In spite of all of the basic research that has been conducted in the area of mineral metabolism and eggshell formation, problems which are associated with the eggshell still exist. Even the slightest structural flaw has the potential to compromise the primary biological function of the eggshell, which is to provide a chamber for embryonic development. The eggshell must be structurally sound enough to provide physical protection to the developing embryo throughout incubation while at the same time allow for adequate movement of respiratory gasses and water vapor. Since the developed chick has to emerge from within the egg, the eggshell must also be able to be penetrated during the hatching process.
Commercial table eggs and hatching eggs are continually monitored for quality. Assessing eggs for quality routinely requires that they be evaluated externally and internally. The external evaluation begins with an examination of the overall appearance of the egg and considers factors such as size, shape, color, texture, cleanliness and structural integrity. Internal evaluation requires the use of a light source over which the egg is passed so that certain quality characteristics can be assessed. It is during this "candling" process that flaws in the eggshell can be detected more easily.
One flaw in the eggshell, which can easily be detected during candling, is shell spotting or "mottling", as it is sometimes called. When candled, it can be observed that the shells of these eggs have many translucent spots that impart a "marble like" appearance to their shell surface. These spotted/marbled eggs can easily be distinguished from eggs with normal shells. In order to understand the underlying cause of the shell spotting/mottling problem, an appreciation of the basic architectural structure of the eggshell is essential.
A cross section of an eggshell will reveal that it consists of several morphologically distinct layers. Simply speaking, from the innermost layer towards the outermost layer these are the 1) shell membranes, 2) mammillary layer, 3) palisade layer and finally 4) the cuticle. The inner membrane is very compact and its inner surface has a smooth appearance. On top of the inner shell membrane sits the outer membrane which consists of a network of coarse protein fibers which allows for the initial anchoring of the inner portion of the shell mineral during the first crucial steps of shell formation once the egg reaches the portion of the hen's reproductive tract known as the uterus (shell gland). The anchoring of the mammillary layer base into the outer shell membrane at specific sites is essential for a proper mineral to membrane cohesion which is the backbone of proper eggshell formation and strength. Once the mammillary layer is securely anchored in the outer membrane and eggshell formation continues, the synthesis of the mammillary layer extends outward as "cones" until it makes up approximately 30 percent of the total shell thickness. As shell calcification continues, the palisade layer is formed and uses the mammillary layer as its foundation. The palisade layer makes up approximately 70 percent of the shell thickness and is composed of tall, erect, self-supporting columns of calcium carbonate crystals, commonly called "calcite".
Eggshell strength and its gas/moisture transport properties are highly influenced by the basic structural organization of the mammillary and palisade layers. Normally, the columns of calcite in these layers are erect, organized and closely associated with one another which results in good shell strength. However, if the columns are not as erect, disorganized and there are a large number of random disorganized air spaces among the calcite columns, the strength of the eggshell is compromised. The presence of air spaces among the normally formed calcite columns in the shell is essential because they allow for gaseous exchange between the developing embryo and the outside environment. However, as the components of the eggshell become disorganized, the functional properties of the shell are compromised and the disorganization associated with the air spaces will lead to more rapid moisture loss through the shell.
Coating the outer portion of the eggshell is a cuticle which is sometimes referred to as the "bloom". The cuticle, a slightly water soluble mucopolysaccharide consisting of carbohydrate, protein and fat, is important in shell strength and seals the pore openings on the surface of the shell. The coating of cuticle also serves to minimize shell bacterial penetration and contains a high concentration of the pigments found in brown eggshells.
A normal eggshell contains approximately 94 percent calcite, 4 percent protein and the remaining 2 percent consisting of a mixture of other minerals of which phosphorus and magnesium dominate at a concentration of approximately 0.4 percent each. Research has shown that the eggshell consists of more than columns of calcite crystals. There is a protein matrix which plays a very important role in helping establish and maintain the normal structure and quality characteristics of the eggshell. Factors such as stress and disease in laying hens not only affect the synthesis of these matrix proteins, but also have a negative effect on the positioning of these proteins in the shell. Disorganization of these shell matrix proteins ultimately affects the columnar structure of the calcite crystals and the size and quantity of the air spaces among the crystalline components of the eggshell. Thus, the normal formation of the protein matrix in the shell, as well as the normally formed shell membranes acting as supports for the mammillary layer, are essential for the proper formation and functional properties of the eggshell.
Altered Eggshell Architecture as Cause of Spotting/Mottling
The amount of marble-like spotting of the eggshell, which is seen during candling, is directly proportional to the amount of structural disorganization within the eggshell. The majority of the disorganization associated with the eggshell occurs in the mammillary and palisade layers. When the amount of structural disorganization in these areas is high there is an increase in moisture loss through the shell due to the large number and size of disorganized air spaces. Thus, an uneven pattern of dehydration occurs once the egg is laid.
As a layer flock ages the quality of their eggshells decline and the percentage of cracks increases. Also, the relative number of eggs with heavily spotted shells increases with age of a layer flock. Research has shown that as the hen ages the egg gets bigger and the shell becomes thinner. Simply, the size of the egg increases and the amount of shell deposition remains somewhat constant and, therefore, the shell thins. The calcite columns in the palisade layer of the shell become shorter with hen age and research in eggshell quality has shown that fracture strength of the eggshell is directly related to the thickness of the palisade layer. In older flocks, the combination of increased spotting and thinner shells has a cumulative negative effect on egg breakage as well as moisture loss. The increased moisture loss has a negative effect on hatchability.
Egg hatchability can be decreased by 3 percent to 14 percent when spotting of the eggshell is high, especially in older breeder flocks. The decline in eggshell quality in older flocks is also highly correlated with an increased bacterial penetration of the eggshell. Minimizing the number of stressors and their intensity in the environment of commercial egg-type hens and breeders will keep the stress level low in the birds and this is usually associated with a decrease in the amount of eggshell spotting/mottling, especially in older flocks. Maintaining proper temperature and humidity in the egg storage area will also help to minimize moisture loss from the egg, but even with proper egg storage conditions certain eggs will still exhibit the characteristic translucent marble-like spotting/mottling. The weight of a chick that emerges from a heavily spotted egg is usually less than that of a chick from a non-spotted/marbled egg of the same egg weight. This is probably related to the higher amount of dehydration which occurs in these eggs during incubation.
The water and gas exchange capacity of an egg is a direct function of the conductivity and porosity of its shell. In heavily spotted/mottled eggs the protein fiber matrix of the shell membrane layers, especially in the mammillary layer, is fragile, disorganized and loose resulting in a less dense shell. This leads to the disorganization of the crystalline structure of the mammillary and palisade layers and is the major reason the water and gas exchange capacity of the shell is negatively affected, microbial penetration increases, cracks develop more easily and eggshell spotting/mottling occurs.
Dr. Gary D. Butcher, D.V.M., Ph.D., is associate professor and avian veterinarian, Department of Large Animal Clinical Sciences, College of Veterinary Medicine; and Dr. Richard D. Miles, Ph.D., is professor and avian nutritionist, Department of Dairy and Poultry Sciences; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.