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Here’s what you’ll learn when you read this story:
- Lab-grown neuron groups can mimic parts of the human brain and help researchers study both the brain’s development and disorders. Now researchers have started linking these “organoids” to proto-spinal columns to understand the pain pathway.
- As these models grow more sophisticated, a concern has arisen about their possibly developing self-awareness. But these are not true “mini-brains,” researchers say.
- Ethically, the practice of implanting organoids into the brains of living animals is more fraught, and scientists are keeping an eye on its ramifications, as well as watching assembloids themselves for any signs of consciousness in the future.
In many ways, the brain is a black box. While we’ve devised various techniques to measure and track its activity, scientists are still looking for ways to directly observe the brain as it develops. Such advancements could help us better understand conditions like autism spectrum disorder, schizophrenia, and Alzheimer’s.
One relatively recent development in this arena doesn’t involve human brains at all, but rather Petri dishes holding infinitesimally small clumps of tissue called organoids. First developed in 2013 by researchers in Austria, these structures have garnered the nickname “mini brains” because they form rudimentary models of the human brain.
Organoids begin as single cells, sometimes stem cells—but they could also begin as, say, skin cells that undergo a chemical process to become stem cells—before developing into a structure comprising millions of neurons. Developed organoids tend to represent specific areas of the brain, such as the thalamus, albeit more simplified because they’re not integrated within a larger system.
As more labs have adopted this methodology, they’ve created increasingly complex structures, including interconnected systems of organoids called assembloids. One of the most developed assembloids incorporates four different types of organoids to model a pain sensory pathway, connecting brain organoids to a spinal organoid.
As assembloids become more complex and incorporate more organoids, allowing scientists to model biological features like the pain sensory pathway, the technology gives rise to the question of eventual consciousness in organoids. Specifically, at what point is it unethical to continue using them to model disorders of the human brain?
It turns out, as far as organoid ethics are concerned, it isn’t even the most pertinent question.
Part of the issue is that consciousness and its genesis are difficult to define. “There is still substantial debate about both the definition of consciousness and, however defined, what methods could be used to measure it,” says Alta Charo, JD, professor emerita of law and bioethics at the University of Wisconsin Law School.
But as organoids—and even complex assembloids—currently exist, “We can comfortably say there is no reasonable possibility of anything remotely like consciousness,” says Charo.
Researchers who work with organoids emphasize that they don’t resemble cogitating, fully formed human brains. Even the term “mini-brain” is an anthropomorphizing misnomer. “These models are not miniature versions of the brain,” says neuroscientist Sergiu Pașca, MD, a professor of psychiatry and behavioral sciences at Stanford University. “They are simplified, developmentally immature, and lack many defining features of an actual brain,” including a system of blood vessels and sensory input. Pașca and his lab constructed the four-part assembloid that models a pain sensory pathway.
They’re so developmentally immature that these organoids contain at most only 0.002 percent of the neurons in a human brain, according to Madeline Lancaster, PhD, a developmental neurobiologist studying brain size and evolution at the Medical Research Council Laboratory of Molecular Biology in the UK. As such, scale and physical integration are two significant factors for Lancaster, who developed the first organoids during her postdoctoral work in Austria. “If technology arose that could enable organoids to develop to a much larger size, say 1,000-fold larger, and start to form the proper shape and structure, and be integrated in some sort of embodied context, then we should reconsider this,” she says.
Indeed, there are bodies of experts with eyes on organoids. In 2021, the U.S. National Academies of Sciences, Engineering, and Medicine published a report addressing ethics and governance on three human brain models, including organoids. While the report states that they “do not meet any current criteria for consciousness and awareness,” as they become more complex it will “be essential to revisit these questions.” So, organoid consciousness isn’t on the table yet, but experts are very much keeping an eye out in case consciousness—or anything remotely like it—blips into existence.
Expert opinions aren’t the only ones worth keeping in mind. Considering public perception of organoids further complicates the ethics at play. John Evans, PhD, a professor of sociology and co-director of the Institute for Practical Ethics at U.C. San Diego, examines the public’s views on ethically complex topics. Regarding organoids, he has observed that a broader audience “thinks that organoids are an extension of an existing human,” retaining an “ephemeral connection” with the human who provided an organoid’s initial cells. This view, he says, is consistent with how the public regards donated blood, tissue, and solid organs.
Closer at hand than organoid consciousness, however, is the practice of implanting organoids into the brains of living animals. In 2022, Pașca and his team published a paper describing the first time human organoids were successfully transplanted and integrated into newborn rats, influencing their behavior as part of the brain. Animals like these are called chimeras, and the 2021 report describes them, too.
“Transplanting organoids into living animals does carry important ethical concerns, mainly around animal welfare,” Lancaster says. “That’s not because of the organoids themselves, but rather because of course animals do have those features of actual brains that we should care about and that suggest at least some level of consciousness.”
Similarly, Evans recognizes the greater ethical concern when it comes to involving animals, especially from a layperson perspective, and not just because of animal welfare. “While scientists and ethicists tend to not consider there to be a fundamental moral divide between humans and animals, the general public does,” he says. “Therefore, mixing humans and animals—particularly in the brain that is seen as the core part of the human—is more ethically fraught.”
Experts recognize the need for guardrails as this field rapidly progresses; Pașca, Charo, and Evans are all co-authors on a 2025 paper entreating the global scientific community to monitor this progress as it continues apace. But, as the 2021 report brings up, there are also positive ethics of continuing this research. Modeling the brain and its disorders could alleviate a great deal of suffering in the future, creating an ethical imperative to continue studying organoids.
“Their unique value comes from giving us access to human brain biology that is otherwise inaccessible,” says Pașca, “allowing us to study disease processes directly in human cells and tissues and to test potential therapeutics.”
