Mineralization process is represented in nature in different ways: bone formation, teeth formation and eggshell formation when we refer to laying hens.

Mineralization can be defined as the transmission of mineral properties to certain substance. It is a process where minerals are secreted through a solution by particular cells and, subsequently, these minerals crystallize and form a structure.

The shell has an inorganic phase, mainly composed by calcium carbonate. It also has an organic phase which contains approximately 2% of sugars, 3% of lipids and 95% of proteins.

How is an eggshell formed?

The eggshell is produced in the uterus or shell gland by the deposition of calcium carbonate. The eggshell formation roughly takes about 20 hours.

Calcium is deposited onto the shell through the blood stream. Eggshells in high productive laying hens contain between 1.7 to 2.4 grams of calcium (average of 2 grams). This corresponds to 8-10% of total calcium present in the organism. Therefore, if the production of the shell lasts approximately 20 hours, this means that 100-150 mg/hour of calcium is needed, thus, bloodstream has to pass through the oviduct at least 80 times to deliver the required amount.

Calcium demand increases during productive stage and can be covered by an enhancement in the intestinal calcium absorption, bone resorption and/or a reduction of the excreted calcium in the kidneys. This balance is regulated by several hormones such as Parathyroid hormone (PTH), calcitonin, calcitriol or estrogens.

Calcification takes place into the uterus when the egg is surrounded by uterine fluid, where calcium and bicarbonates are in high concentrations (hypersaturated medium), so calcium is able to precipitate.

The interruption of the calcification process happens during the previous 2-4 hours before the egg laying. It occurs due to the increase of phosphorus concentration (phosphates) in the uterine fluid, which produces an inhibition of calcium carbonate crystallization in the most superficial eggshell layers.

Metabolism of calcium in an eggshell formation process

Calcium homeostasis is achieved by different mechanisms such as intestinal calcium absorption, calcium renal excretion and through the animal’s bone mineral metabolism.

98% of calcium is found in bones but its participation depends on the amount of calcium absorbed out of the diet in the intestine. Calcium deposition (150 mg per hour) requires a total renovation of calcium every 12 hours.

The main calcium source is obtained from the diet as calcium absorption increases from 40% to 80% during the eggshell production stage, although the ability to absorb and retain calcium in the organism will be reduced as long as the hen gets older. However, not all the calcium comes from intestine; it is also obtained from the bones.

Under normal conditions, when the contribution of dietary calcium is adequate, most calcium extraction from the bones takes place during the night, since there is not enough available calcium to be absorbed in the digestive tract. Bone-sourced calcium is mainly facilitated by the bone marrow present in certain long bones.

When diet calcium levels exceed the 3.65%, most calcium comes from intestinal absorption. Nevertheless, when diet is poor in calcium – only provides 1.95% of calcium – it is not possible to satisfy the required calcium amount only by intestinal absorption. In those cases, calcium obtained from marrow bones can be 30% to 40%.