Here’s a question about chickens and eggs you may not have heard before: How is it that an egg can be so hard to break from the outside, yet so easy for a weak little chick to peck through from the inside?
It’s a tough question to crack. Eggshells are thought to change as the chick grows inside. As the creature develops, parts of the inner eggshell dissolve and the fuzzy little bird incorporates some of that calcium into its bones. But it remained unclear how this process influenced egg shell microstructure. Now, As Nicola Davis at The Guardian reports, a new study in Science Advances suggests that it's all about the nanostructure of the egg and how it develops with the growing creature inside.
To unscramble the mystery and study egg structures, researchers at McGill University used a new focused-ion beam that allowed them to cut extremely thin sections of shell, according to a press release. They then analyzed these thin sections using an electron microscope to study shell structure.
The team examined the shells of fertilized eggs incubated for 15 days and compared that to ufertilized eggs. As Laurel Hamers at ScienceNews reports, they discovered that the key to the eggs' toughness appeared to be the formation of microstructures, guided into place by proteins. They focused their analysis on one particular protein called osteopontin, which is found throughout the shell and is believed to be vital in the organization of mineral structure.
As Davis explains, osteopontin seems to act as a "scaffold" that guides the structure and density of minerals in the shell, in particular calcium. In a developed egg, the minerals in the outer layer of the shell are densely packed and rich with osteopontin. But inner egg layers have a different nanostructure, which has less osteopontin and lower density of mineral packing.
In non-incubated eggs, the nanostructure didn't change. But in the fertilized and incubated eggs, the structure of the inner egg appeared to shift over time. Calcium was transferred to the chicks and the inside of the shell grew weaker, making it easier for the critter to crack through. The inner shell also became bumpier, which the researchers believe provides more surface area for chemical reactions that release calcium to the chicks.
"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," coauthor Marc McKee of McGill tells Davis. "We are really understanding now at the almost molecular scale how an eggshell is assembled and how it dissolves."
As Hamers reports, the osteopontin likely disrupts the orderly formation of calcium crystals in the shell, creating a stronger shell. At the nanoscale, the introduction of the protein prevents the formation of a smooth, uniform crystal structure. Instead, it causes the structure to be more erratic, which strengthens the outer shell. That’s the reason why a crack in an egg forms a zig-zag pattern instead of breaking open cleanly—the break has to find weak spots on its way through the scrambled crystal structure.
To test their findings, Davis reports that the team created their own eggshell substitute in the lab, with and without osteopontin. “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,” McKee tells Davis. “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.”
Knowing the nanoscale structure of the egg could lead to new types of materials, says Lara Estroff, a Cornell engineer who was not involved in the study, Hamers reports. The researchers think it could even improve food safety for eggs. About 10 to 20 percent of chicken eggs crack in transport, according to the press release, which could lead to salmonella contamination. Understanding why some eggs are stronger than others could help breed chickens with tougher eggs.