ABSTRACT
The endocrine system consists of a collection of glands that are scattered throughout the body and secrete chemical substances called hormones. A hormone may be defined as a chemical messenger produced and secreted by a specific endocrine cell and circulates in trace amounts in the blood and acts on specific target cells, usually remote from the endocrine cell. The polypeptide is concentrated in the Golgi apparatus and then stored in storage granules. Large protein hormones are usually retained in the storage granules, whereas small peptide hormones are bound to specific binding proteins within the granules. Tyrosine derivatives are synthesized within the cytoplasm of cells and stored either in storage granules attached to binding proteins or as part of thyroglobulin in the colloid of the follicles. Steroid hormones are synthesized from cholesterol, released immediately and not stored in the endocrine cell.
