Confocal microscopy to detect cancer
Image: Standard confocal microscopy detects constant, jiggling motions of a cell’s particles. [Jose-Luis Olivares/MIT]
MIT engineers have devised a method to assess a cell’s mechanical properties using standard confocal microscopy to detect the 'jiggling motions' of a cell’s particles.
These tell-tale movements can be used to decipher a cell’s stiffness, which reveals whether the cell is healthy or diseased.
Current methods for doing this involve directly probing cells with expensive instruments, such as atomic force microscopes and optical tweezers, which make direct, invasive contact with the cells.
Unlike optical tweezers, the team’s technique is noninvasive, running little risk of altering or damaging a cell while probing its contents.
“There are several diseases, like certain types of cancer and asthma, where stiffness of the cell is known to be linked to the phenotype of the disease,” says Professor Ming Guo from MIT’s Department of Mechanical Engineering. “This technique really opens a door so that a medical doctor or biologist, if they would like to know the material property of cell in a very quick, noninvasive way, can now do it.”
Within a cell, organelles such as mitochondria and lysosomes are constantly jiggling in response to the cell’s temperature.
However, proteins and molecules can also contribute to this movement, making it difficult for researchers to discern which motions are due to temperature and which are due to the protein- and molecule-related processes.
However, Guo and colleagues realised that cell particles energised solely by temperature would exhibit a constant jiggling motion, and hypothesised that these movements would require studying a cell over a relatively long timeframe.
To test their hypothesis, the researchers carried out experiments on human melanoma cells, injecting small polymer particles into each cell and then tracking motions under a standard confocal fluorescent microscope.
They also varied the cells’ stiffness by introducing salt into the cell solution.
The researchers recorded videos of the cells at different frame rates and observed how particle motions changed with cell stiffness.
When they watched the cells at frequencies higher than 10 frames per second, they mostly observed particles jiggling in place; these vibrations appeared to be caused by temperature alone.
Only at slower frame rates did they spot more active, random movements, with particles shooting across wider distances within the cytoplasm.
For each video, they tracked the path of a particle and applied a custom-algorithm developed to calculate the particle’s average travel distance.
They then plugged this motion value into the Stokes-Einstein equation, which uses particle movement to calculate a material’s mechanical properties.
On comparing their stiffness calculations with actual optical tweezer measurements, the researchers noticed the calculations matched the measurements only when particle motion had been captured at frequencies of 10 frames per second and higher.
Guo says this suggests that particle motions occurring at high frequencies are indeed temperature-driven.
The researchers' results also suggest that if they observe cells at fast enough frame rates, they can isolate particle motions that are purely driven by temperature, determine average displacement and calculate cell stiffness.
“Now if people want to measure the mechanical properties of cells, they can just watch them,” highlights Guo. “People have an idea that structure changes, but doctors want to use this method to demonstrate whether there is a change, and whether we can use this to diagnose these conditions.”
Guo and colleagues are working with medics at Massachusetts General Hospital, who hope to use the new, noninvasive technique to study cells involved in cancer, asthma, and other conditions in which cell properties change as a disease progresses.
Research is published in the Journal of the Mechanics and Physics of Solids.