Human Brain Organoids Connect With Mouse Brain Circuits in Study

The human brain contains billions of cells linked by millions of circuits. That complexity makes it difficult for scientists to understand how the brain develops and what happens at the cellular level when problems occur.

How human brain tissue was grown in mice

Although this is not the first study involving human neurons transplanted into laboratory rodents, external, the researchers took the approach further.

First, they genetically engineered mice to develop very little of their own cerebral cortex—the brain’s outer layer, also known as gray matter. The cerebral cortex plays an important role in higher-order thinking, memory and sensation.

The researchers then took human skin cells and “reprogrammed” them so they could develop into brain-like pieces of tissue. These structures, known as organoids, are collections of connected living cells—not whole brains grown in a dish.

When the organoids were transplanted into the mouse brains, the human cells divided and organized within the animals’ existing brain circuits. They also connected with the rest of the mice’s brains and spinal cords.

Human cells began to resemble the mouse cortex

The transplanted mouse cortex was not complete. A normal cortex develops into organized layers, while scans of the human and mouse brain tissue looked “a bit messy,” according to neuroscientist Dr Irali Allodi.

However, after several months, the human cells began to look and function more like the outer layer of a mouse brain.

About six months after surgery, the researchers conducted basic behavioral tests. They observed the mice as they moved around a small tabletop arena.

Pashka said the mice “mainly performed as normal mice did.”

“They don’t have any enhancements,” he added.

Study does not show mice can think like humans

Dr Sarah Chan, a reader in bioethics at the University of Edinburgh who was not involved in the study, told BBC News: “There is no indication that what we are creating here is a mouse that can think like a human, or a human brain inside a mouse.”

However, she said the research “prompts us to think about what it means when an animal’s cognition begins to change.”

“How do we know what it’s like to be one of these mice? And how do we take that into account when working with laboratory animals?” she added.