Scientists are growing human brain tissue in living mice—could this make their welfare harder to assess?

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: What happens when scientists start changing the brain—the very system through which an animal's mental states are expressed in behavior? In a recent study published in Nature, researchers implanted human brain organoids—small, simplified structures grown from human stem cells—into the brains of living mice.

Over time, these organoids developed and integrated with the mouse nervous system. Transplanting organoids composed of human cells into animals, including their brains, is not new. However, previous experiments faced an obstacle: human brain cells grow much more slowly than mouse brain cells, and so organoids are quickly "outcompeted" before they can fully integrate with the mouse brain.

In the new study, scientists genetically altered mice so that parts of their brain did not develop, then transplanted the human organoids into the vacant space. The idea of creating human-animal chimeras—animals containing human cells—raises ethical questions, particularly when involving the brain. Various philosophers have argued that as the proportion of human brain tissue increases, we should consider whether the animal could acquire humanlike mental capacities—and whether this should change how we treat it.

But I argue that serious questions about our ability to recognize animal suffering will arise long before mice gain increased intelligence. The philosopher Thomas Nagel asked a version of this question about bats. He argued that conscious beings have a specific experience of the world, but that others cannot access that experience from the outside.

Instead, we must infer what others experience. With animals, we do this through their behavior and physiology. But this approach assumes that we understand how what we observe relates to what the animal experiences.

There is nothing morally important about a cell simply because it came from a human. Imagine two mice with identical mental lives and cognitive abilities, one produced using transplanted human cells and the other through a modification involving only mouse cells. It is difficult to see why the first should matter more simply because some of its cells are human.