The fine structure of the pores in the shell of the hen's egg

1. No fibrous or crystalline material was present in the lumen of pore canals which were exposed by radial fracture and examined with a scanning electron microscope.

2. The walls of the canals were rough, but they did not have a characteristic ultrastructure.

3. The campanulate outer orifice of the pores was plugged with cuticle in which radial cracks formed channels through which the respiratory gases presumably diffuse.     

Selection for egg shell strength in laying hens using shell membrane characteristics

1. Divergent selection for attachment strength between the shell membrane and the calcium shell was performed in a White Leghorn strain. Multivariate analysis was used to estimate genetic parameters for shell membrane measurements and shell thickness. The aim was to investigate the possibility of improving shell strength in laying hens by selecting for increased attachment strength.

2. A significant direct selection response for attachment strength was achieved. There was a favourable correlated selection differential in the frequency of cracked eggs which resulted in a nonsignificant but favourable, correlated response. Selection for a strong attachment resulted in a thinner egg shell, and vice versa. Heritabilities were all relatively high (0.30 to 0.70). Most of the genetic correlations were in agreement with the achieved correlated responses.

3. Two factors that probably contributed to the responses in egg shell thickness were an unfavourable genetic correlation between attachment strength and shell thickness, and natural selection against changes in attachment strength, in the form of increased mortality during incubation and hatching. Problems connected with breeding for increased shell strength are discussed.     

Localisation and characterisation of egg shell porphyrins in several avian species.

1. Electron microscopy, infra-red analysis and histochemical techniques have been used to determine the site of pigment deposition, the cellular localisation of pigment and the type of pigment precipitated in the egg shells of quail, White Leghorn, Brown Ranger and Black-Headed gull. 2. Transfer of pigment occurs via the surface epithelial cells of the shell gland. 3. Porphyrins are probably synthesised in the blood. 4. Possible pigment deposition mechanisms are discussed.     

The hen's egg: Density of egg shell and egg contents

One hundred eggs were used, from hens of seven widely differing strains: two commercial brown‐egg hybrids (one British, one American), two commercial white‐egg hybrids (one British, one Canadian), one broiler strain, one highly inbred strain of laboratory White Leghorns and one laboratory strain of Brown Leghorns. The volume occupied by the shell of each egg was estimated from its surface area—itself estimated by means of a three‐parameter model (Carter, 1968)— and its mean thickness, measured with an anvil micrometer. The volume occupied by the egg contents was estimated by subtracting the shell volume from the egg volume, which was also estimated by means of the three‐parameter model. Mean overall shell density (counting as “shell” all mineral matter and spaces between the outer surface of the mineral shell and a surface through the tips of the mammillae) was estimated, by regressing shell weight on shell volume, to be 2.241 ±0.004 g./cm.³; covariance analysis showed that the strains were homogeneous in this respect. Mean incremental shell density (i.e. the density of shell distal to the mammillary region) was estimated, by regressing shell weight per unit surface area on shell thickness, to be 2.386±0.004 g./cm.³; the strains were homogeneous in this respect too. The mean depth of the intermammillary spaces was estimated to be 19.9 μ. All the residual deviation from the common regression line can be attributed to measurement error. The estimated density of incremental shell is lower than that of calcite; the packing fraction of the crystals in the shell aggregate and/or the atoms in the crystals was estimated to be 92.8 per cent. The density of the egg contents (at the temperature of the bird) was estimated by regressing weight of contents on volume of contents; covariance analysis revealed significant differences between strains, one of the brown‐egg hybrids having the highest density of egg contents, 1.045 g/cm.³, and the laboratory Brown Leghorns the lowest, 1.033; both commercial white‐egg hybrid strains gave a value of 1.040 g./cm.³.     

Extraneous egg shell calcification as a measure of stress in poultry

The incidence of eggs showing abnormal shell calcification amongst those produced by hens kept in individual battery cages, hens kept in battery cages in groups of 3 and hens kept in battery cages in groups of 4 was studied. There was good agreement between observers in the assessment of abnormal eggs. Hens kept in individual cages laid fewer abnormal eggs than hens kept in groups of 3 or 4. There was no significant difference in the incidence of abnormal eggs between hens kept in groups of 3 or 4. The results of this study support the idea that the incidence of eggs showing abnormal calcification may provide a quick and reliable method of measuring stress in hens which lay brown shelled eggs.     

Studies on the avian shell gland during egg formation: mucosal intracellular pH.

1. The intracellular pH and the extracellular fluid volume of the shell gland mucosa were determined. 2. The pH value varied little during egg formation, the overall mean being 7.11 +/- 0.02 (n = 34). 3. The lowest pH, 7.01, was found 10 h after oviposition and the highest, 7.22, 18 h after oviposition.     

Studies on the avian shell gland during egg formation: The effect of acetazolamide on the composition of the mucosa

1. The effect of acetazolamide, an inhibitor of carbonic anhydrase activity, on the intracellular composition of the shell gland mucosa was determined.

2. The normal cyclic hydration of the mucosal cells was found to be reduced when the egg was in the uterus and the partition of the water between the intra‐ and extracellular compartments was modified.

3. Acetazolamide decreased the accumulation of Na+ within the cells, inhibited the Na+‐independent movement of Cl‐ and reduced the transfer of Cl‐ linked with Na+.

4. It is concluded that acetazolamide blocks the intracellular generation of HGO₃‐ and the reabsorption of Cl‐.     

Studies of membrane,mammilary cores and cuticle of the hen egg shell

Mammillary cores and outer shell membrane have been studied by comparing the weight and chemical composition of outer membrane with cores still attached and of outer membrane free from cores, from the same shells. Inner membrane and cuticle have also been investigated.

