ENDOCRINE SYSTEM

any of the systems found in animals for the production of hormones, substances that regulate the functioning of the organism. Such a system may range, at its simplest, from the neurosecretory, involving one or more centres in the nervous system, to the complex array of glands found in the human endocrine system. Comparative endocrinologists investigate the evolution of endocrine systems and the role of these systems in animals' adaptation to their environments and their production of offspring. Studies of nonmammalian animalshave provided information that has furthered research in mammalian endocrinology, including that of humans. For example, the actions of a pituitary hormone, prolactin, on the control of body water and salt content were first discovered in fishes and later led to the demonstration of similar mechanisms in mammals. The mediating role of local ovarian secretions (paracrine function) in the maturation of oocytes (eggs) was discovered in starfishes and only later extended to vertebrates. The important role of thyroid hormones during embryonic development was first studied thoroughly in tadpoles during the early 1900s. In addition, the isolation and purification of many mammalian hormones was made possible in large part by using other vertebrates as bioassay systems; that is, primitive animals have served as relatively simple, sensitive indicators of the amount of hormone activity in extracts prepared from mammalian endocrine glands. Finally, some vertebrate and invertebrate animals have provided model systems for research that have yielded valuable information on the nature of hormone receptors and the mechanisms of hormone action. For example, one of the most intensively studied systems for understanding hormone actions on target tissues has been the receptors for progesterone and estrogens (hormones secreted by the gonads) from the oviducts of chickens. An understanding of how the endocrine system is regulated in nonmammals also provides essential information for regulating natural populations or captive animals. Artificial control of salmon reproduction has had important implications for the salmon industry as a whole. Some successful attempts at reducing pest insect species have been based on the knowledge of pheromones. Understanding the endocrinology of a rare species may permit it to be bred successfully in captivity and thus prevent it from becoming extinct. Future research may even lead to the reintroduction of some endangered species into natural habitats. group of ductless glands that regulate body processes by their secretion of chemical substances called hormones, which are carried to specific target organs and tissues by the bloodstream. Hormones are necessary for normal growth and development, for reproduction, and for homeostasis (a constant internal environment). They stimulate or inhibit various biochemical processes by combining with specific receptors on the membranes of target organs. Thus, although a hormone circulates in the bloodstream, it does not affect every cell with which it comes in contact but only those cells that contain a specific receptor site. The major endocrine glands in humans are the hypothalamus, the pituitary, the thyroid, the islets of Langerhans in the pancreas, the adrenals, the parathyroids, the ovaries, and the testes. Almost every other organ or tissue of the bodyincluding the intestinal tract, the stomach, and the hearthas been found to be involved in endocrine secretions. In the muscle cells of vertebrate hearts, for example, there are bodies that release a hormone that affects the regulation of blood pressure, blood volume, and the salt-and-water balance. Tumours may be sites of hormonal secretions. Hormonal secretions are generally regulated by negative-feedback loops. In the simple loops, the concentration of another hormone or a metabolite (such as calcium) influences sensitive regulators in an endocrine gland to inhibit or stimulate hormonal secretions in the target organ. The complex loops involve a mechanism called the hypothalamo-pituitary-target organ axis, in which the hypothalamus secretes releasing hormones that stimulate the pituitary to secrete a target hormone, which then enters the circulation and binds with the receptors of the target organ. Because the hypothalamus is an integral part of the brain and central nervous system, and because it is involved in negative-feedback loops in the endocrine system, the hypothalamus forges a critical link between the body's two control systemsthe nervous system and the endocrine system. In addition, stimulation by the autonomic (involuntary) nervous system partly regulates the adrenal medulla and pancreas, and the two systems are again linked. The interrelationship of the two systems provides an opportunity for the body to respond to changes in the internal and external environments. Diseases of the endocrine system result from too much or too little hormone secretion or from the inability of the body to utilize a hormone effectively. Growth hormone, for example, is an anterior pituitary secretion that is necessary for normal growth in children. An excess of this hormone during childhood can lead to gigantism; a deficiency, on the other hand, can result in dwarfism. One of the most commonand seriousendocrine diseases is diabetes mellitus. In its severest form, which usually begins during childhood or adolescence, diabetes results from insufficient production of the pancreatic hormone insulin. A milder form of the disease, which usually strikes middle-aged adults who are overweight and sedentary, results from an inability to utilize insulin effectively. See also hormone. Additional reading General works A comprehensive historical and biographical survey is provided by Victor Cornelius Medvei, A History of Endocrinology (1982). Comprehensive standard texts include Leslie J. DeGroot et al. (eds.), Endocrinology, 3 vol. (1979); and Francis S. Greenspan and Peter H. Forsham (eds.), Basic & Clinical Endocrinology, 2nd ed. (1986). For modern research in the field, see Recent Progress in Hormone Research: Proceedings of the Laurentian Hormone Conference (irregular); and Current Therapy in Endocrinology and Metabolism (biennial). Peter H. Wise, Endocrinology (1986), is a useful atlas.Briefer coverage is provided in Jay Tepperman and Helen M. Tepperman, Metabolic and Endocrine Physiology: An Introductory Text, 5th ed. (1987); Robert Volp (ed.), Autoimmunity and Endocrine Disease (1985); C. Donnell Turner and Joseph T. Bagnara, General Endocrinology, 6th ed. (1976); C.R. Kannan, Essential Endocrinology: A Primer for Nonspecialists (1986); Brian K. Follett, Susumu Ishii, and Asha Chandola (eds.), The Endocrine System and the Environment (1985); and T.S. Danowski, Outline of Endocrine Gland Syndromes, 3rd ed. (1976). A survey of medical literature can be found in The Year Book of Endocrinology. Comparative endocrinology David O. Norris, Vertebrate Endocrinology, 2nd ed. (1985); Ari Van Tienhoven, Reproductive Physiology of Vertebrates, 2nd ed. (1983); Kenneth C. Highnam and Leonard Hill, The Comparative Endocrinology of the Invertebrates, 2nd ed. (1977); Geoffrey W. Bennett and Saffron A. Whitehead, Mammalian Neuroendocrinology (1983); E.J.W. Barrington and C. Barker Jrgensen (eds.), Perspectives in Endocrinology: Hormones in the Lives of Lower Vertebrates (1968); and Aubrey Gorbman et al., Comparative Endocrinology (1983). Theodore B. Schwartz