Revised: August 24, 2011;
Glossary for Terms in Gerontology
Aging -- A gradual but relentless process by which sexually-reproducing organisms lose their youthful capacity for homeostasis. Aging is a physical process that doesn't normally reveal itself until after the completion of a species-specific interval of reproductive competence during which adults rear their progeny from childhood to independence (See Life History). As a result of aging, older organisms become increasingly vulnerable to a variety of age-related diseases and conditions, culminating in death. The tradeoff between aging and repair processes is extremely complex and observed to operate systematically within a hierarchy of at least seven different interacting levels: (1) molecules; (2) organelles (small membrane-bound cellular components with specialized functions); (3) cells; (4) tissues with various histological architectures; (5) organs; (6) organ systems; and ultimately (7) the entire organism itself. Intrinsic aging or senescence occurs silently from within starting at the molecular level, in the same sense that termites, if unchecked, will, sooner or later, destroy the structural integrity of even the largest wooden house. Extrinsic aging corresponds to external trauma or predation, in the same sense that a lightning bold from above can start a fire that burns your house down. Aging is not normally observed in wild populations, but typically manifests itself in zoos, as virtually all post-reproductive feral creatures are removed from the population by predators once they lose their agility. Aging is a physical process that affects inanimate objects as well as living creatures (in the same way that the Egyptian pyramids are subject to wind and rain erosion). The related concept of Longevity determination, however, is the result of a species-specific genomic expression during early development that positions the somatic tissues of an organism to survive long after its reproductive period has been completed.
Apoptosis -- Programmed Cell Death (PCD). This process gets rid of unneeded cells and is particularly important for “sculpting” tissue and organ structure during development of the embryo (or larval metamorphosis in insects), but may occur at any time even in adult cells when a tissue needs to be remodeled. Signals to trigger apoptosis may come from within the cell or from outside, by stimulating suicide receptors in the cell’s external membrane. Internal signals producing apoptosis depend on interactions of several proteins and may serve to protect the organism from cancer by killing cells that have pre-cancerous changes.
Average Life Expectancy -- The age at which 50 percent of the members of a population have died, when plotted on a standard survival curve. This statistic is normally calculated from birth, but may be recomputed in terms of expected years remaining at any age.
Bases – These are molecules with one or two nitrogen containing ring structures. The biologically important bases are the purines Adenine and Guanine and the pyrimidines Cytidine, Thymine, and Uracil. DNA and RNA are composed of linked sequences of nucleotides. In DNA, the purine nucleotides are Adenosine (A) and Guanosine (G); the pyrimidine nucleotides are Thymidine (T) and Cytosine (C). In RNA, the pyrimidine nucleotide Uridine (U) is substituted for Cytosine.
Biomarker -- A measurable parameter of physiological age that is a more useful predictor of remaining life expectancy than chronological age. The ability to measure biomarkers is extremely important in evaluating the efficacy of any potential life-extending intervention.
Blastocyst – A preimplantation embryo that contains a fluid-filled cavity called a blastocoel.
Caloric Restriction (CR) – A diet in which calorie intake is reduced, compared with ad libitum (eat as much as you like) diets, without any reduction in nutritional requirements (protein, water, vitamins, or minerals). CR is not the same as starvation or famine. CR is the only known intervention that systematically extends maximum lifespan. CR has been effective in all species in which it has been tried (although the jury is still out on humans).
Cancer -- A clonal growth (cells all descended from one ancestral cell) that undergo continuing mitotic divisions and are not inhibited in their growth when they come in contact with neighboring cells (contact inhibition). Thus, cancers obliterate the normal architecture of the host tissue. Cancer cells often spread (or metastasize) throughout the body by way of the blood stream or lymphatic vessels to form tumors in new locations beyond the primary site of origin. Cells become cancerous by accumulating, stepwise, a series of several mutations that alter the function of genes important for cell growth.
