How is it that fertilized chicken eggs manage to resist fracture from the outside while at the same time are weak enough to break from the inside during chick hatching? It's all in the eggshell's nanostructure, according to a new study led by researchers at McGill University in Montreal, Que.

The findings, reported in Science Advances, could have important implications for food safety in the agriculture industry, the university said.

Millions of years of evolution have allowed birds to make the perfect eggshell: a thin, protective, biomineralized chamber for embryonic growth that contains all of the nutrients required for the growth of a baby chick. The shell -- while not too strong but also not too weak -- is resistant to fracture until it's time for hatching.

To find out what exactly gives birds' eggshells these unique features, Marc McKee's research team in the McGill University faculty of dentistry, together with Richard Chromik's group in engineering and other colleagues, used new sample preparation techniques to expose the interior of the eggshells so their molecular nanostructure and mechanical properties could be studied.

"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," said McKee, who is also a professor in the McGill department of anatomy and cell biology. Thanks to a new focused-ion beam sectioning system recently obtained by McGill's Facility for Electron Microscopy Research, we were able to accurately and thinly cut the sample and image the interior of the shell."

Eggshells are made of both inorganic and organic matter -- calcium-containing mineral and abundant proteins. Graduate student Dimitra Athanasiadou, the study's first author, found that 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.

The results provide insight into the biology and development of chicken embryos in fertilized and incubated eggs, the university said. 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 occur during egg incubation, the announcement explained.

In parallel experiments, the researchers were also able to recreate a nanostructure similar to what they discovered in the shell by adding osteopontin to mineral crystals grown in the lab. McKee said he believes that a better understanding of the role proteins play in the calcification events that drive eggshell hardening and strength through biomineralization could have important implications for food safety.

"About 10-20% of chicken eggs break or crack, which increases the risk of salmonella poisoning," McKee said. "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."

A video explaining the research can be viewed on YouTube.