sheets of human brain cells alive in a dish, researchers are keeping alive and studying larger and larger chunks of human brains, taken from neurosurgical discards or from the recently dead. There are even some efforts, so far only in non-humans, to keep whole brains from dead animals “alive” apart from their bodies (7).

Other than being creepy, what do these all have in common? They are efforts to understand human brain function, and ultimately human brain diseases, better by making human brains that do not have the rights of human “persons” and hence can be used more broadly, and more roughly, in labs.  But as noted below, there’s a dilemma: the closer the surrogate comes to the living human brain in a living human, the more questions it raises about whether the surrogate has a moral status – and, if so, what status?

A retinal organoid

Image courtesy of Flickr

The Comment lays out some, but by no means all, of the questions these surrogates raise – lays them out but does not answer them. Some of those questions are about the fully human persons whose cells or tissues are used in the research and some are about the “thing” being studied itself.  

There is much work to be done on the ethics of this kind of neuroscience research, but, like the work on disclosure of MRI findings to subjects, it is work that requires the union of scientific, medical, ethical, legal, and other expertise. To know what to think of a human neural organoid, it is important to know something at least of what, if anything, that organoid can sense, perceive, do, or, possibly at some point, think. But the science is only a start to the question. We know mice, rats, and monkeys sense, perceive, do, and think things but we will allow some research with them. The ethical questions need to inform the scientific questions about these things; the scientific findings need to inform the ethics answers.

Happily, this is happening. The authors of the Comment, scientists and ethicists, recognized the appropriateness of concern about this research – not so much as it exists today but for what it may (or may not) become in five or ten years. And they recognized the need to work together. This kind of collaboration, indeed, is built into the NIH BRAIN Initiative, an effort that, at heart (and brain) is fundamentally about creating new tools for neuroscience research, and ultimately brain treatment. Its Multi-Council Working Group, made up of directors and outside advisory council members from 10 NIH Centers and Institutes as well as some at-large representatives (I’m one). It contains a Neuroethics Division (8), chaired by Dr. Christine Grady, chair of the Bioethics Department at the NIH Clinical Center, and myself. That division includes both scientists and ethicists among its members and the Duke workshop had its origin in some of the Division’s work. 

But there is much more to be done, on these topics and others, by that group and many others. Think about possible issues raised by using CRISPR to modify non-human primates by giving them human versions of some genes. (A National Academies of Medicine forum will do so this October (9)).  Or the questions of another human brain surrogate, an in-silico version, if it approaches close enough to consciousness to prompt concerns about its moral status. 

All scientific revolutions are ultimately based on revolutions in tools. The MRI, fMRI, animal models, thin slices, and other tools have taken us far but the next generation of tools is coming. It will make possible much scientific progress, but will undoubtedly raise many ethics issues, issues that will be important, complicated, and (usually) fun. Come and play!

P.S.   And join the International Neuroethics Society! Our Annual Meeting on November 1 and 2, 2018 in San Diego features a panel on these issues:

*Actually, I was wrong about that, a mistake it may be useful to explain. I have been an author, from 2003 to 2017, on six pieces involving human/non-human chimeras (10-15), at least two of which were specifically about brain chimeras. (11, 12) But, in spite of being involved in neuroethics since its modern birth, I put those in a different category – they were part of my stem cell work with some connections to my “weird life forms” interests. I – and we – need to remember to define neuroethics broadly.


Hank Greely is the Deane F. and Kate Edelman Johnson Professor of Law at Stanford University, where he directs its Center for Law and the Biosciences as well as the Stanford Center for Neuroscience and Society. He began serving a two-year term as president of the International Neuroethics Society in November 2017. He chairs the California Advisory Committee on Human Stem Cell Research; and serves on the Neuroscience Forum of the National Academy of Medicine; the Committee on Science, Technology, and Law of the National Academy of Sciences; and the NIH BRAIN Initiative’s Multi-Council Working Group, whose Neuroethics Division he co-chairs. And he likes playing (doubles) tennis even though he is, to be charitable, not very good.


1.  Roskies, Adina L., 2002, “Neuroethics for the New Millennium”, Neuron, 35(1): 21-23. doi:10.1016/S0896-6273(02)00763-8

2.  Kim, B.S., Illes, J., Kaplan, R.T., Reiss, A., Atlas, S.W. Neurologic findings in healthy children on pediatric fMRI: Incidence and significance, 23 Am J Neurorad.1674 (2002);

3.  Illes, J., Desmond, J., Huang, L.F., Raffin, T.A., Atlas, S.W. Ethical and practical considerations in managing incidental neurologic findings in fMRI,  50 Brain and Cognition 358 (2002).

4.  Nita A. Farahany, Henry T. Greely, et al., The Ethics of Experimenting with Human Brain Tissue, NATURE, 556:429-32 (April 26, 2018)

5.  Society for Neuroscience, BRAIN FACTS at 5 (2018), available for download at file:///Users/hgreely/Downloads/Brain%20Facts%20Book%202018%20PDF.pdf, accessed July 12, 2018.

6.  Mansour, A. A. et al. (2018) An in vivo model of functional and vascularized human brain organoids Nature Biotechnol. 36:432041,

7.  Antonio Regalado, Researchers Are Keeping Pig Brains Alive Outside the Body, Technology Review (April 25, 2018),

8.  The BRAIN Initiative, Neuroethics Division of the BRAIN Multi-Council Working Group,, accessed July 12, 2018

9.  Forum on Neuroscience and Nervous System Disorders, National Academy of Medicine, Transgenic and Chimeric Neuroscience Research: Exploring the Scientific Opportunities Afforded by New Nonhuman Primate Models – A Workshop, at, accessed July 12, 2018

10.  Henry T. Greely, Defining Chimeras – and Chimeric Concerns, American Journal of Bioethics, Vol. 3, issue 3:17-20 (2003)

11.  Mark Greene, et al., Moral Issues of Human-Non-human Primate Neural Grafting, Science 309:385-386 (July 15, 2005)

12.   Henry T. Greely, Mildred K. Cho, Linda F. Hogle, Debra M. Satz, Thinking About the Human Neuron Mouse, American Journal of Bioethics:  NEUROSCIENCE 7:(5) 25-40 (May/June 2007)

13.  Henry T. Greely, Human/Nonhuman Chimeras: Assessing the Issues, in Oxford Handbook of Animal Ethics (ed. Tom Beauchamp and R.G. Frey, Oxford Univ. Press, 2011)

14.  Henry T. Greely, Academic Chimeras?, The American Journal of Bioethics, 14:2, 13-14 (2014)

15.  Jun Wu, et al., Stem Cells and Interspecies Chimeras: Past, Present, Future, Nature 540:51-59 (Dec. 1, 2016)

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Greely, H. (2018). Organoids, Chimeras, Ex Vivo Brains – Oh My! The Neuroethics Blog. Retrieved on , from


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