Endocrinology of the avian embryo: an overview.

The full endocrine system is developed during the embryonic stages of the domestic fowl and probably in all avian species. This involves the genesis and differentiation of endocrine cells and progressive acquisition of interactions between endocrine tissues leading to integrated series of control systems. The embryonic endocrine organs produce hormones with a variety of physiological roles (including homeostatic, homeorhetic, and anticipatory). The present chapter provides an overview of the recent advances in the endocrinology of the avian embryo. Particular attention is focused on the general phenomenon of endocrine development and on the ontogeny of the hypothalamo-pituitary (growth hormone)-somatomedin-growth axis, the hypothalamo-pituitary-thyroid axis, and the hormones of calcium homeostasis.