Microscopic structure and distribution of various elements in the eggshell of the Black-tailed Gull, Larus crassirostris, as revealed by scanning and transmission electron microscopy and X-ray compositional microanalysis

Abstract

The present paper describes the microscopic structure and distribution of various elements in the eggshell from the Black-tailed Gull Larus crassirostris. Scanning and transmission electron microscopy (SEM and TEM) in combination with light microscopy demonstrated four major zones in the eggshell: the shell membrane (SM), the mammillary zone (MZ), the palisade region (PR), and the cuticle layer (CL). The SM was composed of a further three layers: i.e. the limiting membrane and the inner and outer SMs consisting of thin fibers. The MZ was a layer lined with conical knobs, the mammillare, each of which had a core portion characterized by a dense matrix with a few vesicles and aggregated fine fibrils. The PR showed a spongy feature depicted as numerous vesicles embedded in a calcified matrix. The CL appeared as a simple structure different from that seen in the eggshells from other birds, such as grebes, cormorants and domestic fowl. X-ray compositional microanalysis (XCM) revealed differences in the distribution patterns of certain elements (Ca, Mg, and P) in the radial face of the eggshell. The concentration of Ca was markedly high throughout the true shell (MZ and PR), whereas that of Mg was locally high in the MZ. The concentration of P was slightly higher in the surface crystal layer of the PR than elsewhere.