A Blossoming Tree
To watch neurons in the living brain, Nedivi and her collaborators put windows in the skulls of mice genetically engineered to produce a fluorescent dye in a few randomly selected brain cells. Through the windows, they take pictures of the fluorescent neurons using a two-photon microscope, an instrument that creates very high-resolution images.
An ultrafast titanium-sapphire laser sends packets of light through a complex series of lenses and mirrors, which directs the light onto individual cells in the brain of the anesthetized mouse. The fluorescent dye in the selected neurons glows only if two photons hit a dye molecule at exactly the same time, allowing for more precise imaging of the cells. (This is why the room must be pitch black: any extraneous light would be picked up by the microscope’s photon detector, muddying the resulting image. Researchers wear head lamps in case they need to adjust equipment.)
Neurons consist of a central cell body and a series of branching projections that extend into different parts of the brain to send and receive electrical signals. To capture the entire structure of a given neuron, the laser scans it in horizontal cross sections, plunging deeper into the brain with each sweep. Researchers go through the images–which resemble Jackson Pollock paintings–and select the shapes that correspond to the projections. A computer program then stitches the images together to create a 3-D model.
To record how the neurons change over time, the MIT researchers take pictures of the same neuron every week for several weeks, using nearby blood vessels to help locate it. In a paper published earlier this year in Public Library of Science Biology, the team showed that dendrites–the projections that neurons use to receive information from other brain cells–could grow and twist and bend, sending out new shoots like a blossoming tree. “It’s very powerful–you can actually see the changes,” says Lee. And because the images are collected from a living animal, he says, they capture the brain’s behavior much more accurately than images of a brain slice, in which many of the neural connections are severed.
This type of growth had never been seen before in studies of living animals. Previous studies using two-photon imaging found small structural changes in dendritic spines, tiny bumps on the surfaces of dendrites. But these studies reconstructed only small sections of each neuron. By modeling entire cells, Nedivi and colleagues were able to see larger-scale changes that may previously have gone unnoticed. “What is most exciting about their work is that it shows an unanticipated amount of dynamism in neurons,” says Josh Sanes, a Harvard University neuroscientist whose lab engineered the mice used in the study.
In addition, Nedivi’s team found that only a certain type of neuron undergoes these changes. Previous studies focused on excitatory neurons, which send electrical signals that cause other neurons to fire. Inhibitory neurons, on the other hand, release chemicals that stop other neurons from firing. It is these neurons that can extend and retract new projections. Inhibitory neurons are less prevalent in the brain than their excitatory cousins and are less well studied.