Diversity of the Cuticle Layer of Avian Eggshells

Abstract

The structure and mineral composition of eggshell cuticles were studied in 5 species of birds. The approximate thickness of the cuticle layer at the top of shell columns was about 1μm in the Red Junglefowl (Gallus gallus), about 130μm in the White Pelican (Pelecanus onocrotalus), about 10μm in the Japanese quail (Coturnix japonica), about 110μm in the Greater Flamingo (Phoenicopterus ruber roseus) and about 45μm in the Humboldt Penguin (Spheniscus humboldti). The matrix of the cuticle layer decalcified with EDTA was composed of vesicles in a variety of sizes in all birds. Major elements in cuticle materials detected by X-ray microanalysis were O, C, Ca and P, and their percentage numbers of atoms decreased in this order. The concentration of P was significantly higher in the cuticles of the quail, flamingo, and penguin than in those of the junglefowl and pelican. Ca mapping on electron-microscopic images showed strong signals in the shell layer and weaker ones in the cuticle layer, whereas P mapping showed that signals were mostly confined in the cuticle layer. X-ray diffraction analyses on the inside of the shell layer showed a profile of calcite crystals of calcium carbonates in all birds. In the cuticle materials, the profile was of calcite in the junglefowl, and a mixture of calcite and vaterite in the pelican. The profiles in cuticle materials of the quail, flamingo and penguin showed no specific signals, indicating that mineral compounds are amorphous in these forms. It was suggested that the diversity of mineral structures in the cuticle layer is caused by the presence of phosphorous, in addition to the structure of the cuticle matrix.