Dynamic, rapid changes in the heart that can be seen from the outside are dramatic, but they’re nothing compared to what can be seen inside heart cells themselves. And a research team from the University of St. Andrews has proven exactly that by placing microlasers inside these cells and monitoring changes at a microscopic and localized level.
In the team’s experiments, the microlasers were inserted into the cardiac cells of both neonatal and adult mice and zebrafish, as well as into cardiac slices from rats. The microlasers were internalized by neonatal cells, or were placed on top of the adult cells, measuring the contractility of the myofibrils inside those cells.
These devices were 10- to 20-µm microspheres using the “whispering gallery mode” effect, which accumulates lightwaves similar to sound waves. This technology captures rapid changes during heartbeats at high resolution. Usefully, only cells involved in that action registered any changes in color; even neighboring cells did not light up. This effect was seen through an inverted fluorescence microscope.
Doctors typically perform a number of standard procedures to monitor the heart, such as measuring blood pressure or taking an electrocardiogram. But these do little to tell clinicians what is happening in specific portions of the heart. And traditional forms of imaging cannot easily resolve changes that may last only microseconds.
Researchers noted that lasers, while particularly useful in bioimaging, tend to be large and consume considerable power, and thus only capture changes barely below the heart’s surface. The work of the St. Andrews team changes that.
“The majority of modern microscopes use CW [continuous-wave] laser to excite fluorophores inside biological tissue and then reconstruct an image based on the captured fluorescence,” said Marcel Schubert, a Royal Society Fellow in the School of Physics and Astronomy at the University of St. Andrews and one of the leaders of the research. “However, biological tissue strongly scatters visible light, making it impossible to look deeper than about 50 µm. Multiphoton microscopy can improve penetration, but for heart tissue the limit is currently only 100 µm. What we show in the paper is that we can recover the signals from our microlasers even through 400-µm heart tissue.”
One of the benefits of this technology, Schubert said, is that there is no aging effect on the microlasers in cells. “Photobleaching might become an issue, but depending on the dye used in the microlaser we have found that they can survive millions of pulses,” he said.
Researchers have already developed even smaller lasers, nanolasers, for ongoing research to track changes inside cells, as well.
“Even though we have not seen detrimental effects from the current 15-µm lasers in cells, they are probably too large for long-term experiments, for example in stem cells, and may affect cell behavior in currently unknown ways,” Schubert said. “Besides being less intrusive, smaller lasers can obviously provide higher spatial resolution, and loading several lasers into one cell may give us insights into variations in contractility within individual cells.”
The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0631-z).