Quality time: Tests assess embryo viability Credit: Newscom

Making a healthy baby through in vitro fertilization (IVF) requires choosing the best sperm, the best egg and, of course, the best embryo. For the past 25–30 years, fertility experts have judged these components primarily on their outward characteristics, but now they are probing deeper.

To ensure fertilization, scientists can inject sperm directly into the egg. This method, called intracytoplasmic sperm injection (ICSI), is particularly useful for men with defective sperm or low sperm counts. “We can even get sperm directly from testes of men who have no sperm in their semen,” says Zev Rosenwaks, who directs the Center for Reproductive Medicine and Infertility at the Weill Cornell Medical Center in New York.

But because ICSI is expensive and technically difficult, it would be helpful to have tests that could screen men to see whether ICSI is even necessary for them, notes Andrew La Barbera, the scientific director of the American Society for Reproductive Medicine in Birmingham, Alabama. Scientists have developed the hyaluronic acid–binding test, which looks for sperm membrane receptors that signal health and maturity. This test might serve as one of many parameters to assess sperm quality before men decide to go through with ICSI. “But you cannot assess the ability of sperm to fertilize an oocyte based on one component,” cautions La Barbera. A study published earlier this year suggests that the hyaluronic acid–binding test might even improve ICSI success rates (J. Assist. Reprod. Genet. 25, 197–203; 2008). However, “there is insufficient clinical pregnancy data to support this test at the moment,” says Denny Sakkas, director of the embryology laboratory at the Yale Fertility Center in New Haven, Connecticut.

When it comes to choosing the best eggs for IVF, scientists are beginning to probe the intercellular machinery of the cells, Sakkas explains. For example, a polarized microscope relying on special software can examine the integrity of the cell spindle—a structure inside the egg that has a pivotal role in cell division. Researchers are also characterizing the specific proteins that healthy eggs secrete into their environment and looking at gene expression markers in the cumulus cells surrounding the egg.

According to Rosenwaks, however, “at the end of the day, when it comes to IVF, the embryo is the main target” of quality checks. Typically, experts examine the embryo for symmetry, structural integrity of cells and overall growth two to five days after fertilization. Now scientists have begun looking beyond the embryo to focus on the medium in which it grows. Sakkas, who in addition to his position at the Yale Fertility Center also serves as the chief scientific officer of the New Jersey–based Molecular Biometrics, is using chemical analyses and mathematical formulas to create a 'metabolic fingerprint' of a healthy embryo that might be used as barometer for gauging the survival potential of an embryo. Others are trying to identify the proteins secreted by thriving embryos and to measure the amount of oxygen consumed, which is usually a signal of growth, La Barbera explains. “All three approaches are in their infancy, but it's exciting.”