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The shell of an egg is made with millions of perfectly organised calcium carbonate microcrystals. It is a marvellous structure, a perfect protein container and, at the same time, the ideal environment for the development of an embryo.

Would you like to know how this structure, so attractive yet also so efficient, is made? Do you know what the study of its crystalline texture is used for? Can you think why such a thin shell is so robust?

Eggshell is truly a prodigious structure. It fulfils its function so perfectly that it seems to have been purposefully designed by a genius engineer. But it was not. There has never been such an engineer. But it is true that there isn’t an engineer on Earth who wouldn’t dream of one day creating a structure with these properties. Let’s have a look at what an eggshell is like.

Eggshell is made up of 96-98% calcium carbonate (from the mineral calcite in birds), while the remaining 2-4% is an organic matrix that is situated within and between the calcium carbonate crystals.

Maybe you think the egg is just food. This it certainly is: the eggshell is a perfect protein container. In fact, it is the only animal food product that keeps all its properties for long periods of time at room temperature.

But to understand the design of the shell it is worth remembering that the function of the egg is the extrauterine development of an embryo. For this reason the inner crystals of the eggshell are very small, so that they can be easily dissolved to provide the calcium that the embryo needs. But the outer part of the shell has to be robust to withstand blows from predators and so the crystals there are larger. The embryo must obviously be able to breathe, hence the structure must have pores for air to pass in and out. But the pores cannot be very large because the embryo must be protected from bacterial contamination.

 

OLYMPUS DIGITAL CAMERA

Optical microscope picture of a thin sheet of a chicken egg. The thickness is 0.3 mm.

Scanning electron microscope photographs of an eggshell: a) general view; b, c & d) details of the external vertical layer of the membrane; e) the mammillary cones that are formed at the beginning of the shell formation.

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A hen creates this highly sophisticated structure in less than 24 hours. It does so mobilising up to 10% of the calcium from its skeleton to make the shell. And it does this every day. How?

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Timeline diagram of egg-laying, Yves Nys

The reproductive system of the hen is made up of the oviduct, which in laying hens measures between 60-80 cm long, weighing approximately 40 g and extends from the ovary to the cloaca. By morphology and function the oviduct can be divided into six regions:

Infundibulum: measures 8-9 cm long with a wide funnel-shaped end. The yolk takes about 15-30 minutes to pass through this region.

Magnum: measures around 35 cm. The egg, on passing through this region by means of mechanical stimulation, provokes the release of the albumen. The egg stays in this region varies between 3-4 hours.

Isthmus: 10 cm long. In this region the formation of the inner and outer membranes of the shell is completed. Both membranes are deposited over the albumen in approximately 60-75 minutes.

The tubular glands are located at the base of the red isthmus. They produce keratan sulphate, present in the mammillary nuclei of the eggshell. The mammillary nuclei are deposited during the time that the egg stays in this region and they are the nucleation centres out of which the mineralization of the eggshell begins.

Uterus: The non-ciliated epithelial cells of this region produce dermatan sulphate, the production of which coincides with the start of the calcite crystal growth. The egg stays approximately 20 hours. In this part two phenomena occur:

  1. in the initial phase of shell formation, the distal part of the uterus secretes ions and water, which are pumped toward the inside of the egg;
  2. shell mineralization is produced by means of the precipitation of calcium carbonate in association with an organic matrix.

The mineral phase is deposited, intimately associated with the organic matrix.

This matrix is formed by macromolecules, among which are soluble and insoluble proteins such as lysozyme and collagen, by polysaccharides such as chitin and by lipid membranes.

Many of the proteins involved in biomineralization processes contain a high proportion of amino acids (aspartate) and phosphorylated groups. All these organic components play a very important role in the biomineralization process: during nucleation, growth, control of the polymorphic phase, morphology and orientation of the individual crystals and thus in the mechanical properties of the shell. The extraordinary structure with tiny crystals in the interior and large shells on the exterior, as described above, is the result of the competitive growth of the calcite crystals.

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The antibacterial protection of the outer cuticle of the egg is fundamental to making it such an effective protein container.

The integrity of the eggshell is therefore essential to preserve the egg in perfect state. A rupture of the shell is deadly for the embryo and critical for food use.

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Regrettably, 6 billion eggs, 8% of the 80 billion eggs produced each year, are not suitable for consumption due to shell defects. It is therefore very important to know the eggshell structure, the relation between the crystallographic and the textural and mechanical properties. And how all of these are related to the feeding, genetics and age of the hen.

Did you know that…

  • The World Health Organization has chosen the egg as the ideal food source of protein for children.
  • Europeans consume 210 eggs a year per capita.
  • Every year 80 billion eggs are produced.
  • The shells of bird eggs are all made of calcite, whereas the shells of reptile eggs are of aragonite.
  • The textural properties of eggshells are the same for the same species, which means that there is genetic control.
  • Eggshell is precipitated in a cavity with controlled chemical conditions, the shell gland or uterus of the hen, and out of an acellular medium, the uterine fluid.
  • Eggshell creation is the fastest known process of biomineralization.
  • The shell of an ostrich egg has a thickness of 3 mm and can support a weight of 70 kg.
  • The weight of an egg can vary between less than 1 gram (several species of hummingbird) and more than a kilo and a half in the case of the ostrich.

To find out more

  • If you want to learn more about biomaterials, the Wikipedia entry is a good place to start.
  • You can learn more about the crystallization of aragonite, a crystalline form of calcium carbonate present in a large variety of biomineral structures in nature.

 

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