NORTHWESTERN (US)—New research reveals that healthy eggs need a tremendous amount of zinc to reach maturity and be ready for fertilization—a finding that may ultimately help physicians assess which eggs isolated from a woman will produce the best embryos.

“Understanding zinc’s role may eventually help us measure the quality of an egg and lead to advances in fertility treatment,” says Alison Kim, a postdoctoral fellow in obstetrics and gynecology at Northwestern University and lead author of the study featured on the cover of the September issue of the journal Nature Chemical Biology.

“Currently we can’t predict which eggs isolated from a woman produce the best embryos and will result in a baby. Not all eggs are capable of becoming healthy embryos.”

There’s no link yet to zinc content in the egg and the nutritional status of women, but Kim plans to research that area.

Working with mice, Kim and colleagues discovered the egg becomes ravenous for zinc and acquires a 50 percent increase in the metal in order to reach full maturity before becoming fertilized.

The flood of zinc appears to flip a switch so the egg can progress through the final stages of meiosis. (Meiosis is when the egg sheds all but one copy of its maternal chromosomes before it can be fertilized by a sperm and become an embryo.)

“Zinc helps the egg exit from a holding pattern to its final critical stage of development,” says Tom O’Halloran, a coauthor of the paper and a chemistry professor at Northwestern.

“It’s on the knife’s edge of becoming a new life form or becoming a cell that dies. It only has 24 hours. Zinc seems to be a key switch that helps control whether the egg moves forward in its development stage. ”

Kim found there were approximately 60 billion zinc atoms in a mouse egg just before the egg was ready to be fertilized. She measured the zinc content of the eggs using a technique called synchrotron-based X-ray fluorescence microscopy that allowed detection of multiple metals in single eggs using the characteristic X-ray signature of each element.

Zinc levels were significantly higher in eggs than other important metals such as iron and copper. Zinc was the only metal to change significantly in concentration during the maturation process.

The researchers also used small molecules to block the accumulation of zinc by the maturing egg. They found an insufficient accumulation of zinc caused all the eggs to pause prematurely at the beginning stage of meiosis. The progression of meiosis was restored by returning zinc to the eggs.

The work was funded by a W.M. Keck Foundation Medical Research Award and the National Institutes of Health. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy.

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