MIT scientists are on the road to creating 3-D images of the internal
workings of cells—including how they communicate.
Anton Van Leeuwenhoek, considered the father of modern microscopy, created a way for humans to see beyond the boundaries of the naked eye and observe the teeming life within a drop of water or that within the human circulatory system. Van Leeuwenhoek took the first step to view these ‘invisible’ worlds, but each year researchers across the globe step up and offer new instruments and techniques needed to peer even deeper.
Scientists at the Massachusetts Institute
of Technology have discovered a way to view and capture images of the inner
workings of live cells. They hope that by using this process called quantitative
phase imaging, they will one day be able to uncover how cells communicate
with each other.
The matters at hand
The transmission electron microscope (TEM) was the first type of electron microscope to be developed and is patterned exactly like the light microscope but uses electrons instead of light. When pushed to their limit, electron microscopes can make it possible to view objects as small as the diameter of an atom. Indeed, most electron microscopes used to study biological material can “see” down to about 10 Å—in effect, magnifying objects up to 1 million times. Although this does not make atoms visible, this method does allow researchers to distinguish individual molecules of biological importance nevertheless.
The drawback, however, is in the sample prep. Since living specimens must be dehydrated, frozen, or fixated using a negative staining material such as uranyl acetate or plastic embedding, this makes it difficult to see the ever-changing movements that characterize a living cell.
Answering the call
Researchers at the George R. Harrison Spectroscopy Laboratory, at the Massachusetts Institute of Technology (MIT), Cambridge, have discovered a breakthrough imaging method, called quantitative phase imaging (QPI), which allows researchers to view live cell membranes and create images of the internal systems of living cells and their movements—in their natural states, without sample pre-processing. It is hoped that this will one day enable researchers to develop a greater understanding of diseases such as sickle cell anemia and malaria.
QPI techniques build on interferometry without the sensitivity challenges. Interferometry is a technique to measure lengths or distances precisely based on the interference of two light beams. With interferometry, the apparatus is so sensitive that even breathing near the equipment can disrupt the system. In a typical lab environment, attempting to measure such tiny optical signals that red blood cells emit is akin to “sensing the movements of a jellyfish in a stormy ocean,” according to Gabriel Popescu, a postdoctoral associate in MIT’s Spectroscopy Laboratory.
Up to this point, the researchers have been able to create images with a resolution of 0.2 nm in normal laboratory conditions with no isolation or stabilization of the microscope. And although QPI hasn’t reached the level of resolution that electron microscopy offers, Michael Feld, MIT professor of physics and director of the Spectroscopy Laboratory, says he believes that someday it will.
Good vibrations
As grand as all this is, the MIT scientists have an even loftier goal in mind; to use QPI to discover and capture how cells communicate with each other and what triggers them to turn cancerous.
“One of our goals is to create 3-D tomographic images of the internal structure of a cell,” says Feld. “The beauty is that with this technique, you can study dynamic processes in living cells in real-time.”
So far, the team has focused their attention on red blood cells, neurons, and epithelial cancer cells. Red blood cells are an especially good model to study cell membrane dynamics as they are simplistic by nature, with no nuclei or internal structures. Red blood cell movement, or flickering, was observed more than a century ago. This continuous undulation, says Popescu, is “effectively a drum in perpetual vibration,” and offers scientists the opportunity to study the mechanical properties of the cells’ membrane—including how the membrane provides the cell with the information it needs to squeeze through narrow capillaries.
Feld, Popescu, and their colleagues have also been able to illuminate these vibrations and quantify the elasticity in normal red blood cells, compared with abnormal, less elastic cells, such as the twisting deformation seen in sickle-cell anemia. They have shown that the frequency of cell membrane vibration is directly related to its elasticity. “The elasticity of these cells is crucial for their function,” says Popescu.
The research led them to consider how sickle cell anemia and malaria infection affect the mechanical properties of red blood cell membranes.
“In malaria, our first measurements indicate that the membrane stiffens as the parasite ages inside the cell,” says Popescu. “In fact, the cell membrane behaves much like a guitar string. The tighter or stiffer it is, the higher the pitch produced. So, our technique can be regarded as an incredibly sensitive microphone.”
The key to a cure?
Cell membrane integrity plays an important role in many diseases—including chronic alcoholism—that involve membrane abnormalities in red blood cells. The researchers at MIT believe QPI may yield a way to help reverse the effects of such diseases.
And, as John Sedat, professor of biochemistry at the Univ. of California, in San Francisco, points out, optical imaging is an ever-evolving field. “There’s a kind of miniature revolution taking place in microscopy,” he says. “This is an example of physicists coming into biology and bringing in a lot of new ways to see things.”
