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aging, in biology, cumulative changes in an organism, organ, tissue, or cell leading to a decrease in functional capacity. In humans, aging is associated with degenerative changes in the skin, bones, heart, blood vessels, lungs, nerves, and other organs and tissues. The branch of medicine that deals with the disorders of aging in humans is geriatrics.
Biologists have advanced a variety of theories to explain aging, but most of them agree that this process is largely determined by genes. This view is suggested by the great range of lifespans among different animal species—from a few days in the fruit fly to more than 100 years in some tortoises. Scientists have recently learned how to double the lifespans of such laboratory organisms as roundworms and fruit flies through genetic manipulation, and mutant genes in mice have been observed to have a comparable effect in postponing aging.
At the cellular level, an important recent finding has been that the lifespans of cells in the human body are determined by strings of DNA (genetic material) called telomeres, which are located at the ends of the chromosomes. Each time a cell divides, the telomere becomes shorter; the senescence and death of the cell is triggered when the telomere is reduced to a certain critical length. Telomerase, an enzyme that can intervene in this process, is being closely studied in relation to cancer as well as aging.
Environmental factors have been observed to affect aging as well. Scientists have discovered that they can significantly postpone aging in mice by providing them with very low-calorie diets, and recent studies of rhesus monkeys on low-calorie diets appear to be having the same results. It is believed that these diets slow the aging process by lowering the rate at which tissue-damaging substances called free radicals are produced in the body. One aim of these studies is the development of antioxidant drugs that could slow the aging process in humans by protecting against free radicals. The use of testosterone, melatonin, human growth hormone, and other hormones as “anti-aging” treatments is medically unproved and potentially dangerous, as the hormones can have damaging side effects.
See L. Hayflick, How and Why We Age (1994); J. Silvertown, The Long and the Short of It: The Science of Life Span and Aging (2013); publications of the National Institute on Aging.
the gradual changes that occur in the course of the development of an organism, which begin long before old age and result in the progressive weakening of an organism’s adaptive capacity. Aging is the final stage of ontogeny. It is studied in gerontology. The rate of aging determines the life-span of different animal species, which is genetically determined, and the relationship between the metabolic, structural, and functional manifestations arising in the various systems of an organism.
The development of ideas on the nature of aging is closely associated with the conflict between various philosophic schools over the origin of life, the evolution of the animal world, and the relationship between life and death in individual development. F. Engels defined the relationship between the categories of life and death as follows: “Life is always thought of in terms of its inevitable result, already fixed in the embryo—death.” Aging is an inherent part of individual development during which phenomena may arise that are similar to the early stages of ontogeny but have a different mechanism.
There is no generally accepted explanation of the mechanisms of aging. More than 300 hypotheses have been advanced, many of which are of purely historical interest. Most of the modern hypotheses can be divided into large groups. According to the first group, aging is a programmed process of quantitative and qualitative changes that naturally occur in the genetic apparatus, which is controlled by genes, as are all the other stages of an organism’s development. According to the second group, aging is a result of a disturbance or an injury to the genetic apparatus in the course of vital activities—an accumulation of “errors” caused by many factors, including cross linkages, free radicals, and cellular metabolic products. A compromise view also exists, which holds that initial genetically programmed changes create “vulnerable” places which undergo the injurious effects of metabolites accumulated in the course of vital activities.
Aging hypotheses greatly emphasize the exogenous and endogenous factors that contribute to the development of aging. Some hypotheses attempt to elucidate the sequence of changes that occur with aging. The American scientist H. Curtis, the British scientist C. Minot, and the Soviet scientist I. I. Shmal’gauzen have stated that postmitotic, highly differentiated cells are responsible for initial aging mechanisms. According to L. Hayflick, mitotically active cells have a limited potential for division, which also results in the initial aging of an organism.
The aging process has been studied primarily in vertebrates, including man. Some investigators believe that aging starts with the fertilization of a cell, after the cell’s first division (the Soviet scientist M. S. Mil’man) or after the cessation of growth (G. Bidder). Others believe that aging occurs during all age periods (the Soviet scientists A. V. Nagornyi, V. I. Nikitin, I. N. Bu-lankin) or develops in the climacteric period (I. V. Davydovskii). Cells differ in their capacity for division, in the length of their life-span, and in the period of the commencement of their aging. The extremely close interrelationships and interdependence between age-related changes at different levels of the organization of living organisms and different levels of vital processes determine both the onset of aging after an organism’s conception and the course of aging in the later stages of ontogeny.
