Intricate experiments using mice have allowed scientists to visualise the formation of new brain circuits that form after birth. They also found that mice exposed to greater sensory stimulation formed more neural connections.
The brain is a complex web of billions of long nerve cells called neurons. Connections between these cells, at sites called synapses, allow neurons to communicate with one another. These connections are believed to be the basis of the brain’s functions, such as memory and the sense of smell.
It was long believed that neurons cannot grow after birth. But studies have shown that, like other part of the body, the brain contains stem cells, which can form new nerves. In the current study the researchers were interested in how these stem cells contributed to the formation of new connections after birth.
They first genetically engineered mouse embryos to contain a gene that when activated would make cells glow red. When the mice were born they were immediately anaesthetised and a small bit of DNA injected into a compartment of the brain where stem cells are found. These stem cells are known to develop into nerves that connect with the region of the brain involved in smell. The DNA entered the stem cells and turned on the gene making the cells glow red. Any cells that develop from the stem cells will also glow.
The baby mice were returned to their mothers and reared normally, except that half were exposed to a constantly changing range of odours. A month later they were injected with a harmless modified virus that glows green and is targeted to the cells that have developed from the stem cells that glow red.
The green-glowing virus inside the red neurons turned them yellow. But the virus was also able to pass across the synapse into connected neurons making them glow green.This allowed the scientists to see the connections made by the new neurons which had originated from stem cells and formed after birth. This shows clearly that brains are still developing even once the baby is born. And interestingly, baby mice exposed to a range of smells formed more of these new connections revealing how the environment affects brain development.