Ultrastructure of the eggshell of selected Palaeognathae species – a comparative analysis

The study aimed at comparative analysis of the eggshell ultrastructure, indicating differences and similarities in its structure depending on the bird species. The study was carried out in ostrich, emu and rhea breeding flocks. The birds were kept under open system. The ostrich flock comprised 6 females and 3 males, the emu flock included 22 birds with equal sex ratio, while the rhea flock consisted of 16 females and 4 males. Emus and rheas were 5 years old and in their 3rd laying year, whereas ostriches were 4 years old and in their 2nd laying year. The analysis of eggshell ultrastructure and porosity was performed on the post-hatching eggshells being obtained after the incubation of eggs from around peak laying period of these birds. In total, 27 eggshells were evaluated, 9 of each species. Analysis of the ultrastructure of ostrich, emu and rhea eggshells showed their different architecture being dependent on the bird species. The cuticle in ostrich eggs adhered firmly to the vertical crystal layer, it mainly occurred around pore canal orifices in rhea eggs, while only its residual presence was observed in emu eggs. The percentage of the vertical crystal layer was similar in the ostrich and rhea eggshells (2-3%) but significantly higher in the emu eggshells (9%). The largest number of mammillae per unit of inner eggshell surface area was recorded in emus, while the broadest palisades in rheas. The ostrich eggshells were shown to be characterised by the least number of pores per unit of surface area, with significantly narrower pore canals.