It's mind-blowing. These fuzzy green blobs show how brain cells move around inside a baby's brain.
Continuing for months after birth, armies of neurons migrate through a baby's brain, until they reach their destination. Now researchers have had the best look yet at how these cells move in human brains.
Eric Huang at the University of California, San Francisco, and his team looked at the brains of infants who had died because of heart defects and other problems unrelated to their brains. They took thin slices of the brains and kept the cells alive in a dish for up to two days.
Under the microscope, the team watched some neurons with the characteristic elongated shape of migrating cells as they continued their journey. Sometimes the neurons seemed to be using blood vessels as tracks to guide their path.
After birth, new brain cells are only generated at a few sites deep inside the brain before migrating to where they are needed. By placing the slices in an MRI brain scanner, the team saw evidence of large numbers of cells sweeping into the frontal lobes of the brain, which are responsible for higher thought processes. "They form this very beautiful arc," says Huang.
The team found most moving cells in the brains of newborns, with numbers largely tailing off by about 5 months of age. However, other studies have shown that small numbers of neurons continue to be made into adulthood – sparking hope they could be used to heal the brains of people with Parkinson's disease, for example, or after a stroke.
By using antibodies that bind to particular types of neurons, Huang's team found that most of the migrating cells in babies are a type called inhibitory neurons. These cells "turn down" the activity of other neurons – the opposite of excitory brain cells which make other neurons more likely to fire.
Michael Kohl at the University of Oxford speculates that the inhibitory neurons move into place once the baby is getting inputs from the outside world, helping to fine-tune the balance between excitation and inhibition. "You just add inhibitory neurons and see where you need them to balance excitation," he says.
Next, the team plans to see if inhibitory neuron migration is different in children with various neurological problems. It is possible that epilepsy is caused by inhibitory neurons failing to work properly, while autism has also been linked to a lack of balance between these cells and excitatory neurons.