Establishing a good foundation is the basis of a good egg, whether it is laid for consumption or for reproduction purposes. To a large extent, this depends on the diet on which the bird is fed. Evidence is now accumulating that organic selenium influences structural changes in the mammillary layer of shells, which leads to improved shell quality.
By Professor Sally E. Solomon, Senior Research Fellow, University of Glasgow Veterinary School, Scotland
In the language of the estate agent, "Look out for cracks near windows and doors and unexplained damp patches as these problems can be expensive to correct, and it might be easier and cheaper to look at another property!" If you are a developing chick and the shell is your temporary home, you have no choice but to continue your early development in the "property" you were created in. As consumers we reject cracked eggs prior to purchase. However, many of the eggs that make it from the supermarket to our kitchens still give cause for complaint, like the thin shelled egg with its watery white, or the egg that when boiled and peeled refuses to separate from its contents. The defects that reduce the efficiency of the shell as an embryonic chamber or serve to irritate the cook occur during the early stages of shell formation – in housing terms, when the cement has been poured for the foundations and the first bricks laid.
So what constitutes the foundation layer of the egg with respect to the ‘true shell’ i.e. the calcified portion of the latter? The mammillary layer or basal layer supports the growth of the palisade columns, which comprise the major fraction of the fully formed shell followed by the vertical crystal layer and cuticle. However, before shell formation begins, thick egg white and the paired shell membranes have a ‘foundation’ role to exert, and failure at this level, as at any of the other levels discussed, can compromise quality (Figure 1).
The shell membranes
Egg white is encapsulated in the first of the paired shell membranes. The inner membrane is amorphous and it, together with the outer mesh-like membrane onto which the ‘true shell’ will grow, serves to contain the nutrient components of the egg. Occasionally the integrity of the membrane layers is breached and the calcium salts, which should have been restrained in their inward movement by the topography of the two layers, pass through to the egg white and, attracted by calcium binding proteins that are an integral part of albumen, create a bond between the shell and its contents. When an egg such as this is boiled and peeled, a plug of egg white will accompany the broken shell. The origin of the holes in the shell membranes is difficult to explain. That they are present prior to the main phase of calcification is unequivocal and so may reflect some dysfunction of the isthmus. It is acknowledged that the total thickness of the paired membranes decreases with increasing bird age and indeed fluctuates during the laying year. These aberrations may predispose the membranes to a temporal weakness, which encourages changes in their normal random organisation.
What about the outer shell membrane and its role in the nucleation process of the ‘true shell’? At ultra structural level it resembles unravelled knitting wool. Each fibre consists of a protein core surrounded by a carbohydrate mantle. It is this randomly organised front that presents to the supersaturated solution of calcium carbonate in which the egg is bathed, the countless nucleation sites thatwill attract calcium ions in association with other minerals, the whole enabled by a myriad of protein types to form the mammillary layer. Frequently the random distribution of fibres fails and the latter can become aligned. In such a situation, the nucleation sites will also be aligned, and although calcification will proceed, the normal growth process will be impaired (Figure 2).
The mammillary layer
As the foundation layer of the ‘true shell’, the mammillary layer is the site of a range of structural defects, which ultimately impact performance. Consider in the first instance, the timing of fusion of adjacent mammillae. If the process is delayed because the nucleation sites are too widely separated, or because of structural changes in the morphology of the mammillae, then voids are created and fluid from the albumen migrates outwards and collects in the spaces provided. These clear areas are best visualised by candling. Despite the signal they are giving of weakness within the shell, such eggs are rarely rejected by packers.
Meat spots in a variety of forms, such as tissue debris, tissue debris associated with blood, and debris in association with calcite crystals, are most often found in egg white, but these deposits can also settle on the membrane fibres. In such a position, they mask the nucleation sites and the normal growth pattern is not established. Whether these areas of ‘erosion’, as they have been described, have a major influence on quality will depend on the degree of incidence of their occurrence. In the context of this article, however, they have altered the foundation layer and as such merit inclusion (Figure 3).
It is perhaps relevant at this stage to point out that the defects already described rarely occur in isolation, and in combination their effect is of course more catastrophic.