Chromosome – The structures in the nucleus of the cell, consisting of DNA bound to histones and other proteins. The genes are made of DNA (although the majority of the DNA sequence is not part of any gene). Genes are arranged along the chromosomes in a continuous sequence. Chromosome protein structure allows for selective activation (genes are transcribed into protein) or silencing (genes are not expressed), and thus for differential expression of the genome in different cell types and expression of genes in appropriate sequences during development of the organism or under various metabolic conditions. Chromosomes exist in pairs, one inherited from the mother (egg) and the other from the father ( sperm). Thus, normal somatic cells carry two, usually slightly different, versions of each gene (alleles) and are called diploid, while the germ-line cells are called haploid.
Cleavage – The mitotic divisions of the early embryo that occur in the absence of growth to divide the embryo in to many smaller nucleated cells.
Cloning -- The use of the chromosomes from an adult cell to create an identical twin (copy) of an organism by inserting the adult nucleus into an egg from which the nucleus has been removed, stimulating embryogenesis, and implanting the embryo into the uterus of a surrogate mother. Reproductive cloning of sheep, mice, goats, cows, pigs, and mules have been widely accomplished. However, attempts at cloning of rats, dogs, and horses have not yet met with success. Laws banning human reproductive cloning have been proposed in many countries, including the US. Therapeutic cloning (without the intent to implant the embryo into a surrogate mother) but with the aim of creating a large collection of embryonic (totipotent/pluripotent) stem cells to treat the original donor is a potentially a significant medical intervention for the future.
Diploid Cell -- A cell with pairs of homologous chromosomes.
DNA -- An abbreviation for Desoxy Ribonucleic Acid. Double stranded DNA molecules consist of antiparallel (running in opposite directions) chains of nucleotides in which the sugar component is desoxyribose. The chains are arranged in a double helix with the two chains wrapped around each other and bound together so that each "A" is paired with a "T" (A:T pair) and each "G" is paired with a "C" (G:C pair). Thus, when the chains unwind and separate, new identical antiparallel sequences can be copied along their lengths. Thus, DNA is self-replicating.
Compression of Morbidity Hypothesis -- Prof. James F. Fries, M.D., a rheumatologist at the Stanford University School of Medicine, established the “compression of morbidity” hypothesis in 1980. He suggested that if the onset of disability due to age-related diseases and conditions could be postponed to a greater degree than average life expectancy would increase, then total lifetime disability could be compressed into a shorter average period and the cumulative average lifetime disability would also be reduced. Although it was quite controversial when it was first proposed, this hypothesis has now proven to be valid over the last 20 years.
Disposable Soma -- From an evolutionary point of view, the Prime Directive of any organism is to transform available energy from the environment into the maximum number of progeny. Part of the energy is consumed in the maintenance of the organism's somatic (body) tissues (for growth and repair of injury) and part is used to propagate the germ-line tissues. Natural selection favors genetic combinations that produce the most efficient trade-off between these two forms of energy utilization in such a fashion as to maximize evolutionary fitness (i.e., reproductive performance). As a consequence, less energy is directed to somatic maintenance than would be required for the indefinite survival of any individual. This results in the death of individuals, but the immortality of the germ line. The details of the trade-off are largely a function of the ecological niche in which the organism propagates, including predators, prey, parasites, and other environmental factors. According to this theory, the least energy would be devoted to somatic maintenance in a hazardous niche; conversely, more energy would be dedicated to somatic renewal in a relatively protected niche.
Egg -- A female haploid germ cell.
Entropy -- A measure of the level of disorder or randomness in a closed system. It can be thought of either in the sense of thermodynamic/metabolic processes or the increasing molecular disorder in a structure. It can be thought of as the same process by which erosion occurs when soil is exposed to rain and wind.
Evidence-Based Medicine -- The practice of medicine with treatment recommendations that have their origin in objective tests of efficacy published in the scientific literature rather than anecdotal observations.
Fecundity -- The ability to produce offspring. High fecundity means the ability to produce progeny rapidly and in large numbers. In the demography of human populations, fecundity is the physiological ability to reproduce, as opposed to fertility.
Fertility -- Reproductive Potential. In demography, the number of births per year divided by the number of women of childbearing age, expressed as a rate.
