A Blog by Virginia Hughes

Videos: A (Very) Close Look Inside the Zebrafish Brain

About a year ago I wrote a story about the hottest new animal model in neuroscience: baby zebrafish. The critters are not much to look at. They’re the size and shape of a curled eyelash, with big bulging eyes. But when some neuroscientists look at the fish, they see a lot of potential. The fish have around 300,000 neurons — enough to perform relatively complex behaviors, such as swimming in different directions and learning to fear certain stimuli. And most importantly, the embryonic fish are transparent, making it easy to watch their brain cells in action, all at once.

This week Eric Betzig of HHMI’s Janelia Farm Research Campus in Ashburn, Virginia, reports in Nature Methods a new technology that dramatically sharpens those microscopic images. You can see for yourself in the video below, which shows neurons deep in the midbrain of a living, 3-day-old zebrafish:

(Credit: Betzig Lab, HHMI’s Janelia Farm Research Campus)

The new microscopy can not only show the activity patterns of groups of neurons, but tiny structures within each cell. For example, the video below zooms into a neuron in the deep hindbrain of a 4-day-old fish. Mitochondria, the structures that provide the cell’s energy, are in pink, and the plasma membrane, or outer covering, is in green:

(Credit: Betzig Lab, HHMI’s Janelia Farm Research Campus)

I won’t pretend to totally understand the physics of how the microscope works, but apparently the researchers borrowed a technique from astronomy called “adaptive optics.” Here’s a good explanation from HHMI (which funds Betzig’s work):

Over the last decade, Betzig and others have taken a cue from astronomers in using adaptive optics to correct for the light-bending heterogeneity of biological tissues. Astronomers apply adaptive optics by shining a laser high in the atmosphere in the same direction as an object they want to observe, Betzig explains. The light returning from this so-called guide star gets distorted as it travels through the turbulent atmosphere back to the telescope. Using a tool called a wavefront sensor, astronomers measure this distortion directly, then use the measurements to deform a telescope mirror to cancel out the atmospheric aberrations. The correction gives a much clearer view of the target object they want to observe.

A microscopy technique that Betzig developed in 2010 with Na Ji, who is now also a group leader at Janelia, achieves similar results by using an isolated fluorescent object such as a cell body or an embedded bead in the tissue as the “guide star.”

…The team created a guide star by focusing light from the microscope into a glowing point within the sample. Using a technique called two-photon excitation, they could penetrate infrared light deep within the tissue and illuminate a specific point. The wavefront sensor would then determine how the light that returned from this guide star had warped as it passed through the tissue, so that the appropriate correction could be applied.

Go read the whole article to find out more.