The chicken egg and its shell have been the subject of research by various groups of scientists. Typically, a hen lays about 300 eggs per year, so close to one a day, during which it takes about 25-26 hrs for the egg and its shell to form fully. After laying the egg, it takes about 21 days for the laid egg to hatch. Dr. Marvin A. Tung’s Ph.D. thesis at the University of British Columbia, Vancouver, Canada, in 1967 was devoted to the physical, chemical and rheological studies of the hen’s egg (<http://open.library.ubc.ca >media >pdf>), where he points out that the egg can withstand a vertical load of up to 4 kg, when pressed along its minor axis and that the shell stiffness is most important in offering it protection against crushing. Each hen’s egg is about 60 grams in weight and has about 6 grams of mineral in the shell which provides hardness.
About 200 years ago, the German mineralogist Frederich Mohs devised a scale to estimate the hardness of materials. Taking diamond to be the hardest (by giving it a scale value of 10), the precious stone topaz was assigned a value of 8, quartz 7, and the mineral stone apatite (calcium phosphate) the value of 5.The tooth enamel in our mouth thus has a value of 5, since it is largely made of hydroxyl-apatite; it is thus the hardest mineralized tissue in our body; fingernails are only 2.5 while calcite (calcium carbonate) is harder, having a value of 3. The eggshell of chicken, which has calcite in it, is thus sufficiently hard to be protected from breaking. But then how does the fertilized newborn baby chick with its tiny beak manage to break the egg and come out into the world?
The answer to this puzzle has recently been published by a group led by Dentistry and Anatomy and Cell Biology Professor Marc McKee of McGill University inMontreal, Quebec, Canada, in the journal Science Advances (Sci Adv.2018;4:eaar3219). The group collaborated with Professor Richard Chromik of Mining and Materials at the same university, along with other scientists in Canada, Spain, Germany and the US, to determine the fine structure of the chicken eggshell. The shell is about 0.36 mm overall thickness, but has sub-layers, with different structure and protein composition.
The structure, composition and mineral content of the eggshell has been studied in some detail, previously, by an international group led by Professor Maxwell Hincke of the University of Ottawa, Canada, in collaboration with colleagues from France, Germany, Spain and Canada (see Hincke et al., Frontiers in Bioscience, 17,1266-1280,2012; free access on the web). Their work, as well as that of others, has determined that as many as 500 proteins are identified in the eggshell, and they have specifically focused on the major proteins ovocleidin (ovo-egg, kleidon-lockup) and ovocalyxin (ovo-egg, kalyx-bridge) and a third one called osteopontin (osteo-bone, and pont-a Latin word meaning bridge).
The shell is thus a composite material having a combination of minerals (calcite at 96% by weight) and the rest containing trace elements and so-called matrix proteins. The shell has been optimised by evolution for hundreds of millennia since the time of the dinosaurs and even before that during the transition of animals from marine to terrestrial environments. The positive surface charges of calcium in calcite crystals bind the negatively charged matrix proteins like osteopontin, making a functionally cohesive biomineral composite.
The researchers have sliced the eggshell to view five sub-layers and examined each of those with advanced microscopy techniques and hardness testing. They discovered that, in the very thin slices, there was a nano-scale structure to the biomineral. The dimensions of the nanostructure varied from being the smallest in the outermost layer (30 nm) through 33, 59 and 74 nm, way down to the innermost layer which is 68 nm thick, all of which intimately covers the egg where development and growth of the chick is occurring. The innermost layer, called the mammillary layer, was softer, and slowly dissolved away its nanostructure to provide the growing chick with the calcium necessary to make its skeleton. The shell thus not only protects the growing chick, but also provides essential calcium mineral ions. This dissolution and thinning of the shell also allows for the hatching chick to break open and emerge to the outside world.
Over 50 years ago, a team of Japanese scientists had filmed the complete development of a chick from the egg, showing what happens day by day in the 21 days that it takes for the chick to form from the egg and come out to the world. Alas, that remarkable film is not readily available now, but a shorter (2 minute-long) video, called “Chick Embryo Development”, produced by the Poultry Hub in Australia, is available on YouTube, and is well worth watching. Looking at it, one marvels at how evolution has made sure how to protect the birth, growth and emergence of a new generation by encasing the embryo safely inside a hard-to-break-shell. With the understanding of each of the layers of the shell, we now have a better understanding of this remarkable protective structure, within which a new life is born.