Gene -- a functional unit of heredity. It is a segment of DNA located at a specific site on a chromosome whose length is typically several thousand base pairs long. A gene directs the formation of an enzyme or other protein by means of processes of transcription and translation. More precisely, a gene is a sequence of DNA that can be activated and copied into messenger RNA (or mRNA) by the process known as transcription. mRNA is then processed (spliced and reassembled), migrates through a pore in the nuclear membrane into the cell cytoplasm, and then is translated by ribosomes into a sequence of amino acids, which are joined together by peptide bonds to form a protein at cell organelles called endoplasmic reticulum (rough and smooth). Each triplet of bases (or codon) in mRNA specifies a different amino acid (out of 20 possible choices). It's this sequence of amino acids that determines the identity of a protein (its primary structure). The folding pattern (secondary [Alpha helix or Beta pleated sheets], tertiary, and quartinary structure) of the primary sequence – with the help of so-called chaperone proteins -- determines its three-dimensional morphology and ultimately its function (e.g., a structural vs. an enzymatic function).
Genotype -- The genetic makeup of a cell, organism or group of organisms, with respect to a single trait or group of traits; the sum total of genes transmitted from parents to their offspring.
Genome -- The complete collection of genes in the nucleus of each cell of our bodies. There are now known to be somewhat less than 25,000 genes in the human genome.
Geriatrics -- A branch of Internal Medicine concerned with the care and treatment of older persons and the treatment and amelioration of diseases of old age and frailty.
Germ Cell – Either an egg or a sperm cell.
Gerontology -- A branch of biology focusing on the common mechanisms of aging across all multicellular species. Gerontologists, for example, are keen to understand species that appear to exhibit very gradual or negligible senescence over a long time interval. In this context, gerontologists may study yeast, worms, fruit flies, mice, rock fish, tortoises, bats, parrots, humans, and other creatures exhibiting exceptional longevity.
Gerontome -- The subset of the genome whose genes affect longevity, either significantly reducing or increasing the average lifespan of an organism.
Gompertz Model -- A class of statistical models first proposed by the nineteenth-century British actuary Benjamin Gompertz, in which the hazard rate for death rises exponentially with increasing age of the organism (at least after an initial period of high risk of mortality at birth and infancy and a much lower risk in late childhood and adolescence). Today, the Wibel Model is superior to the Gompertz Model, as it more accurately explains observed demographic data.
Grandparenting Hypothesis – This is the supposition that abruptly terminating reproduction at a particular age (menopause) and prolonged survival of human females after menopause may have been selected for because of better success in child-rearing (and hence survival of the gene pool) when older women focus their resources on the welfare of their grandchildren and thereby increase their likelihood of survival of the tribe, rather than investing energy in producing more and more children of their own, and potentially compromising the reproductive success of their more mature offspring.
Haploid Cell -- A cell with half the normal compliment of chromosomes, typically a germ cell.
Hayflick Limit – The limit to the number of times a cell is can divide during serial cell culture. The value of this limit as a predictor of maximum lifespan of the organism is still unproven. In cultures of normal human fibroblasts, for example, the Hayflick Limit = 50 (+ 10) cell doublings. Cancer cells grown in culture, however, exhibit no such limit and continue to divide indefinitely. Normal cells grown in culture that have been instructed to manufacture telomerase (to relengthen their chromosomal telomeres after each division) can achieve replicative immortality and do not obey the Hayflick Limit, but they do not appear to manifest other pathological characteristics of a cancer cells (like loss of contact inhibition).
Homeostasis -- The physiological capacity of an organism to regulate itself by rapidly restoring internal conditions following a sudden perturbation in the external environment.
Inner Cell Mass – Cells that give rise to the embryo proper and that arise from the inner cells of an early preimplantation embryo.
Life History -- The combination of age-specific survival probabilities and fertilities characteristic of a species; the time table of individual development and aging for a representative organism (e.g., in humans, from fertilization [conception], to embryogenesis, implantation/placentation; organogenesis/fetogenesis, birth, infancy, adolescence, puberty, adulthood, menopause, loss of vitality, frailty/morbidity, and ultimately, death [mortality]).
Lifespan – The maximum lifespan of a species is the characteristic observed age of death for its very oldest individual(s) (e.g., for humans 122+ years). On the other hand, average lifespan is the age at which 50 percent of the members of a species or group has died. Over the last two centuries, average life expectancy has risen significantly, while maximum lifespan as demonstrated in the Table above has hardly changed at all.
