Eggshell Pigments–from Formation to Deposition

Abstract

The earliest avian eggshells probably lacked pigmentation but, in part to help protect the eggs from predators, many species of bird have evolved a diverse range of coloured shell markings. However, the wide variation in shell colour and pigment pattern arise out of probably no more than three molecules, protoporphyrin, biliverdin and zinc biliverdin chelate. These molecules are constructed from four pyrrole rings (tetrapyrroles) but, while their synthesis pathways are well defined, the site(s) of shell pigment synthesis is less well defined. It is probable that biliverdin is synthesised in the uterus. While there is evidence that protoporphyrin is also synthesised in the uterus the evidence for this is more circumstantial. What is known is that pigment is secreted from the surface epithelial cells of the uterus into the uterine fluid and hence onto the shell. It is notable that the protoporphyrin content the uterine tissue of white-egg laying birds is not dissimilar to that of brown-egg laying birds, the control mechanisms being exerted at the level of protoporphyrin release from the surface epithelial cells into the lumen of the uterus. Similarly white-shelled eggs are not necessarily devoid of pigment. While pigment is normally concentrated on the outer surface of the shell there are many species of bird that incorporate pigment into the outer half or third of the shell. This has stimulated research into potential functional roles for shell pigments, including substituting for calcium carbonate when calcium is limiting and improving shell strength through 'lubricating' the calcite crystals.

Condition-dependent strategies of eggshell pigmentation: an experimental study of Japanese quail (Coturnix coturnix japonica)

“…We also found that eggshells containing more biliverdin displayed bluer spots. These results confirm the complexity of eggshell pigment distribution in different areas of the eggshell (Sparks, 2011). Eggshell pigment synthesis and deposition are still hotly debated mechanisms in heavily spotted eggs, but the proposition that protoporphyrin IX and biliverdin are involved in the synthesis of haem is now gaining credence (e.g.…”

iTRAQ-Based Quantitative Proteomics Identifies Potential Regulatory Proteins Involved in Chicken Eggshell Brownness

“…Market tests suggest that consumers positively perceive brown eggs as being more nutritious and flavorful, and that hens laying brown eggs are thought to be organically fed [2]. Consumers who prefer brown eggs also pay attention to the intensity of eggshell brownness, and exhibit a preference for uniformity in eggshell brownness [3]. …”

“…Porphyrin and its derivatives, originally called the coloring of life, play an important role in life living organisms [6]. Porphyrin contains four pyrrole rings (tetrapyrrole)connected by methane groups and there are fifteen kinds of possible isomers formed by the side chains on the porphyrin rings through different combinations [3, 7]. The structure of tetrapyrrole is highly conjugated which produces absorption bands in the visible region.…”

Modelling Eggshell Maculation

“…There are two main types of pigments responsible for the colouration and patterning on eggshells: biliverdin IXα produces a blue-green hue, while protoporphyrin IX produces brownish hues (Kennedy and Vevers, 1975). Homogeneously deposited pigment is responsible for the underlying base colour of the egg, while distinct patterns of maculation result from the heterogeneous deposition of relatively large amounts of pigment, typically protoporphyrin IX (Sparks, 2011). During the production of maculation, pigment granules in the apical cells of the shell gland epithelium (Tamura and Fujii, 1966;Poole, 1967) are secreted into the lumen shortly before oviposition (Poole, 1965;Tanaka et al, 1977;Soh et al, 1989;Tamura and Fujii, 1966) where they gather to form pigment masses between folds of the mucous membrane lining the shell gland (Soh et al, 1989).…”

“…During the production of maculation, pigment granules in the apical cells of the shell gland epithelium (Tamura and Fujii, 1966;Poole, 1967) are secreted into the lumen shortly before oviposition (Poole, 1965;Tanaka et al, 1977;Soh et al, 1989;Tamura and Fujii, 1966) where they gather to form pigment masses between folds of the mucous membrane lining the shell gland (Soh et al, 1989). Contraction of the shell gland membrane then deposits these accumulated pigments onto the eggshell's surface (Soh et al, 1993), resulting in the speckles, spots, blotches and streaks, if the egg is in rotation (Solomon, 1987;Sparks, 2011), characteristic of the particular species or individual.…”