The amniotic egg provides an environment suitable for the development of the embryo. This environment, produced by the mother in the form of yolk and albumen, is enclosed by the eggshell. The eggshell consists of an underlying layer of proteinaceous fibres and a surface layer predominantly of calcium carbonate, whereas albumen consists primarily of proteins, although carbohydrates and lipids are found in trace quantities. Proteins produced by the oviduct of the mother, therefore, are largely responsible for the environment in which the embryo will develop.
The albumen and eggshell create a relatively homeostatic environment for embryonic development, the eggshell reducing the influence of the external environment but allowing gases and water to be exchanged. The composition of albumen is not static, but changes during embryonic development by interactions between the external environment and developing embryo. The composition and function of albumen alters during the course of embryonic development. In later development, the embryo is able to interact more directly with its environment through the apposition of extra-embryonic membranes to the eggshell and, therefore, plays a greater role in its own homeostasis.
This chapter will review the variety of proteins produced by the maternal oviduct and examine their influence on embryonic development. By far, the most studied is the albumen and eggshell membrane proteins of avian eggs, particularly those of the fowl (Gallus gallus).