Longevity Genes – Gerontic genes that extend or shorten the maximum lifespan of a species.
Multipotent Cell -- A stem cell that has limited capabilities for specialization, normally within a specific tissue type.
Mutation -- Any change in DNA structure which alters the established order of the bases. This may cause a gene (or series of genes) to fail to be activated normally (either to be silenced or, conversely, to be expressed inappropriately) or may cause a gene to express a protein with abnormal structure (and hence abnormal function). Most mutations that have any effect are deleterious. Rarely, mutations may produce some advantage for the organism carrying them. Alleles with advantageous mutations are selected for and tend to become more common in the species’ genome. The opposite is true for deleterious mutations. Some mutations move the cell carrying them toward a malignant phenotype ( cancer).
Necrosis -- Cell death secondary to traumatic injury. Necrosis invariably induces a subsequent inflammatory reaction, as distinguished from apoptosis which does not.
Nucleotides -- molecules which consist of a purine or pyrimidine base, a ribose or desoxyribose sugar, and a phosphate group.
Parthenogenesis – The development of an individual from an egg without fertilization.
Phenotype -- The external manifestations of gene expression whether at the level of the cell (e.g., muscle cells are long and thin and contain contractile fibrils; nerve cells have excitable membranes and communicating processes) or the organism (e.g., the giraffe has a long neck; a leopard, spots, and elephants, trunks).
Pluripotent Cell -- A cell capable of giving rise to most tissues of an organism.
Progeria -- A human disease or syndrome in which some characteristics of senescence are accelerated so that relatively young individuals appear prematurely aged. Examples include Hutchinson Guilford Syndrome (HGS is a rare autosomal-dominant disorder with a classic withered presentation leading to an early death in the teenage years. It is
now known to be caused by a de novo point mutation in the Lamin-A gene that causes a premature truncation in a protein that coats and organizes the interior surface of the nuclear envelope), Rothmund's Syndrome, Cockayne's Syndrome, Bloom's Syndrome, Marfan's Syndrome. Down's Syndrome, Huntington's Disease, and Werner's Syndrome (an enzymatic [helicase] defect in DNA repair).
Protein -- A linear sequence of Amino Acids whose three-dimensional shape determines a particular function in the body.
Proteome -- The collection of all proteins in the body of an organism. For humans, it is estimated that there are 250,000 - 300,000 proteins, of which fewer than half have been catalogued thus far. Only about 10,000 proteins have been fully characterized to date, but systematic efforts to identify and characterize them all are now underway and could be completed before the end of this decade. Understanding their functions and interactions could take much longer.
Reproductive Cloning -- The creation of an embryo using SCNT with the aim of creating a new (identical twin) individual of that species.
Reprogenic -- Techniques at the intersection of reproductive medicine and genetics for manipulating gametes and embryos.
RNA – Ribonucleic acid. RNA is a sequential chain of the nucleotides Adenosine, Guanosine, Thymidine, and Uridine. In RNA, the sugar molecules are ribose. RNA is typically single stranded. The sequence of most RNA molecules is copied from specific DNA sequences by enzymes in a process called transcription.
Senescence - - That portion of aging that begins at older ages, after one passes the point of minimum mortality, around age 12 for humans. More specifically, senescence is not a single disease. It is characterized as a generic, increased susceptibility to a myriad of age-related chronic diseases and a reduced ability to repair damage. These chronic diseases include heart disease, cancer, stroke, COPD, Diabetes, autoimmune diseases, and certain life-long infections.
Somatic Cell -- A diploid cell of the body; a cell other than a germ cell (an egg or a sperm).
Somatic Cell Nuclear Transfer (SCNT) -- The transfer of a cell nucleus from a somatic cell into an enucleated egg (one from which the nucleus has been removed).
Sperm -- A male haploid germ cell.
Stem Cell – An undifferentiated cell that possesses the ability to divide for indefinite periods in culture and may give rise to highly specialized cells of each tissue type. There are embryonic stem cells found in the blastocyst that are known to be totipotent, while adult stem cells found in bone marrow, for example, may only be pluripotent (not able to produce an entire new organism but are able to produce all three tissue types).
