Abstract

A hormone is a chemical substance synthesized and secreted from a cell, which acts in low concentrations by binding to a stereospecific target-cell receptor to activate a response. Vertebrate hormones are classified by chemical structure; most hormones are polypeptides, steroids, or amino acid-derived compounds. Classically, hormones are secreted from endocrine glands into the blood, where they act on remote target tissues to affect cellular metabolism, growth, and function. The anterior pituitary gland has been called the master gland because it secretes multiple hormones, including adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and growth hormone. These hormones have diverse and profound effects on cellular function, and also stimulate secretion of hormones from the adrenal cortex, the thyroid gland, and the gonads. In addition to glandular secretion, hormones are released from secretory cells in the gut, heart, kidney, and brain. Hormones include substances released into the extracellular fluid to act locally on adjacent cells, ie, paracrine control, or on the cell that released the hormone, ie, autocrine control. Identified vertebrate hormones include atrial natriuretic hormone; endothelin, ie, polypeptides that control blood pressure and volume; and inhibin and activin, ie, proteins that affect reproductive function. Hormones identified in invertebrates and in plants have significant and varied effects in these species.