Oil droplets turn cells into tiny lasers

Scientists have turned individual cells into miniature lasers by injecting them with droplets of oil or fat mixed with a fluorescent dye that can be activated by short pulses of light. The finding, reported in Nature Photonics, could help to broaden how light is used for both medical diagnosis and treatment. The system was devised by Seok Hyun Yun and Matjaz Humar, both optical physicists at Harvard Medical School in Cambridge, Massachusetts, and uses droplets of fat or oil within a cell to reflect and amplify light, generating a laser.

Seok had previously reported a method for generating laser light by engineering cells to express a fluorescent jellyfish protein, then placing a single such cell  between a pair of external mirrors. His latest work goes a step further, producing a cell with a self-contained laser.

Conventional luminescent probes, which include fluorescent dyes and  proteins, have relatively broad emission spectra. This limits the number of probes that can be used simultaneously, because it is often difficult to distinguish these sources of luminescence from the broad background emissions of naturally occurring molecules in tissue.

The microlasers could change this because their emission spectrum is narrow, making it easier to label cells with light, says Jeffrey Karp, a bioengineer at Brigham and Women’s Hospital in Boston, Massachusetts. “One of the greatest  implications of the work is to track thousands of cells simultaneously with a single technique,” he says. Seok and Matjaz report that they can vary the wavelength and tag individual cells using fluorescent polystyrene beads of different diameters, rather than injected droplets of oil or fat. In theory, using different combinations of beads and dyes with different spectral properties should make it possible to individually tag almost as many cells as exist in the human body. “It will be fun or very exciting to adapt the knowledge that’s in the traditional laser community and explore that in this platform to optimise laser characteristics,” says Seok.

He cautions that the technique is not yet ready for therapeutic use. But
eventually the modified cells could be used to locate target tissue, such as a cancerous tumour, and active pre-loaded, light-sensitive drugs only in that area. The eggs of the spined soldier stink bug vary in colour depending on how much light is reflecting from the surface onto which they are placed, according to a new study.

“The female stink bugs are using a visual assessment of the laying surface to determine the colour of the egg,” said Paul Abram, an entomologist at the University of Montreal. The species they studied, Podisus maculiventris, is found in fields and backyard gardens across North America. It is the only species known to vary egg colouration based on environmental cues, Paul said. Some birds and insects lay eggs in various colours, but the differences are usually influenced by nutrition or age.

Paul first noticed the variously coloured eggs because of a crossword puzzle in a newspaper lining the bottom of a stink bug cage in his lab. Darker-coloured eggs were deposited on the black squares of the puzzle, and lighter-coloured eggs appeared on the light squares. In nature, stink bug eggs deposited on the tops of leaves are darker and thus better protected from UV radiation. Eggs laid on the underside of leaves do not require the protective pigment and tend to be lighter.

Exactly how the female stink bug manages to vary the colour of her eggs is a mystery. “What is actually going on inside the female stink bug at a physical level is simply unknown,” Paul said. “Whatever it is, it’s strange and unique.”