Stress and egg quality
Over the years many scientific articles have been written on the subject of stress and egg quality. What constitutes stress, how does it manifest itself in terms of bird behaviour, and is the egg an adequate indicator of external traumatic events? Research carried out several years ago demonstrated that the seemingly simple movement of one bird from a cage of four to another cage (now holding five birds) for a period of one hour was sufficient to dramatically alter shell structure for some considerable time. However, what aspect of this transient change in stocking density encouraged such deterioration in quality – the handling, the change in pecking order?
It is difficult to isolate one particular factor. Under stress, the oviduct breaks down, meat spots increase, and the shell invariably becomes thinner. By way of explanation of this change in thickness, one need look no further than the morphology of the mammillary layer following an event such as that described. Normal mammillae are sporadic in their occurrence, their place being taken by rounded crystal forms, which neither attach to the underlying fibres nor contribute to the growth of the palisade layer. In answer to the question of whether the shell is an adequate indicator of stress, the answer is a qualified ‘Yes’. Shell thinness is not in itself an indicator, but thinness taken in conjunction with other features such as ultra structure and the breaking strength of the shell will confirm the bird’s exposure to unfavourable conditions.
Occurrence of confluence
Of the structural variants that commonly occur within the mammillary layer, ‘confluence’ has a significant effect on shell function. It is a common feature in the small eggs of young birds, but is perhaps best demonstrated in the target egg and particularly the second egg in the sequence that, when it arrives in its soft shelled state in the shell gland pouch, is confronted by a resident fully calcified shell. In such a situation complete calcification of the soft shelled egg is not possible; squeezed as it is against the hard egg, it fails to plump properly and the nucleation sites are pushed together. The effect is to create an area of fusion or confluence in which individual palisade columns are reduced in terms of thickness and both shells present externally with rounded areas of calcium dusting where they were contiguous. Both eggs will be downgraded.
Gas exchange pores
The shell is of course a porous structure, estimated at 10,000 - 20,000 pores per shell. The laying hen does not discriminate between the growth chamber and the table egg in terms of design and so any modification in the structure of the inner layers of the shell that impact pore patency will ultimately influence pore function. Under stress, though, the rounded crystal forms that dominate the mammillary layer can and locate at pore sites (Figure 4). Likewise, mammillae growing together as confluent masses influence both the number and distribution of pores and as such alter the functional capacity of these sites for gas exchange and water vapour conductance.
In terms of albumen quality, membrane strength and organisation, as well as factors influencing the early phase of calcification, is there a "one cure fits all", and more to the question, is it possible to favourably influence one aspect of the shell forming process without exerting a negative effect on the other foundation layers?
If such a cure exists then it is an investment in bird health. Enabling the oviduct to perform its diverse functions on a 24-hour basis is a delicate balance between environment and diet. A healthy bird in an environment compatible with its behavioural requirements will access those nutritional elements which will ultimately transform the lipoprotein rich yolk mass surrounded by its protective albumen and membranes into a shelled product suitable for its diverse roles.
The key ingredients in a balanced diet in addition to macro-nutrients include calcium, phosphorus, vitamin D3, linoleic acid and trace minerals. As their name suggests, trace elements are required at extremely low concentrations in most living tissues. Of these, selenium as well as cobalt, copper and zinc fall within the category ‘essential’, and in terms of egg quality, have been found in all parts of the fully formed egg from yolk to shell. The functional significance of selenium varies, thus in the membrane-bound yolk it facilitates sperm penetration and with respect to egg white, it appears to improve albumen quality and maintain its form and viscosity during storage, a feature required of both the table and hatching egg.
Evidence is now accumulating to show that when organic selenium (Sel-PlexSRTm, Saccharomyces cerevisiae CNCMI-3060, Alltech) is fed to both breeders and layers, structural changes occur in the mammillary layer of their shells consistent with improved quality. In the breeder eggshell the positive changes in structure began to appear towards the end of lay at which time hatchability also increased.
Whether the mechanisms governing the improvement in the structure of the foundation layer of the ‘true’ shell are specific to the shell gland pouch, or whether the earlier improvement in albumen quality facilitated the deposition of a more ‘normal’ mammillary layer has still to be resolved. What is evident, however, is that structural changes in the shell can be encouraged through the use of organically bound selenium without any adverse influence on other parameters of shell quality.