Some approaches to in vitro fertilization involve mixing sperm and egg in a test tube and letting nature take its course. But in about half of all infertility cases, a problem with the man's sperm may require a more direct method. In these cases, a different process, called intracytoplasmic sperm injection (ICSI), in which a single sperm cell is injected directly into an egg, is sometimes used. With this one-shot opportunity, it's important to choose a sperm cell with the best potential for success. A team at the University of Edinburgh, Scotland, has now announced a new technique to ensure that the best sperm win: analyzing their DNA for potential damage beforehand, and choosing those that are structurally sound.

"It's a new development that could be very promising," says Alan Penzias, a reproductive endocrinologist at Boston IVF and Harvard Medical School, who was not involved in the research. Penzias explains that current standards for choosing a single sperm cell for an ICSI procedure usually depend on assessing how well the sperm swims; if none of the sperm can swim, a chemical test can find those that are intact and alive. "It's been really pretty crude," he says.

Alistair Elfick, lead scientist for the Edinburgh team, explains that by choosing a single sperm rather than allowing many sperm to swim to and compete for a place in the egg, "you have very much become the arbiter of the quality of that sperm. So clearly, there's a motivation to have a more rigorous selection procedure." With this new technique, the researchers can rank different sperm and choose the one with the most intact DNA. "The endpoint we're moving towards is having a score of DNA quality," Elfick says. But he adds that the approach is an overall measure of the sperm's health; it's not sensitive enough to pick and choose traits.

The method that Elfick and his colleagues developed relies on Raman spectroscopy, a technique that measures the way that molecules scatter photons from a beam of laser light, revealing the molecules' vibrational properties. In order to probe a single sperm cell with Raman spectroscopy, the researchers first pin it down with optical tweezers--a focused laser beam that is able to "trap" a small object like a living cell. The unique scattering produced by each molecule creates a fingerprint of the contents in a sample, allowing scientists to analyze its chemical makeup. In this application, the researchers use Raman spectroscopy to look at the structure of a sperm cell's DNA and determine whether that DNA is broken or intact. Elfick explains that when DNA breaks, a chemical group forms at the ends of the breaks, and they can be detected with Raman spectroscopy.