Biomineralization: protecting baby birds for 500 million years

Eggshells, of course, have the important job of providing adequate protection for the developing chick it contains, allowing it to develop outside of a hen’s uterus. But, it doesn’t just have to be strong — it also needs to allow the chick to escape when its development is complete.

Biomineralization is a 550-million-year-old process by which organisms make hard structural features. The eggshell of a domestic chicken is roughly 95 percent the mineral calcium-carbonate and about 3.5 percent organic material by weight. The incorporation of organic material and the specific nanostructure lead to mechanical properties that provide distinct advantages for the animals that produce them.

Even though an eggshell is only a third of a millimeter thick, it is not uniform. It is in fact comprised of different layers, each with distinct nano-structures.

Biominerals have complex nano-structures that can’t form without the help of specific proteins. There are hundreds of these proteins that are critical to the precise crystallization patterns found in nature. The structures help improve the shells' mechanical properties, making it strong and preventing cracking (before it becomes necessary).

A team of researchers from institutions from Canada, the U.S., Germany, and Spain worked to find out how these protein-mineral interactions make eggshells what they are.

In their March 2018 paper published in Science Advances, the researchers identified the existence of a nano-granular structure in eggshells. The size of these nano-building blocks varied depending on the shell region. The changes in structure through the shell are thought to stem from variations in protein content though the exact processes are not well understood.

By testing across the shell cross-section, the scientists found that hardness and elasticity gradually decreases from the outside of the eggshell towards the central region, which fits logically with what scientists predicted about material hardness. Curiously, as they approached the innermost layer, the measurements no longer matched the prediction; the team is still unsure of exactly why.

An eggshell’s composition is not static over the course of the chick embryo’s development: it actually partially dissolves throughout the incubation period. The scientists compared the nano-structures of the shells from incubated and un-incubated fertilized eggs. They found smaller nano-structure sizes in the innermost layer; they believe this special structure enables it to partially dissolve. This dissolution serves two important purposes: the dissolved material is a source of calcium required for the development of the chick’s skeleton, and it reduces the shell's thickness, facilitating the chick’s escape.

To explore the effects of protein on shell development, the team grew synthetic calcium carbonate crystals in the presence of the protein osteopontin (OPN), which is so named because of its initial discovery in bone. The crystals grown in the control experiment did not show any specific internal nano-structure, but when grown with OPN, a nano-structure was clearly present. They found that the size of the nano-structure was dependent on the amount of protein in the initial mixture. These findings can inform the development of bio-inspired nano-composites with unique properties. The higher the OPN concentration, the smaller the size — a sign that the nano-structure size, and the resulting material properties, is tunable.

These results have some promising applications. Eggshell mechanical properties abruptly decrease after a hen has been laying for a year, which is a substantial problem for the egg industry. Understanding the mechanisms by which eggshells get their strength can help find a way to extend the egg laying period of hens while maintaining eggshell strength.