Human brain cells grown in a lab can reveal a great deal about development and disease. But they don't reveal much about the brain as a whole, about how it forms a network that is the brain.
Scientists at Stanford University, led by Sergiu Pașca, tried growing human brain tissue inside mice. The transplanted cells survived and connected with it's nervous system. The researchers called this method as "Developmental Xenocortication". The aim of this method is to study how brain tissue grows and work inside a living body.
It is very difficult to do this in a living humans body, the only other alternative until now is to grow small brain tissue in labs. This is not enough. The brain depends mainly on connections between it's nerve cells called neurons. They send and recieve signals constantly. As the develop, nearby cells and the kind of signals they recieve help shape how they grow and what kind of connection they can make. Studying brain tissue in a dish provides only part of the process.
A brain tissue in a petri dish will stay alive and can even form some connections, send and recieve signals. To create brain tissue, researchers start with cells that have been reset into a felxible state, these are called induced pluripotent stem cells, which are then guided to become brain cells. They grow into small three dimensional clusers called organoids.
These organoids are sometimes called "Mini Brains". They do reproduce some features of brain development. But they are not complete brains. They are useful in observing the processes that will be difficult in studying in a living person. But they are not actual brains and such whatever information gathered from them will not be enough
In 2022 Pașca's team transplanted human brain organoids into young rats. Blood vessels from the rats supplied oxygen to the tissues, these organiods developed more fully that they would have if kept in a dish. They also responded to sensory information. These human brain cells changed the rat's reward seeking behaviour. This showed that the transplanted cells are not part of the central nervous system of the mice.
The biggest obstracle the researchers faced was that the mice brain already contains it's own growing tissue. There is not much room for a transplant to expand. In a new study, the researchers changed the conditions inside the brain of the mice. They used genetically engineered mice that had lost many neurons in regions of the brain such as the cortex abd the outer region, these regions help process information and guide the behaviour of the mice.
They transplanted organoids soon after the birth of the modified mice.Now that more space is available, the transplanted brain tissue grew to occupy most of the cortical volume. It produced several types of bain cells and formed structures that are similar to human brain cortex.
These cells also connected with the nervous system of the mouse.
Growing human brain tissue in mice itself is a big achievement, but when the behaviour of these mice is compared with the other mice, the researchers could see clear difference in behaviour between the groups.
These changes are very small and does not mean they developed human like intelligence, but this is a great leap in science. Now scientists can watch human brain develop, connect and respond to injury in a living system.