Innate defenses of the avian egg

The remarkable reproductive success of avian species relies upon the integrated defensive features of the eggshell, acellular membranes, physicochemical characteristics, and antimicrobial molecules. These egg structures are dramatically modified during embryonic development: solubilization of internal eggshell components, disintegration of acellular membranes that serve as substratum for the growth of extraembryonic structures, and assimilation/transfer of egg white and yolk antimicrobials to other compartments. The apparent degradation in basic egg defenses is counterbalanced by the establishment of other protective and functional structures (namely, the amniotic, the allantoic, and the yolk sacs) that support several vital functions until hatching. Their cellular complexity, as well as the functional and molecular specificity of these structures, adds an additional defensive layer. This chapter reviews the different levels of these interconnected egg defenses that assist the embryo throughout its development, beginning with the egg defenses from the maternal origin to the development of extraembryonic protective structures. © 2022 Elsevier Ltd. All rights reserved.