Eggs are weird, and not just because they come out of a chicken’s cloaca, the versatile hole that expels both feces and eggs. Eggs are weird because they’re a unique combination of strength and weakness. They’re so synonymous with fragility that boxes containing breakable items are often labeled: “Handle like eggs.” But at the same time, the calcium shell of an egg can handle a lot of pressure without cracking. A hen can lay on her eggs without breaking them, but a tiny newborn chick can peck its way out with relative ease. There’s so much more to eggshells than meets the eye, and scientists want to get to the bottom of what’s so special about them. In a paper published Friday in the journal Science Advances, a team of researchers outlines how specialized proteins and crystalline microstructures give eggshells their unique properties.

The key to this strange mix of strength and weakness is due to the effects of a specific protein on the calcite crystals that make up the three layers of eggshells, writes the team led by Dimitra Athanasiadou, a Ph.D. candidate at McGill University and the first author on the study.

For anyone who hasn’t used expensive laboratory equipment to examine an eggshell, you may be surprised to learn that it’s made up of three distinct layers. In order from outside to inside, they’re the vertical crystal layer, the palisades layer, and the mammillary layer. These layers are all made up primarily of calcium carbonate crystals, and according to the new study, their differences in hardness are due to variations in their nanostructure, a feature that’s mediated by the presence of a protein called osteopontin.

Using several microscopic imaging techniques, the researchers found that the vertical and palisades layers, the hardest layers, have a finer crystalline structure than the mammillary layer, a property that’s associated with smaller crystalline molecules. By growing calcium carbonate crystals in the absence and presence of osteopontin, they found that the protein is an essential part of a finer crystal structure, meaning that it seems to play an important role in the hardness of eggs’ mineralized shells.

Poultry and eggs contribute billions of dollars to the U.S. economy each year, so scientists are eager to learn more about the biological functions that lead to stronger and weaker eggshells. The researchers hope that their findings will help the egg industry find ways to select for chickens that produce stronger eggs, possibly by identifying genetic markers for higher levels of osteopontin.

Abstract: Avian (and formerly dinosaur) eggshells form a hard, protective biomineralized chamber for embryonic growth — an evolutionary strategy that has existed for hundreds of millions of years. We show in the calcitic chicken eggshell how the mineral and organic phases organize hierarchically across different length scales and how variation in nanostructure across the shell thickness modifies its hardness, elastic modulus, and dissolution properties. We also show that the nanostructure changes during egg incubation, weakening the shell for chick hatching. Nanostructure and increased hardness were reproduced in synthetic calcite crystals grown in the presence of the prominent eggshell protein osteopontin. These results demonstrate the contribution of nanostructure to avian eggshell formation, mechanical properties, and dissolution.