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How eggshells stay strong until the chick wants out

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An eggshell’s amazing nanostructure explains how it manages to resist fracture from the outside, while still letting a hatching chick break through.

The finding, reported in Science Advances, could have important implications for food safety.

Birds have benefited from millions of years of evolution to make the perfect eggshell, a thin, protective bio-mineralized chamber for embryonic growth that contains all the nutrients required for the growth of a baby chick.

The shell is resistant to fracture right up until it’s time for hatching. But what exactly gives bird eggshells these unique features?

To find out, researchers used new sample-preparation techniques to expose the interior of the eggshells to take a look at their molecular nanostructure and mechanical properties.

“Eggshells are notoriously difficult to study by traditional means, because they easily break when we try to make a thin slice for imaging by electron microscopy,” says Marc McKee, professor of anatomy and cell biology at McGill University.

The researchers used a focused-ion beam sectioning system. “We were able to accurately and thinly cut the sample and image the interior of the shell.”

“About 10-20% of chicken eggs break or crack, which increases the risk of Salmonella poisoning.”

Eggshells are made of both inorganic and organic matter—calcium-containing mineral and abundant proteins. A factor determining shell strength is the presence of nanostructured mineral associated with osteopontin, an eggshell protein also found in composite biological materials such as bone, says graduate student and first author Dimitra Athanasiadou.

The findings offer insight into the biology and development of chicken embryos in fertilized and incubated eggs. Eggs are sufficiently hard when laid and during brooding to protect them from breaking.

As the chick grows inside the eggshell, it needs calcium to form its bones. During egg incubation, the inner portion of the shell dissolves to provide this mineral ion supply, while at the same time weakening the shell enough to be broken by the hatching chick.

Using atomic force microscopy, and electron and X-ray imaging methods, McKee’s team found that this dual-function relationship is possible thanks to minute changes in the shell’s nanostructure that occurs during egg incubation.

In parallel experiments, researchers were also able to re-create a nanostructure similar to what they discovered in the shell by adding osteopontin to mineral crystals grown in the lab.

McKee believes that a better understanding of the role of proteins in the calcification events that drive eggshell hardening and strength through biomineralization could have important implications for food safety.

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“About 10-20 percent of chicken eggs break or crack, which increases the risk of Salmonella poisoning,” says McKee. “Understanding how mineral nanostructure contributes to shell strength will allow for selection of genetic traits in laying hens to produce consistently stronger eggs for enhanced food safety.”

Support for the work came from the Natural Sciences and Engineering Research Council and the Canadian Institutes of Health Research.

Source: McGill University