How to build the perfect egg
Imaging reveals nanostructures that make eggshells simultaneously strong and weak. Fiona McMillan reports.
The changing nanostructure of bird eggs enables them to be sturdy enough to provide protection, yet fragile enough to be cracked open by a fledgling chick when the time comes, new research has shown.
A study, published in the journal Science Advances, provides a new understanding of the mechanical properties of eggshells and could help improve the food safety of eggs.
Avian eggshells, which are almost entirely made of biomineralised calcium carbonate, provide a protective and nutrient-rich growth chamber for growing chick embryos. Indeed, the shell itself is an important source of calcium for the chick.
The egg is an evolutionary strategy that has been around for hundreds of millions of years. However, exactly how a cocktail of calcium carbonate crystals mixed with proteins forms such a robust and functional structure has until now been unknown because viewing eggshell structure at the nanoscale is challenging.
“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 lead author Mark McKee of McGill University in Quebec, Canada.
To solve the problem, he and his colleagues used a focused-ion beam sectioning system, which enabled them to make thin, accurate cuts of an eggshell and image the interior structure.
They found that calcium carbonate crystals form granules that vary in size across the outer, centre and inner layers of the shell. The outer layer has the smallest “nanogranules”, while those on the innermost later are much larger.
Hierarchies of nanoscale-to-macroscale structures are found in many other biomineralised structures in nature — including bone, mollusc shells, and coral skeletons — and are known to confer toughness to the overall structure.
Dimitra Athanasiadou, the paper’s first author, discovered that a mineral-binding protein called osteopontin (OPN), enables eggshells to achieve their nanostructure size hierarchy.
When she made synthetic eggshells by adding OPN to calcium carbonate, she found that the higher concentrations of the protein led to the formation of smaller calcium carbonate nanogranules – and thus a tougher shell. Conversely, less OPN yielded larger granules, and a weaker structure.
The resulting variation in across the layers doesn’t just give an egg its overall strength, but also provides unique benefits for a growing chick. The small nanogranules of the outer layer deliver resistance to impact, while the larger granules on the inner layer are easier to dissolve, enabling the embryo to absorb calcium as it grows.
The same process also gradually weakens the shell from the inside out, making it easier, when the time comes, for the baby bird to peck its way out.
Understanding how proteins influence shell toughness could have practical benefits for the egg industry.
“About 10 to 20% 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.”