Shedding Light on Life
Advances in optical microscopy reveal biological processes as they unfold.
by Courtney Humphries
The scenes are familiar from biology textbooks. A long string of DNA is copied to form a matching strand. A virus infects a cell by stealing through its membrane. Two white blood cells meet and confer before launching an immune attack.
In textbooks, all these processes that are so fundamental to the lives of cells are typically depicted in drawings or static snapshots captured by powerful electron microscopes. But that’s changing. A growing revolution in imaging is making it possible for biologists to watch small-scale events as they unfold in living cells and tissues.
“The human brain is vision-focused,” says professor of molecular and cellular biology Jeff Lichtman. “If we see things, then we think we know what they mean.” To be able finally to see events that were known only in theory is incredibly satisfying for scientists. Even more important, this revolution also opens up the possibility of learning things about life that could never be studied before.
Ironically, the technology enabling much of this change is the same one that launched the study of modern biology centuries ago: the light or optical microscope. A congruence of factors has shuttled these instruments back into the forefront of biology in recent years, after almost a half-century during which they were overshadowed by more powerful techniques such as electron microscopy and x-ray crystallography, which are able to create images on the level of single molecules.
New technologies—more sophisticated imaging techniques, fluorescent molecules that act as beacons of light in the cell, and the computing power to gather and stitch together multiple images and create videos from high-powered microscopes—make it possible to harness one of light’s key advantages: gentleness. Unlike higher-resolution techniques, light microscopes can image biological structures without killing them or chemically fixing them. At Harvard, the resurgence of light microscopy is making it possible to see structures and events that have never before been seen in the context of living cells and organisms. New discoveries are emerging at many scales of life, from the activation of a single gene in DNA to the development of disease in an organ.
Pushing Microscopes Further
At the molecular scale, Xiaowei Zhuang, professor of chemistry and chemical biology and of physics, is pushing the boundaries of what light microscopes can capture. A typical light microscope can easily image a single cell and some internal structures, but most other objects—viruses, clusters of proteins, DNA—cannot be seen in great detail. That’s because these smaller details lie within light’s diffraction limit—the point at which light waves begin to interfere with one another, blurring the image. The question of how individual molecules in cells interact is fundamental in biology, but for the most part these interactions lie beyond the reach of light microscopes. Zhuang’s lab has been working on a new “super-resolution” optical imaging method that uses clever tricks to image objects a tenth the size possible using normal light microscopy.
Courtesy of Xiaowei Zhuang Laboratory
Conventional immunofluorescence images (labeled A) are paired with three-dimensional STORM images (labeled B) of different intracellular structures. Above: A pairing of microtubules, with details (C,D, and E) showing three different cross-sections of the boxed area in image B.
Courtesy of Xiaowei Zhuang Laboratory
Above: Microtubules (green) and clathrin-coated pits (red). The pits are indentations in a cell’s surface that mediate certain extracellular interactions.
Courtesy of Xiaowei Zhuang Laboratory
Above: Clathrin-coated pits are much clearer using the STORM technique (B). And by combining two STORM cross-sections, a single pit (B inset), can be seen in three dimensions, revealing its half-spherical, cage-like structure.
Like many new optical imaging techniques, Zhuang’s takes advantage of small fluorescent molecules called fluorophores to create an image. Scientists can determine the location of a single fluorophore under a microscope with great precision, even though it is much tinier than a microscope’s resolution. Light emitted from the molecule will produce a blurry image, but the center of this blur indicates where the actual molecule resides.
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