Egg Shell

The shell acts as an egg isolation environment and regulates the egg breathing, process through which CO2 will be removed after the egg laying.

It is a hard shell formed by a network of calcareous proteins (3%) where minerals are deposited and which corresponds approximately to 10% of egg weight. Calcium carbonate is crystallized in the form of aragonite, which gives it its hardness. Presents an outer cuticle waxy nature with a number of pores (ca 10.000-50.000/per egg) are allowing gas exchange between the interior and exterior.

Egg White (Albumen)

There is a denser and smoother egg white (albumen). However, when we open the egg, we see two distinct areas, clear and dense gelatinous elsewhere less dense, clear liquid. The difference between the two is that the dense egg white has a concentration 4 times higher ovomucin giving stability through a three-dimensional network. Then higher dense albumin egg has, the fresher the egg is.

In the transparent zone, the pellucida chalazas are distinguished which are eggwhite condensation fixing the yolk and kept in the middle while the egg is fresh, and two egg white liquid areas and a thick one around the yolk.

Yolk

Yolk is a type of emulsion, lipo in hydro, oil in water, composed basically 1/3 protein and 2/3 lipid. Large amount of proteins have high concentration of enzymes, also lipids.

The yolk is the yellow portion of the egg. It is covered by the vitelline membrane that separates it from the eggwhite and protects it from breakage. The color is determined primarily by the diet of the hen. It can show a red spot, which corresponds to the germ disc, from which the chicken will be developed given the case that egg had been fertilized.

Diagram of an egg showing its different structures

The presence of all these layers and the egg shape produce a very rigid structure.

Membranes

The membranes (inner and outer) are two "envelopes" which together form the "chorion", one is attached to the shell and other contacts with the eggwhite. The organic matter of shell membranes contains proteins as major constituents. The membranes act as filters of defense against the entry of microorganisms. In the more blunt egg pole are separated and form an air cell greater as the eggs ages.

Air Cell

The air cell is formed on the banks of the egg with membranes attached, immediately, to the shell. It is relatively small in the new-laid egg (3mm) and increases as time passes. Therefore, it intervenes importantly to determine the egg quality. The smaller the air camera is the fresher the egg is.

Chalazas

The chalazas are two formations similar to transparent-white color cords, located in the longitudinal axis of the egg which is formed in the uterus by twisting fibers mucin, secreted with the magnum. The main function is to set the egg yolk to the center of the egg.

Mammillary Layer

It is the inner portion of the more calcified eggshell.

Spongy Layers

They consist of parallel crystal columns extended to approach the shell surface.

Cuticle

It is the end of the shell exterior. It has a thickness of 10 to 30 micrometers (mm). It is composed of organic matter called mucin. Its function is to prevent the ingress of liquid or solid particles and prevent microbial invasion into the egg. It is the first and most important barrier against bacterial invasion. The cuticle regulates the exchange of gases through the shell and prevents microbial invasion.

Germinal Disc

It is a structure that looks like a depression located on the yolk surface, whose size and development are related to the fertile egg and the embryo development.