Theories of Aging (Part 2) - The Programmed Theories of Aging
Programmed theories propose that the body's genetic code regulates cellular reproduction, aging, and death. Aging is based on an internal built-in genetic "clock" and follows a preset biological timetable. As such, growing old is not a result of a series of random events, but genes program age-related molecular and cellular changes.
Aging, according to programmed theories, represents a natural continuation of the cycle that regulates childhood growth and development. The internal genetic programs contain information for the regulation of cellular reproduction and death.
Somatic cells have a fixed number of times they divide in life. After this predetermined point has been reached, they are no longer able to replicate. This, in turn, triggers cell death (apoptosis). This theory postulates a built-in impairment in the ability of the cell to continue dividing, which is the result of sequential switching on and off of certain genes and shortening of the telomeres. "Growing old" (or senescence) is defined as the point in time when resultant age-associated functional deficits become manifest.
Proponents of this theory are interested in studying the human genome, to see if the "aging code" can be "cracked" to circumvent aging by interventions at the genetic level, such as application of the enzyme telomerase.
A major milestone in aging research was the discovery of a fixed lifespan for cells in laboratory culture. In 1961, Leonard Hayflick found that normal human cells in tissue culture go through a finite number of cell divisions and then cease. This maximum number of divisions is called the "Hayflick Limit." Hayflick found that fetal cells replicate around 100 times. But cells taken from a 70-yr old reach their limit after only 20 to 30 divisions.
Cells from older organisms divide proportionately fewer times than cells taken from younger organisms. Cells have the ability to remember this programming under harsh conditions, such as being frozen and later thawed. Furthermore, transplanted donor cells have limited survivability in a new host.
Cancerous cells were removed from the body of a terminally-ill young woman named Henrietta Lacks (HeLa), before she died in 1951. These cancerous "HeLa" cells were cultured in a glass lab dish, where they kept dividing and growing and survive to this day. A little bit of Henrietta Lacks achieved immortality, in a sense, in a laboratory dish. We see, by this tragic example, that cancer cells do not conform to the "Hayflick Limit." Similarly, embryonic stem cell lines are exempt from the "Hayflick Limit" and can renew themselves perpetually through cell division.
Neuroendocrine and neurochemical theories
A change in hormone secretions, through the hypothalamic-pituitary feedback system, controls the pace of aging. Changes in the endocrine glands, such as the adrenal and thyroid glands, ovaries, and testes, are accompanied by corresponding declines in functional capacity. An example of this is the thinning of bones in women after menopause.
Proponents of this theory use various natural and synthetic hormones, such as human growth hormone, DHEA, or estrogen replacement, to slow or reverse the aging process.
Some theorists believe certain hormones, such as insulin or cortisol, actually accelerate aging and the development of degenerative diseases. Others claim an imbalance of neurochemicals in the brain hinders healthy cell division throughout the body. Aging occurs because of the decline in neurons and associated hormones.
A programmed decline in the immune system leads to increased vulnerability to infectious disease, aging, and eventual death. The decreased immune function, especially in the thymus and bone marrow, is the basis for the lack of general well-being that leads to aging.
Declines in immune system function can negatively affect the outcomes of many illnesses such as pneumonia, urinary tract infections, postoperative infections, and cellulitis in the elderly adult. There is an age-associated decline in T-cell functioning, which is accompanied by a decrease in resistance.
The reduction in immunologic activities also leads to an increase in autoimmune dysfunction with age. Examples of autoimmune diseases are rheumatoid arthritis, lupus, and multiple sclerosis. A breakdown in the body's immunological memory system causes it to mistake normal cells as foreign and these are attacked by the person's own immune system.
Proponents of the immunological theory believe that a healthy diet and lifestyle coupled with preventive health measures, such as influenza and pneumonia vaccinations and limiting exposure to pathogens, can support immune function and help prevent aging and untimely death.
Eliopoulos, C. (2010). Gerontological nursing (7th ed.). Philadelphia: Lippincott Williams & Wilkins.
Moody, H. R. (2010). Aging: Concepts and controversies (6th ed.). Thousand Oaks, CA: Pine Forge Press.
Tabloski, P. A. (2006). Gerontological nursing. Upper Saddle River, NJ: Pearson Prentice Hall.Last edit by Joe V on Jan 9, '15