Eggshell pigments - from formation to deposition

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.