HormonesHormones are organic substances that are secreted by plants and animals and thatfunction in the regulation of physiological activities and in maintaininghomeostasis. They carry out their functions by evoking responses from specificorgans or tissues that are adapted to react to minute quantities of them. Theclassical view of hormones is that they are transmitted to their targets in thebloodstream after discharge from the glands that secrete them.
This mode ofdischarge (directly into the bloodstream) is called endocrine secretion. Themeaning of the term hormone has been extended beyond the original definition ofa blood-borne secretion, however, to include similar regulatory substances thatare distributed by diffusion across cell membranes instead of by a blood system. . Among animals, the hormones of the vertebrates–particularly those of humansand other mammals–are the best known.
Most vertebrate hormones originate inspecialized tissues, called endocrine tissues, and are carried to their targetsthrough the bloodstream. Endocrine glands. A major endocrine gland invertebrates is the pituitary, which consists of two distinct sections: theanterior pituitary (or adenohypophysis) and the posterior pituitary (orneurohypophysis). The anterior pituitary is sometimes called the “mastergland,” because it secretes several hormones that affect the otherendocrine glands. For example, the anterior pituitary hormones thyrotropin andadrenocorticotropic hormone (ACTH) regulate endocrine activity in the thyroidand the outer region (cortex) of the adrenal glands, respectively. The anteriorpituitary also secretes hormones that affect the sex glands.
One of these isfollicle-stimulating hormone (FSH), which stimulates egg production in theovaries and sperm production in the testes. Another is luteinizing hormone (LH). In females, LH works in conjunction with FSH to regulate the female reproductivecycle and the secretion of female sex hormones. In males, LH controls theproduction of the male sex hormones. Other hormones produced in the anteriorpituitary include growth hormone, which is responsible for normal body growth,and prolactin, which promotes milk production in female mammals. Its designationas the master gland notwithstanding, the anterior pituitary itself is regulatedby substances called releasing hormones that are secreted by the hypothalamus,the part of the brain located directly above the pituitary.
These hypothalamichormones stimulate–or, in some cases, inhibit–the secretions of the anteriorpituitary. The posterior pituitary stores and releases two hormones: oxytocin,which causes the uterus to contract during birth, and vasopressin, which acts onthe kidneys to restrict the output of urine. These two hormones are actuallyproduced by the hypothalamus, which is linked directly to the posteriorpituitary. Other endocrine glands in vertebrates include the thyroid,parathyroids, adrenals, pancreas, and gonads (sex glands). The thyroid produceshormones that control metabolic rate and oxygen consumption.
Hormones from theparathyroids are concerned with calcium concentration in the blood, and thepancreas releases insulin and glucagon, hormones that, respectively, lower andraise the blood-sugar level. Hormones from the adrenal cortex regulate glucoseand sodium metabolism. Those secreted by the central portion (medulla) of theadrenals affect the heart and the circulatory and respiratory systems; thesehormones are important in helping an individual cope with stress. The heartitself releases a hormone– atrial natriuretic peptide–that helps regulateblood pressure, blood volume, and the salt and water balance within the blood.
(see also Index: thyroid hormone, parathormone) The female sex hormones–theestrogens and progesterone–are produced by the ovaries. Together with FSH andLH, these hormones control the cyclical changes in the female reproductivesystem–the menstrual cycle in human females and the estrous cycle in otherfemale mammals. The estrogens also are responsible for female sexualcharacteristics. Progesterone is concerned with the maintenance of pregnancy. Male sex hormones–known as androgens–include testosterone, which is secretedby the testes. Testosterone is responsible for the maintenance of male sexualcharacteristics.
Hormone chemistry. Structurally, vertebrate hormones fall intotwo main classes. Those of the adrenal cortex and the sex organs are steroids, amajor class of lipid compounds. Virtually all other known vertebrate hormonesconsist of amino acids.
Most nonsteroidal hormones are composed of chains ofamino acids–either short chains (polypeptides) or long chains (proteins). Thehormones of the adrenal medulla, however, are composed of amino acid derivativescalled amines, those of the thyroid of a single amino acid combined with atomsof iodine. It is believed that hormones achieve their effects on target tissuesand organs through either of two mechanisms. The steroid hormones and thehormones of the thyroid can, as a result of their chemical structures, passthrough cell membranes. These hormones apparently enter a target cell andcombine with an intracellular receptor protein.
The hormone-receptor complexthen enters the cell’s nucleus, where it apparently affects the activity ofspecific genes. Genes carry the cell’s hereditary blueprint for proteinsynthesis, and so the interaction of the hormone-receptor complex with the genesinfluences the cell’s production of proteins. Because many proteins function asenzymes within the cell, this influence on protein synthesis can havefar-reaching effects on the cell’s activities. The polypeptide, protein, andamine hormones are believed to operate by a different mechanism.
These hormonesdo not enter the target cell; instead, they combine with a receptor protein onthe cell’s outer membrane. This hormone-receptor complex apparently triggers anenzyme in the membrane, causing the synthesis of a so-called”second-messenger” compound within the cell. This second messenger–inmany cases, cyclic adenosine monophosphate (cyclic AMP)–apparently activatesenzyme systems that bring about the desired action by the cell. It isinteresting to note that both proposed mechanisms of hormone action involve thebinding of the hormone to a specific receptor molecule. This feature accountsfor the specificity of hormones; a hormone can have an effect only on cells thatpossess the appropriate receptor.
Hormones probably exist in all invertebrates. In insects, neurosecretory cells in the brain produce thoracotropic hormone. This hormone stimulates glands in the thorax to secrete the hormone ecdysone,which causes the periodic molting, or shedding, of the hard exoskeleton. Anotherinsect hormone, called juvenile hormone, maintains the larval state. A decreasein juvenile hormone triggers the development of the adult characteristics. Plants also have a hormonal system, which includes auxins, gibberellins, andcytokinins, all of which promote growth.
Plant hormones also include severalgrowth inhibitors, which regulate such activities as the fall of deciduousleaves in autumn and the development of dormancy in buds and seeds.