HORMONE

organic substance that is produced in one part of a multicellular organism and is able to exert effects on other, sometimes far distant, parts. Hormones regulate a variety of physiological activities concerned with growth, reproduction, and the maintenance of a constant internal environment (homeostasis). Among animals, the hormones of the vertebratesparticularly those of humans and other mammalsare the best known. Most vertebrate hormones originate in specialized tissues, called endocrine tissues, and are carried to their targets through the bloodstream. organic substance secreted by plants and animals that functions in the regulation of physiological activities and in maintaining homeostasis. Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system. Additional reading Comprehensive works are Henry R. Mahler and Eugene H. Cordes, Basic Biological Chemistry (1968); Abraham White et al., Principles of Biochemistry, 6th ed. (1978); Albert L. Lehninger, David L. Nelson, and Michael L. Cox, Principles of Biochemistry, 2nd ed. (1993); Thomas Briggs and Albert M. Chandler (eds.), Biochemistry, 2nd ed. (1992); John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Chemistry and Life, 4th ed. (1993); Lubert Stryer, Biochemistry, 3rd ed. (1988); Donald Voet and Judith G. Voet, Biochemistry (1990); Geoffrey Zubay, Biochemistry, 3rd ed. (1993); and Laurence A. Moran et al., Biochemistry, 2nd ed. (1994). David J. Holme and Hazel Peck, Analytical Biochemistry, 2nd ed. (1993), covers newer methods of analysis. The Editors of the Encyclopdia BritannicaMore specific references on hormones include E.J.W. Barrington (ed.), Hormones and Evolution, 2nd ed., 2 vol. (1979), for students and general readers, concerned with the molecular structure and mode of action of hormones in relation to evolutionary theory; G.K. Benson and J.G. Phillips (eds.), Hormones and the Environment (1970), a wide-ranging review and research symposium; Max Hamburgh and E.J.W. Barrington (eds.), Hormones in Development (1971); W.R. Butt, Hormone Chemistry, 2nd rev. ed., 2 vol. (197576), a correlation of research on the major mammalian hormones, requiring some knowledge of chemistry; Sheldon Aaronson, Chemical Communication at the Microbial Level, 2 vol. (1981); Rex E. Coupland, The Natural History of the Chromaffin Cell (1965), an integrated treatment of research on structure and function, considered at all levels of analysis, from the gross anatomical to the molecular; John Ebling and Kenneth C. Highnam, Chemical Communication (1969), a concise elementary introduction for high school and first-year university students; G.V. Hoad et al. (eds.), Hormone Action in Plant Development (1987); B.E. Frye, Hormonal Control in Vertebrates (1967), an elementary introduction, with emphasis on general principles and physiological adaptation; John W. Buckle, Animal Hormones (1983); Claude A. Villee, Jr., Human Hormones (1987); M. Gabe, Neurosecretion (1966), an authoritative and comprehensive treatment; G.W. Harris, Neural Control of the Pituitary Gland (1955), an authoritative monograph for students and research workers; Mary Pickford, The Central Role of Hormones (1969), an elementary treatment for students and general readers with some knowledge of vertebrate biology; Clark T. Sawin, The Hormones: Endocrine Physiology (1969), a lucid treatment of hormone action for university students; Charles B. Nemeroff (ed.), Neuroendocrinology (1992); Barry D. Bercu (ed.), Basic and Clinical Aspects of Growth Hormone (1988); and Roman J. Kutsky, Handbook of Vitamins, Minerals, and Hormones, 2nd ed. (1981). Ernest J.W. Barrington The Editors of the Encyclopdia Britannica The hormones of invertebrates Some form of endocrine regulation probably occurs in all invertebrates; in arthropods (as exemplified in insects and crustaceans) it attains a level of complexity similar to that of vertebrates. Hormones of insects Insects secrete hormones from neurosecretory cells and also from endocrine glands. Important neurosecretory centres occur in the pars intercerebralis region of the brain. The several cell types found in these centres indicate that more than one hormone is produced there. The hormones of plants Growth in plants is regulated by four categories of phytohormonesauxins, gibberellins, cytokinins, and inhibitors. Growth promoters Auxins Figure 6: The structures of plant hormones. The distribution of auxins, which promote the lengthwise growth of plants, is correlated with the distribution of the growth regions of the plant. The most important auxin, whose structure is represented in Figure 6, is b-indolylacetic acid (IAA), which is formed either from the amino acid tryptophan or from the breakdown of carbohydrates known as glycosides. The hormone affects plants by its action on chemical bonds of carbohydrates comprising plant cell walls. The process permits the cells to be irreversibly deformed and is accompanied by the entry of water and the synthesis of new cell-wall material. Many animal hormones may exert their effects by influencing protein synthesis, and evidence suggests that auxins may act in a similar way. Many other naturally occurring and synthetic compounds called auxins also have growth-promoting properties, but they are not always as active as IAA. Some of these compounds, however, resist the enzymatic destruction that is the normal fate of IAA within the plant; this feature is of great value in research and in horticulture, because auxin action can be prolonged. Other auxin-like compounds are used as selective weed killers (e.g., to disturb the leaf growth of dicotyledonous plants either in fields containing monocotyledonous cereal crops or on lawns) and as agents that remove leaves from dicotyledonous plants (defoliating agents). The hormonal characteristics of IAA are readily demonstrated in grass seedlings, in which the hormone is synthesized at the tip of the coleoptile (the protective sheath of the emerging plumule, or embryonic bud) and passes downward to its point of action in the growing region, where it evokes elongation of the coleoptile cells; growth stops if the tip is removed. The movement of the hormone downward from the tip of the coleoptile depends upon an interaction between the hormone and the cells through which the movement normally takes place. In addition to promoting normal growth in plant length, auxins influence the growth of stems toward the light (phototropism) and against the force of gravity (geotropism). The phototropic response occurs because greater quantities of auxin are distributed to the side away from the light than to the side toward it; the geotropic response occurs because more auxin accumulates along the lower side of the coleoptile than along the upper side. The downward growth of roots is also associated with a greater quantity of auxin in their lower halves. This effect, which is the opposite to that found in coleoptiles, is attributed to an inhibitory action of auxins on root growth, but this aspect of auxin action is not yet fully understood. Auxins have actions other than those associated with promoting growth; e.g., they play a role in cell division, in cell differentiation, in fruit development, in the formation of roots from cuttings, and in leaf fall (abscission). In experimental conditions, auxins tend to inhibit the progress of plant aging, perhaps because of their stimulating effect upon protein synthesis. The hormones of vertebrates Hormones of the pituitary gland Figure 2: Elements in a generalized mammalian pituitary gland. The pituitary gland, or hypophysis (Figure 2), which dominates the vertebrate endocrine system, is formed of two distinct components. One is the neurohypophysis, which forms as a downgrowth of the floor of the brain and gives rise to the median eminence and the neural lobe; these structures are neurohemal organs. The other is the adenohypophysis, which develops as an upgrowth from the buccal cavity (mouth region) and usually includes two glandular portions, the pars distalis and the pars intermedia, which secrete a number of hormones. The hormones secreted by the adenohypophysis are protein or polypeptide in nature and vary in complexity; as a result, their chemical constitution has not always been as fully characterized as has that of structurally simpler molecules of some other endocrine secretions. Functional analysis of these hormones also is difficult, for the targets of certain hormones of the adenohypophysis, called tropic, or trophic, hormones, are other endocrine glands. The action of such tropic hormones can be understood only in the light of the mode of function of the endocrine glands they regulate. Adenohypophysis Growth hormone (somatotropin; STH) Growth hormone is a protein, the primary structure of which has been fully established for the human and bovine forms of the hormone. It is probably universally distributed in gnathostomes (vertebrates with jaws), in which it is essential for the maintenance of growth, but its presence in agnathans (jawless vertebrates) has not yet been established with certainty. The physical and chemical properties of growth hormone ( Table 1), which differ from species to species, are associated with marked differences in biological activity. Only part of the molecule, however, is actually responsible for its biological activity, for up to 25 percent of it can be lost without causing any decline in potency. Man responds to growth hormones obtained from other primates, but the rat responds to those from a wide range of species. Even more striking, growth of teleost (bony) fishes, which stops if the pituitary gland is removed, can be restarted by treatment with mammalian growth hormone; on the other hand, preparations of pituitary glands from these fishes have no effect on the growth of mammals. The growth hormones of lungfishes, which are closely related to the terrestrial vertebrates, and of sturgeons, which are primitive members of the evolutionary line that led to bony fishes, affect mammalian growth, perhaps because these hormones have a more generalized molecular structure. Growth is such a complex process that definition of the growth hormone's mode of action is difficult. One of its known effects is an increase in the rate of protein synthesis, which is to be expected, since growth involves the deposition of new protein material. In addition, growth hormone affects the metabolism of certain ions (including sodium, potassium, and calcium), promotes the release of fats from fat stores, and influences carbohydrate metabolism in ways that tend to cause an increase in the level of glucose in the bloodstream. The last action creates a demand for an increased output of insulin (a hormone secreted by the pancreas), which acts to return the blood-glucose level to normal. Prolonged treatment of dogs with growth hormone can overstrain the pancreatic tissue in which insulin is synthesized and bring about a diabetic condition, in which insulin is formed in inadequate quantities. It is unlikely, however, that this is a factor in establishing diabetes mellitus in man. Excess secretion of growth hormone does, however, have damaging effects in man, for it produces overgrowth of the skeleton. If this occurs in youth, before the closure of the epiphyses (ends) of the long bones, it results in gigantism. If it occurs afterward, it causes acromegaly, in which the disturbance is more serious, with enlargement of the bones and soft tissues, and consequent distortion of the skull.