Laser beam can pop out single cells.

A laser that cuts out a cell and flips it into a waiting test tube offers researchers a way to remove single cells from a tissue sample in less than 30 seconds.

The technique offers a quick means to choose cells for DNA screening tests while minimizing contamination. This is especially important when applying the polymerase chain reaction, or PCR, a widely used method for making many copies of DNA or RNA (SN: 10/23/93, p. 262). Karin Schutze and Georgia Lahr of the Academic Hospital in Munich describe their new technique in the August Nature Biotechnology.

First, the researchers affix a slice of tissue to a thin plastic sheet and mount it on a glass slide above a highly focused ultraviolet laser. Looking through a microscope, they identify a particular cell, then burn around its edges with the laser, cutting out the cell like a cookie from a sheet of dough.

Then, by doubling the power of the laser but adjusting it to focus below the target cell, they launch the individual cell vertically off the glass slide and catch it with the cap of a test tube. The cell rides the stream of photons from the laser "like a surfer on top of a wave," says Lahr. The cell "can be beamed several millimeters away, even against gravity," she reports.

To demonstrate the selectivity of their technique, called laser pressure catapulting, Schtitze and Lahr collected individual colon cancer cells in test tubes. An analysis of the DNA of a single cell revealed the presence of a mutation that has been linked to colon cancer.

One advantage of the method is that the person preparing the sample never has to touch the cell. Earlier methods used fine needles to dissect cells from a tissue sample--a slow, tedious process. A disadvantage of laser pressure catapulting is that it kills the cell, although the genetic material remains intact.

Other researchers have developed similar laser-based techniques to isolate clusters of cells. In one such method, a team at the National Institutes of Health in Bethesda, Md., uses an infrared laser to heat a plastic film laid on top of a slice of tissue. The film melts above the target cells and sticks them to a plastic cap held above the sample. When the cap is lifted away, it carries the cells with it. This method also kills the cells.

Yet another variation, called anchored cell analysis and sorting, works on live cells growing on a piece of heat-absorbing plastic film in a petri dish. A laser beam is shone onto the film around the cells of interest, fusing a microscopic island of plastic to the dish. When the rest of the film is peeled off, it takes the surplus cells with it, leaving the targeted cells behind. This technique was described in 1985 by Melvin Schindler of Michigan State University in East Lansing.

The live cell then "can be examined, manipulated, and potentially cloned," writes Schindler in a commentary accompanying the German study. All these techniques should ease the preparation of DNA libraries that catalog the patterns of gene expression in cells, he adds.