Moving cells tracked simultaneously in 3-D

Ashley N. Rice, ashley.rice@photonics.com

Tracking moving cells under a microscope just got easier, thanks to a novel lensless computational imaging platform that captures precise motion paths in three dimensions.

“In our lens-free computational on-chip microscopy approach, we can easily track >1500 sperm in 3-D within a single experiment, which led us to observe and digitize >24,000 human sperm trajectories in this work,” Aydogan Ozcan, an electrical engineering and bioengineering associate professor at UCLA, told BioPhotonics. It was the first time such a large sample volume had been recorded for as long as 20 seconds.

This illustration and data set depict the microscopy technique developed by Ozcan and colleagues at UCLA. The top image shows a schematic of the system, which involves two partially coherent light sources (a red 635-nm LED and a blue 470-nm LED) that simultaneously illuminate the microscope field of view from two different angles. A CMOS sensor records the resulting holograms, and software uses that information to encode the exact positions of the target cells. The bottom image shows the reconstructed 3-D trajectories of 1575 human male gamete cells in a volume of 7.9 µl. Images courtesy of Ozcan Research Group at UCLA.

Observing human sperm cells has usually been limited to conventional lens-based optical microscopes. The small size of a sperm head (about 3 to 4 µm) requires a high-magnification lens to observe its motion, and sperm cells’ relatively fast speed (about 20 to 100 µm/s) makes it difficult to track them over time as they move in 3-D.

“Using a conventional microscope with objective lenses would have a limited field of view. This means you can track a limited number of sperm,” he said. The limited imaging area also means a small depth of field, “as a result of which, sperm quickly get out of focus while swimming in 3-D.”

To address these challenges, the UCLA team’s on-chip imaging platform used holographic shadows of sperm cells. Lens-free images of the cells were acquired simultaneously using red and blue wavelengths of light. The offset light beams created holographic information that, when processed using sophisticated software, accurately revealed the paths of objects moving under a microscope.

The researchers discovered that human male gamete cells travel in a series of twists and turns along a constantly changing path that occasionally follows a tight helix – a spiral that, 90 percent of the time, is in a clockwise direction.

These images show, for the first time, that human male gamete cells swim primarily in four types of patterns. The insets show the front view of the trajectories.

“We do not know the reason for the right-handedness preference of human sperm,” Ozcan said. “It was known that sperm of various species can in general swim in helical patterns. However, it was not directly visualized or observed for human sperm before. This is enabled by our work.”

The platform also could be used as a high-throughput tool to track micro-swimmers such as bacteria, algae and sperm of other species, or to rapidly quantify the impact of various stimuli, chemicals and drugs on the 3-D swimming patterns of sperm, he said.

The most practical application – screening of sperm quality – will require technologies that can rapidly image its motion and the motion of other motile micro-organisms before it can be brought to market. This could take one to two years, Ozcan said.

The team is looking to increase the frame rate to observe even faster temporal events that happen around sperm.

“We would like to develop this platform into a high-throughput measurement system so that the effects of various different combinations of drugs or chemicals and other external stimuli on sperm motion can be experimentally probed in a rapid and parallel scheme, on the same chip,” Ozcan said.

The work appears in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1212506109).