It’s been a tough one to crack, but scientists say they have zoomed in, to an unprecedented degree, on the structure of shells surrounding chicken embryos, revealing how they change to allow young birds to hatch.

Before being laid, bird eggs form a hard calcium-rich shell with three main layers. While it was already known that these thin from the innermost out as a chick grows in preparation for hatching – with calcium from the shell being incorporated into its skeleton in the process – quite what happens at the molecular scale has been something of a mystery.

Now scientists say they have discovered that eggshells have a nanostructure, and that it appears to play a key role in the strength of the shell.

“Everybody thinks eggshells are fragile – [when] we’re careful, we ‘walk on eggshells’ – but in fact, for their thinness they are extremely strong, harder than some metals,” said Prof Marc McKee, a coauthor of the study from McGill University in Canada. “We are really understanding now at the almost molecular scale how an eggshell is assembled and how it dissolves.”

Writing in the journal Science Advances, McKee and colleagues describe how they probed the issue by focusing on the role of a protein known as osteopontin. This substance is found throughout the eggshell and was already thought to be important in organising the structure of its minerals.



“Something as different as an eggshell and a tooth and a bone, they all have this protein,” said McKee. “We think it is proteins like that that help guide the mineralisation process to give these tissues their properties.”

Using a number of microscopy techniques, as well as a cutting-edge method known as focused-ion beam for preparing thin sections of the eggshell, the team found that all of the layers appear to be formed from an array of tiny areas packed with a crystalline calcium-containing mineral.

The team also found the areas are smaller and more closely arranged in the outer layer, with the nanostructure becoming larger towards the inner layers. Levels of osteopontin were found to be lowest in the innermost eggshell layer.

“The third discovery was that the outside of the shell is harder as it has the smallest [nanostructure] and then you move inwards and it gets a little bit softer,” said McKee.

The team say the upshot is that osteopontin seems to form a sort of scaffold that guides the arrangement of calcium-containing mineral, generating a nanostructure that affects the hardness of the eggshell layer.

McKee says the theory is backed up by experiments in the lab.

“If you don’t put in the protein in the test tube you get a big giant calcite [calcium carbonate] crystal like you’d find in a museum. If you throw in the protein, it slows the process down, it gets embedded inside that crystal and it generates a very similar nanostructure property in those synthetic crystals and they have increased hardness,” said McKee. Higher concentrations of osteopontin were found to produce a smaller nanostructure.

The team then turned from the eggs that wind up on our breakfast tables to looking at the structure of chicken eggs that had been fertilised and incubated for 15 days. While the nanostructure of the outermost of the three eggshell layers remained unchanged, the nanostructure of the inner layers had become smaller in size. That, said McKee, is a result of calcium carbonate being dissolved in acidic conditions and used in the chick’s skeleton, and the process might be aided by the nanostructure increasing the surface area of the calcium-containing mineral.

The upshot is that the shell weakens, allowing it to crack and the chick to hatch.



While the role of other proteins in the structure of eggshell layers has yet to be unpicked, McKee said the latest findings could prove useful in the design of new human-made materials.

“When you think about it, we should be making materials that are inspired by nature and by biology because, boy, it is really hard to beat hundreds of millions of years of evolution in perfecting something,” he said.