Caught on camera: the cells eating your brain

For the first time, scientists have captured images of immune cells known as microglia eating brain cell synapses.

Although that sounds like the description of some terrifying autoimmune disease, the process – long predicted, and now confirmed – is thought to be wholly beneficial.

Microglia comprise about one tenth of the cells in the brain. They are similar in many ways to macrophages that attack infected, damaged or dead cells throughout the rest of the body and, indeed, function as the brain’s first line of defence against microbial or viral threats.

However, scientists have long suspected they perform a second, equally important function – nibbling away connections between brain cells, known as synapses, when they have become redundant or dysfunctional.

Now, at last, a team of researchers from the European Molecular Biology Laboratory in Hamburg, Germany, and Rome, Italy, have succeeded capturing microglia eating a synapse on film.

To do so, the team, led by Laetitia Weinhard, used a combination of correlative light and electron microscopy (CLEM) and light-sheet fluorescence microscopy to drill deep inside a mouse brain.

In recording the microglia in action, the scientists realised that the immune cells did not engage in a one-sided attack on synapses. Instead, both sides of the interaction engaged in a weirdly beautiful dance.

As the microglia approached, the synapses extended thin projections known as filopodia towards them.

“As we were trying to see how microglia eliminate synapses, we realised that microglia actually induce their growth most of the time,” Weinhard explains.