Survival Function -- The probability that an individual will remain alive at a particular age. The percentage of an experimental cohort that remains alive over the course of the experiment.
Telomere/Telomerase – Repetitive DNA sequences at the four ends of the chromosome, which can be lengthened by an RNA-containing enzyme called telomerase. For mammals, this repeat sequence is "TTAGGG." Telomeres allow the entire functional sequence of the chromosome to be copied during cell division by providing “spare DNA” at the ends. Because the normal DNA replication process cannot be initiated at the very end of a chromosome without telomerase, chromosomes would get systematically shorter each time they were copied. Chromosomes in germ cells are passed on for unlimited numbers of generations, so that germ cells require telomerase to avoid destruction of their chromosomes and extinction of the species. Most somatic cells do not have an active telomerase, so their telomeres shorten at each cell division. When telomeres become critically shortened, the cells either die by apoptosis or cease dividing (see Hayflick Limit). Cancer cells typically develop an active telomerase enzyme as an essential ingredient of the process of becoming malignant. Therefore, a drug that blocked telomerase might function as an important anticancer therapy with very few side effects if such a drug could be identified.
Therapeutic Cloning -- The creation of a several day-old embryo using SCNT with the aim of harvesting the cells for subsequent tissue-culture amplification and injection into a host for therapeutic purposes (presumably without fear of GVH [Graft vs. Host] Disease or immunological rejection).
Totipotent Cell -- A cell having an unlimited capability to create a new organism. A totipotent cell has the capacity to specialize into an embryo, extraembryonic membranes and tissues, and all postembryonic tissues and organs.
Translational Research -- Clinical investigation with human subjects (patients or normal volunteers) in which knowledge obtained from basic research with genes, cells, or animals is translated into diagnostic or therapeutic interventions that can be applied to the treatment or prevention of disease or frailty.
Trophobastic Cells – Cells that contribute to the placenta but not to the embryo itself and which are required for an embryo to implant into the uterine wall.
A few of our even more technical terms are now listed below:
Antagonistic Pleiotropy -- Multiple gene effects in an organism, such that alleles which improve fitness early in life have detrimental effects later in life.
Biological Warranty Period -- For an individual species, such terms as longevity, average life expectancy, maximum duration of life, or lifespan are generic. They measure statistically how long an individual member of a given species could live starting from birth under standard environmental conditions (demographics and epidemiology). They make no distinction between lives that are cut short by premature deaths and lives exhibiting a complete lifespan in which the organism’s genes are permitted to express their fullest potential without predation or other limitations of lifestyle adversity. However, these terms convey no sense of why that species’ lifespan has or should have these limits. These determinations can only be appreciated in the context of a broader marketplace of competing species in an ecological food chain (web), and especially including microorganisms and parasites present in their habitat.
In the context of product manufacturing, say of automobiles, a warranty period explicitly provides a limited time frame within which conspicuous failures would not be anticipated by the engineers who designed the vehicle under conditions of normal use, and the manufacturing company incurs little financial risk by enticing new-car buyers with a type of insurance policy that includes free repairs for the first 30,000 miles or three years, which ever comes first.
The adjective “biological” is inserted to restrict our attention to biological systems and implies that failures of biological origin are likely to occur whenever an individual’s survival extends beyond his or her warranty period (typically the termination of reproductive competence for females [menopause] or the equivalent age for males [andropause]). Afterward, the creature lives on “borrowed time,” as it were. If the net accumulation of damage to the basic molecular building blocks of life (DNA, proteins, carbohydrates, and fats) eventually exceeds the maintenance and repair capacity of the body, this renders the individual vulnerable to forces of morbidity (entropy or The Second Law of Thermodynamics). Rather than being a part of the intentional choices made by manufacturing engineers for cars or other mechanical devices (more expensive, well-made cars do last longer before they fail catastrophically) however, biological warranty periods are a stochastic byproduct of Darwinian neglect and genetic programs for a species’ life history (conception, embryogenesis, development, and sexual reproduction) having run out of new (or interesting) things to do. Humans may have their own personal agendas, but after the biological warranty period, our genetic prime directive to “go forth and multiply” has been exhausted. In other words, by definition, the termination of the biological warranty period coincides with the early onset of aging, and without a fundamental intervention in programming, vitality can only be expected to go down hill from that point. See Life History.
