Early Development of the Avian Immune System

INTRODUCTION Various protective mechanisms exist in the egg that assure the survival of the embryo through hatching. These include the physical and chemical barriers of the egg and the defensive activities of the developing embryo. The various embryonic events in the differentiation and maturation of the immune system will receive special emphasis. The Avian Egg The bird egg is admirably adapted to house, feed, and protect the developing embryo. The outer limy shell and adherent shell membranes provide a physical barrier that excludes most microorganisms but allows free exchange of respiratory gases. Interior to the shell membranes is a thick zone of albumen that provides a sterile fluid medium for the free growth and morphogenesis of the embryo and the extraembryonic membranes. In the center of the egg is the yolk mass that will nourish the embryo through the incubation period (Romanoff and Romanoff, 1949). Protective Mechanisms. Except for rare . . .