Many years ago, S. P. Botkin and E. Metchnikoff substantiated the need to distinguish between physiological (natural) and premature (pathological) aging. In the opinion of many investigators, premature aging results from unfavorable environmental factors and from disease. Significant changes in the aging process develop at the cellular level and are manifested by decreased cellular excitability and lability, by increased duration of the action potential, and by changes in synaptic conduction. With aging, cells and tissues are less affected by nerves and become more sensitive to some humoral factors because of changes in the metabolism of mediators. Characteristically, the level of internal respiration drops because of a decrease in the number of active cellular elements and mitochondria contained in these elements and a decrease in the oxidative capacity of the mitochondria. The increase in the rate of glycolysis cannot compensate for the insufficient production of energy by oxidation-reduction processes, causing a decrease in the concentration and rate of renewal of high-energy compounds. The reactivity of active protein groups changes, thus causing an accumulation of inert protein molecules within the cells.
With aging, changes also occur in different systems, including the nervous system. Internal inhibition is affected sooner than other processes. The subordinating influence of the higher parts of the central nervous system on the lower parts weakens, the lability of nerve centers decreases and their sensitivity to some humoral factors increases, and the relationships between the centers and the periphery change. Neurodynamic shifts underlie the changes that occur in the mind and the behavior of the elderly, whose ability to work and concentrate declines and who are apt to become emotionally unstable. Arterial pressure often increases, the heart beat slows, and cardiac output decreases. Peripheral resistance increases, and vascular walls become less elastic. Pulmonary ventilation and the vital capacity of the lungs decrease. Age-related changes in hemodynamics and respiration become very apparent during strenuous activity, for example, physical work. The enzyme activity of digestive juices and the rate of absorption of fatty acids, amino acids, and glucose decrease, and gastrointestinal motility and the antitoxic function of the liver weaken.
According to the adaptation-regulation theory of V. V. Frol’-kis, aging is inherently contradictory: while some processes weaken, others strengthen as a result of the mobilization of important adaptive mechanisms. The mechanisms include an increase in the number of nuclei in many cells following a change in the activity of the genetic apparatus of each nucleus, an intensification of glycolysis at the same time that the rate of internal respiration decreases, the hypertrophy of some cells and atrophy of others, and an increase in tissue sensitivity to many hormones following a weakening of endocrine function.
Changes in the neurohumoral regulation of functions and vascular permeability play a major role in the aging of a complex organism and in the development of the organism’s adaptive mechanisms (I. P. Pavlov, A. A. Bogomolets, N. B. Man’kovskii). With aging, metabolic and functional parameters do not change smoothly, gradually, and in the same direction. Some parameters, for example, myocardial contractility, the functioning of the digestive, thyroid, and sexual glands, and visual and acoustic acuity, progressively deteriorate with age, whereas other parameters, including blood-sugar levels, cellular membrane potentials, and some parameters of blood morphology, do not change significantly. There is an intensification of still other parameters, including the synthesis of some hypophyseal hormones, the sensitivity of many cells to humoral factors, and the activity of some enzymes.
The irregularity of aging is related to the fact that changes in organs and tissue develop differently in different age periods (especially in the climacteric period). For example, the thymus is active in childhood, the sexual glands of women become less active by the age 50, and some hypophyseal functions are preserved even in advanced old age. Age-related changes steadily intensify, limiting the adaptability of the body and promoting the development of many diseases, which become more frequent with old age. The proper balance of work and rest and a nutritious, well-balanced diet are important factors in delaying the effects of aging.
REFERENCESMarx, K., and F. Engels. Soch., 2nd ed., vol. 20, p. 610.
Nagornyi, A. V., V. N. Nikitin, and I. N. Bulankin. Problema stareniia i dolgoletiia. Moscow, 1963.
Davydovskii, I. V. Gerontologiia. Moscow, 1966.
Berdyshev, G. D. Ekologo-geneticheskie faktory stareniia i dolgoletiia. Leningrad, 1968.
Dil’man, V. M. Starenie, klimaks i rak. Leningrad, 1968.
Frol’kis, V. V. Regulirovanie, prisposoblenie i starenie. Leningrad, 1970.
Frol’kis, V. V. Starenie i biologicheskie vozmozhnosti organizma. Moscow, 1975.
Man’kovskii, N. B., and A. Ia. Mints. Starenie i nervnaia sistema. Kiev, 1972.
Unlike aging in animals and man, aging in plants is accompanied by the formation of new organs, a process that frequently continues throughout the life of a plant. In some plants, all organs age and die simultaneously. For example, the agave dies after flowering. In others, for example, trees in the Central Zone, there is a cyclical, seasonal dying off of leaves at the same time that other organs remain viable. The aging of the lower leaves of many herbaceous plants is accompanied by the formation of young apical leaves.
Many practical methods used in agriculture, including the breeding of early-ripening varieties according to morphological characteristics and the rejuvenation of fruit trees and shrubs by deep pruning, are based on N. P. Krenke’s theory of cyclical aging and rejuvenation.
REFERENCESKrenke, N. P. Teoriia tsiklicheskogo stareniia i omolozheniia rastenii. . . . Moscow, 1940.
Biologiia razvitiia rastenii. Moscow, 1975.
N. L. KLIACHKO