Unique cell imaging method represents biology’s future
December 1, 2016
Scientists and curious minds alike have been observing cells through a microscope since the 17th century.
The advancements in the structure and capabilities of these microscopes have enabled researchers to see things that could not have been observed before. However, the data gathered from these microscopes has remained largely qualitative, which is something that has not changed since they were invented. One would think that biologists would be well-versed in technology since the advent of the computer, but until recently the main tool that has been used to study cells has been the human eye.
“Biology is, I think, shifting clearly towards more engineering, a more quantitative way of analyzing the cells. Not just visualizing the cell, but measuring, extracting the numbers,” said Gabriel Popescu, a professor of electrical and computer engineering and director of the Quantitative Light Imaging Laboratory (QLI Lab) at the Beckman Institute for Advanced Science and Technology.
Popescu, along with his diverse team of researchers, engineers, and biologists, has developed a novel way of imaging cells using light beams called Phase Correlational Imaging (PCI). It is a new method used to observe dynamic systems, such as live cells grown in a lab. One major difference between this imaging technique and other long-standing observational methods is that it does not use labels to see the structure of the cells.
“Most cells are intrinsically transparent, and for more than 100 years the common way to look at them was to paint them with colors, stain them with stains or fluorescent tags,” Popescu said. “So while this is OK, I mean, it gets the contrast up, it affects the function of the cells.”
One example of this limitation is the fact that fluorescence disappears after a while, so there is a limited window of opportunity to see the stained cell. These stains can also affect the function of the cells and even kill the cells. In this new method, it was important to make sure that the function of the cells was not impaired by the methods employed to observe them. The method essentially consists of taking a time-lapse of the cells using light beam correlations.
The research team used a technique called interferometry, Professor Popescu’s specialty that he has written about in one of his books. One light beam is shined through the cells and another light beam, called the reference beam, is not shined through the cells. When these light beams overlapp, they create a refraction index based on minute differences in the density of the cell’s parts, resulting in a clear series of images of the cell.
Professor Popescu explained how the quantitative data can actually be extracted and what the meaning of the word correlation is in PCI.
“Imagine the suspension of small particles in a glass of water. If the particles are small, they will move very fast and if you image that, you’re getting a movie with very fast fluctuations. A correlation system tells you how fast a certain system is fluctuating. A very correlated process will be very slow. When the particles are moving fast, the next frame of the particles will be nothing like the first one, ” he said.
Using these correlation maps, the researcher can tell how fast the particles are moving. Through this method, they are able to extract how fast things are moving inside the cell based on the quantitative maps generated by the PCI instrument. All of these data sets lend to science a multitude of different applications.
For example, just looking at the correlational data sets, the senescent cells can be distinguished from the quiescent cells. Quiescent cells are temporarily inactivated cells, possibly due to a lack of food or some kind of threatening environmental conditions. Senescent cells are irreversibly inactivated cells and are often times much older.
“We’ve found that the senescent cells actually have slower dynamics that the quiescent. And that’s I think the first time that anybody has distinguished between the two without using fluorescent tags, ” Popescu said.
However, PCI does have its limitations. For example, the main advantage of using fluorescence over PCI is that is offers more specificity. PCI does not stain individual parts of the cell for observation like fluorescent tags do. This is why Professor Popescu and his colleagues have put a lot of effort into merging the fluorescence technique with their PCI method. This way they get the specificity that the fluorescence offers, but also the noninvasive quantitative data imaging from their unique method.
Popescu said the main struggle at this point is to get the word out about this novel technology.
“When biologists see the new method, they understand the value right away. The major challenge is that they don’t know this thing exists,” he said.
Popescu does believe, however, that this PCI technology is a part of the future of biology and that the merging of different fields will foster more advancements in science.
“I think the biology of the future will be a more quantitative science than we’ve seen so far,” Popescu said. “This will generate better models. For example, if I’m studying cancer, ideally I’d be able to predict its evolution. So you find the tumor, but it’s not yet killing you, but the ability to predict when it’s going to kill you — that’s the biggest question. That can only be understood if you have a model that has predictable value. That model cannot be built unless you have quantitative data. ”