Dr. Pignolo is Assistant Professor and Director, Ralston-Penn Clinic for Osteoporosis & Related Bone Disorders, Department of Medicine, Division of Geriatric Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA.
Within the past 12 months, Dr. Pignolo reports no commercial conflicts of interest.
Albert Einstein College of Medicine, CCME staff and interMDnet staff have nothing to disclose.
Release Date: 07/12/2010
Termination Date: 07/12/2013
Estimated time to complete: 1 hour(s).
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- Discuss how rates of senescence and primary aging processes influence life span
- Define the differences between average life span, median length of life, maximum life span and life expectancy
- Evaluate the key factors associated with exceptional human longevity.
Aging refers to any time-related process that occurs during the life of an organism including those that promote beneficial, neutral and deteriorative consequences. Although often used interchangeably with the term aging, senescence specifically refers to those deteriorative changes that occur with time during postmaturational life that increase vulnerability to disease and decrease the likelihood of survival.
Rates of Senescence and Primary Aging Processes
There is no strong evidence that prokaryotes (e.g., bacteria) undergo senescence. Populations of single-celled eukaryotic organisms, such as budding yeast, are also immortal, but individual cells within these populations have a limited life span as measured by the number of bud generations or cell divisions. In multicellular organisms, senescence is thought to occur in those species where the germ cell line is separate from the somatic (body) lines, in those displaying a distinction between parent and smaller offspring or, more precisely, in those whose cells undergo somatic cell differentiation (by which cells take on a specialized function during growth and development).
Senescence can be described among different species as rapid, gradual or negligible. Rapid senescence occurs suddenly with deteriorative changes soon after maturation in nematodes, flies and in other short-lived invertebrates, or soon after reproduction in species such as annual plants and Pacific salmon. Negligible senescence is exhibited by long-lived species such as clams, trees, fish and reptiles, in which there is little evidence for postmaturational increases in mortality rates. Gradual senescence progresses persistently but slowly after maturation in all placental mammals including humans. However, measurements necessary to corroborate the occurrence of senescence are complicated and are often confounded by predation, infection, other environmental factors that predispose individuals to very high accidental death rates and relatively long life spans in some species. Despite these limitations, senescence is thought to occur largely as the result of primary aging processes, that is, aging in the relative absence of disease or injury.(1)
Measuring Life Span
Mean or average life span is distinguished from median length of life, which is the age at which there are as many individuals with shorter life spans as there are individuals with longer life spans. Life expectancy is the expectation of a certain mean length of life at birth, or at any age, calculated from the current mortality conditions in a population. For example, the life expectancy for a hypothetical group of 70-year-olds would be based on the currently observed age-specific death rates among individuals age 70, 71 and so on, up to the greatest age attained in the prevailing population. Maximum life span is the age of the longest-lived survivors of a cohort or population; for humans, it is operationally considered to be the oldest age reached by one in 100 million people.
Protection from premature death, rather than changes in aging processes, underlies the survival increases reflected by median life span and life expectancy (Figure 1). Reduction in the high rate of infant deaths by improvements in sanitation, nutrition and immunization account for much of the protection from premature death by environmental hazards and infectious diseases seen early in the last century.(1)
Figure 1. Increase in Average Life Span.
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Increase in average life span (black arrows), but not maximum life span, is the result of protection from premature death.
Median life span and life expectancy are thus influenced by many factors and are thought not to reflect primary aging processes. However, in 2008, life expectancy at birth exceeded 80 years in 11 countries, with twentieth century life expectancy doubling in some developed countries.(2) Although rising life expectancy at birth is not universal, the current highest recorded average life expectancy is for Japan where it continues to rise.(3) In fact, female life expectancy in Japan has risen for the last 160 years at a steady pace of almost 3 m/yr. Figure 2 illustrates the average life expectancy in selected developed countries.
Figure 2. Life Expectancy At Birth (2008).
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Life expectancy at birth in 2008 in five developed countries. Source: U.S. Census Bureau, International Database, accessed on May 27, 2008.
Despite some challenges to the contrary, maximum life span has been considered an index of the rate of aging of a population and the maximum life span of a species tends to be inversely related to its rate of aging. For example, rats, with a maximum life span of about five years, are thought to age more rapidly than dogs, with a maximum life span of about 20 years. Any factor that increases the maximum life span of a species is considered to have influenced primary aging processes.