Isn’t it interesting that birds’ eggs are strong enough to resist fracture from the outside, while also being weak enough to allow the chick to break through from the inside at hatching? How is this possible?
Birds have benefited from millions of years of evolution to make the perfect eggshell, a thin, protective hard chamber for the embryo that contains all the nutrients required for the growth of a baby chick. The shell, being not too strong, but also not too weak, is resistant to cracking until it’s time for hatching.
Eggshells are made of both inorganic and organic matter, this being calcium-containing mineral and abundant proteins. Scientists have found that a factor determining shell strength is the presence of nanostructured mineral associated with osteopontin, an eggshell protein also found in composite biological materials such as bone.
The researchers were able thinly cut a sample of the interior of the shell and image the structure using electron miscroscopy
The results provide insight into the biology and development of bird embryos in fertilized and incubated eggs. Eggs are sufficiently hard when laid and during brooding to protect them from breaking. As the chick grows inside the eggshell, it needs calcium to form its bones. During egg incubation, the inner portion of the shell dissolves to provide this mineral ion supply, while at the same time weakening the shell enough to be broken by the hatching chick. The scientists found that this dual-function relationship is possible thanks to minute changes in the shell’s nanostructure that occurs during egg incubation.