The weights of inner and outer membrane are significantly correlated (P < 0.05). Both membranes contain galactosamine, glucosamine, sialic acid, glucose, mannose and fucose but in different proportions. Uronic acid is probably absent.

An average of 3.3 mg of organic mammillary core material per shell (52 μg/cm²) was present in the shells investigated, but there was a trend suggesting that eggs laid by more mature hens may contain rather more. Cores are rich in hexo‐samine, sialic acid and hexose, but the probable absence of uronic acid suggests that chondroitin sulphates play no part in shell initiation. There were no major differences in the amino acid spectrum of membrane samples with and without cores.

Galactosamine, glucosamine, sialic acid, galactose, glucose, mannose, fucose and an unidentified pentose are present in the cuticle. In some respects these sugars are combined differently from their combination in the adjacent outer levels of the matrix.     

Ultrastructural characteristics of ostrich eggshell: outer shell membrane and the calcified layers

The ultrastructure of the eggshell of the domestic hen has been well researched and structural studies of other avian species, such as the ostrich, often base their interpretation of egg shell structure on that of the chicken. In the ostrich, lowered hatchability and hatching trauma may be due to shell ultrastructural abnormalities. In the present study the ultrastructure of the calcified portion, and the outer shell membrane (OSM), of domesticated ostrich eggshells was investigated using standard electron microscopic techniques. Transmission and scanning electron microscopy studies demonstrated intimate contact between cup-shaped structures present on the OSM and the mammillary layer of the calcified portion of the shell. The initial calcium carbonate growth of the calcified shell was of a dendritic nature with nucleation sites on the surface of the cup's contents. The dendritic growth gave way to a more randomly-orientated, smaller crystallite growth structure, which changed in form as it neared the vertical crystal layer (VCL). The VCL is described as being both amorphous and 'crumbly' depending on the plane of fracture. These observations suggest that firstly, initial calcification is contained within the cups and is then directed outwards to form the shell and that secondly, the VCL may contain an evolutionary, calcified cuticular layer. These observations serve as a baseline for studies investigating the effect of shell structure and strength on hatchling trauma and the influence of maternal diet.     

The effect of water on egg shell strength including a study of the translucent areas of the shell

A series of measurements of egg shell strength was carried out, using a needle to pierce the egg at a number of points at the waist up to a maximum of eight.

It was found that water weakens the egg shell whether it gets in from the outside of a normal egg or from the inside of an emptied egg. This weakening caused by water is not permanent and the shell recovers its strength on drying in air. Absolute alcohol strengthens the empty shell even more than air‐drying, but not to the same extent as oven‐drying. Shell strength slowly increases when a normal egg is allowed to stand in the air for a month.

Translucent areas of the shell are weaker than opaque areas and this difference is more marked at a week old than in a newly laid egg. Soaking the shell in water weakens it but also eliminates the difference between the two types of area, while oven‐drying strengthens the shell but also eliminates the difference.

The possible relationship of these points to the more practical aspects of shell strength and to the accurate measurement of shell strength is mentioned.     

Interior openings of functional pores in the avian egg shell: Identification with the scanning electron microscope

1. In a method to identify interior openings of unobstructed pores, they are stained with uranyl acetate.

2. Back‐scattered electron atomic number contrast imaging indicates the openings of those pores that have been penetrated by the stain, because of the enhanced back‐scattering arising from the atomic number difference between uranium and calcium. Morphologically identical structures that are unstained do not contribute to the total effective pore area.     

The application of shell strength measurements in egg shell quality determination

Comparisons of shell thickness, specific gravity and shell strength (breaking strength and shell deformation under load) for individual loads have been made and correlation coefficients determined. The relationship between egg size, shell strength and shell deformation have been investigated.

Shell strength has been correlated with shell thickness and specific gravity, but due to the complex nature of shell strength, it has been concluded that for routine work the measurement of shell thickness either indirectly or by specific gravity or by direct shell thickness measurement gives the most practical indication of shell quality.     

Studies on the avian shell gland during egg formation: Aqueous and electrolytic composition of the mucosa 1

1. The intracellular composition of the avian shell gland mucosa was studied at six stages of egg formation.

2. Total water content of the mucosa before shell calcification was 4.8 kg/kg dry weight (dw) and rose to 6.6 kg/kg dw during shell formation; 60% of the increase was intracellular and 40% extracellular.

3. The potassium concentration of the mucosa remained constant during egg formation.

4. Chloride was removed constantly from the mucosal cells during egg formation.

5. A model was constructed for the glandular cells which depicts them as primarily NaHCO₃‐secreting cells towards the luminal side and HC1‐secreting cells towards the serosal side, their main function being to provide CO^2‐ ₃ for shell formation. In this model, the columnar mucosal cells are responsible for the calcium used in shell formation.     

The measurement of egg shell strength: a comparison of four methods

A variety of methods exists for assessing egg shell strength. For practical field use, a method is desired which is relatively rapid, easy to use and cheap. It must demonstrate differences between experimental varieties or nutritional treatments with sufficient precision that decision makers can rely on the results. A study has been made of eggs from 10 inbred lines of White Leghorns using non‐destructive deformation, beta backscatter count, deformation at failure, force at failure, shell stiffness and shell thickness to estimate shell strength.

A comparison of these methods indicated that under the conditions of this experiment, backscatter count yielded low coefficients of variation and did not demonstrate differences between inbred lines. The other methods showed differences between lines with non‐destructive deformation possessing an acceptable combination of precision, ease of operation and cost. Deformation and force at failure and egg shell stiffness required expensive non‐portable apparatus but stiffness was nevertheless thought to be a useful parameter. Direct measurement of shell thickness was acceptable but necessitated breaking the eggs.