Diapause – A preprogrammed “time out” in the genome triggered by severe stress in the environment (famine, drought, heat, cold, etc.) The organism can then delay reproduction and tread water so to speak, waiting for things to improve (bacteria, C. elegans, and all mammals, even humans, that have Caloric Restriction mechanisms have diapause built into their genomes). Although diapause may extend lifespan, it doesn’t mean that the organism would chose to do so voluntarily or that it would be in any way desirable.
Ecdysone -- The juvenile insect hormone produced in caterpillars that induces molting and metamorphosis.
E-mortality -- Indefinite life expectancy for individuals of a sexually reproducing species without death secondary to cellular senescence. However, an emortal individual may still die secondary to environmental trauma or an accident. Etymology: This term was first coined by Alvin Silverstein, Ph.D., Professor of Biology at College of Staten Island/CUNY (2800 Victory Blvd.; Staten Island, NY 10314; E-mail: email@example.com or DrASilverstein@aol.com ) to distinguish it from the more usual term of immortality and thereby avoid all of its religious connotation. He states, "I was using it among colleagues and friends back in the 1950's, although it did not appear "in print" until my book, Conquest of Death: The Prospects for Emortality in Our Time (Macmillan, NY; 1979). [Editor's Note: Although this book is now "out-of-print," there are 193 other books authored by Dr. Silverstein that can be found on Amazon.com.]
Gerontic Gene -- a gene affecting longevity, either significantly reducing or increasing average or maximum lifespan of an organism.
Hormesis -- The stimulating effect of a subinhibitory concentration of any toxic substance on an organism. In other words, some members of a population, after a period of illness, may actually benefit from a "poison" that normally shortens the life span of other members of the group, as in an LD50 dose of a poison that is lethal to half the members of a population of mice but then strengthens those mice who manage to survive the injury.
Iteroparity -- The state in an individual organism of reproducing repeatedly or more than once in a lifetime.
Mortality Trajectory -- Plot of death rate against age group over time.
Nebenkern (German Etiology?) -- During spermatogenesis in the testicles, mitochondria aggregate in early postmeiotic spermatids and then fuse into two giant organelles that wrap around each other (like snakes lying on a flat surface, not helically coiled) toward the tail portion of the future sperm. Remember that the life-and-death of a sperm depends on the ATP needed to rotate the cork-screw tail for swimming faster than any competing sperm, including those from other potential male competitors while the female is still receptive. This wrap-around structure is called a Nebenkern and resembles an onion slice when imaged in cross-section under an electron microscope. In fruit flies ( Drosophila melanogaster) the gene called Fuzzy Onions (Fzo-1) is up-regulated just prior to Nebenkern formation late in Meiosis-II and then disappears when mitochondrial fusion is completed. Note that the metaphor with an onion is not really accurate topologically (as concentric spheres), since what we're looking at is two disjointed concentric spirals.
Neoteny– A prolongation of the larval stage in a sexually mature organism as in the Mexican tiger salamander or axolotyl, or in certain termite castes held in the larval stage as future replacements for the Queen (cf. pedogenesis).
Pedogenesis – A permanent larval stage with sexual development, as in certain gall midges.
Pleiotropy – The multiple phenotypic effects of a single gene.
Semelparity -- A life-history pattern that is characterized by a sharp burst of reproductive activity followed by rapid senescence, such as occurs in Pacific salmon.
A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Some plant scientists use the parallel terms monocarpy and polycarpy. In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life. In any iteroparous population there will be some individuals who die between their first and second reproductive episodes, but unless this is part of a Syndrome of Programmed Death after reproduction, this would not be called semelparity. This distinction is also related to the difference between annual and perennial plants. An annual is a plant that completes its life cycle in a single season, and is usually semelparous. Perennials live for more than one season and are usually (but not always) iteroparous.
Many, but not all, of these terms are derived from Kenneth W. Wachter and Caleb E. Finch, Eds., Between Zeus and the Salmon: The Biodemography of Longevity, pp. 269-274 (National Academy Press, Washington, D.C.; 1997).