Reproductive-cell cycle theory
Encyclopedia
Rather than seeing aging as a loss of functionality as we get older, this theory defines aging as any change in an organism over time, as evidenced by the fact that if all chemical reactions in the body were stopped, no change, and thus no aging, would occur. Since the most important change in an organism through time is the chemical reactions that result in a single cell developing into a multicellular organism, whatever controls these chemical reactions that regulate cell growth, development, and death, is believed to control aging. The theory argues that these cellular changes are directed by reproductive hormones of the hypothalamic-pituitary-gonadal axis (HPG axis). Receptors for reproductive hormones (such as estrogens, progestagens, androgens and gonadotropins) have been found to be present in all tissues of the body. Thus, HPG axis hormones normally promote growth and development of the organism early in life in order to achieve reproduction. Hormones levels then begin to change in men around age 30 and more abruptly in women when they reach menopause
, around age 50. When the HPG axis becomes unbalanced, cellular growth and development is dysregulated, and cell death and dysfunction can occur, both of which can initiate senescence
, the accumulated damage to cells, tissues, and organs that occurs with the passage of time and that is associated with functional loss during aging.
Evidence supporting this theory comes from disease studies showing that women who reach menopause later have less heart disease
and stroke
, less dementia
, and less osteoporosis
, supporting the theory that the longer the HPG axis is in balance, the less likely one is to develop age-related diseases. Conversely, early surgical menopause has been demonstrated to increase the incidence of these diseases. However, the most compelling supportive evidence is from studies of Hormone Replacement Therapy
(HRT). Research with women and men undertaking HRT has shown that taking sex hormones that are biologically identical to human hormones delays the onset, decreases the incidence of, and can reverse the course of age related illnesses such as heart disease, Alzheimer’s disease, osteoporosis, and some types of cancer
. However, only biological hormones appear to have these effects. The use of non-human or synthetic hormones has been shown to increase the risk of certain of these diseases. Compellingly, 18 studies have demonstrated an increase in longevity for those women taking HRT.
Further studies in support of the theory have shown that suppressing the HPG axis, such as when organisms experience either caloric restriction, cold, or exercise stress, increases lifespan. This is thought to be an evolutionary conserved mechanism that allows organisms to suppress HPG axis signaling and reproduction, thereby conserving reproductive resources (germ cells) for a later time when the environment is better suited to raising offspring. By having the same hormones regulate both reproduction and aging, an animal is able to modulate its fertility
and its rate of aging based on environmental conditions .
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
, around age 50. When the HPG axis becomes unbalanced, cellular growth and development is dysregulated, and cell death and dysfunction can occur, both of which can initiate senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
, the accumulated damage to cells, tissues, and organs that occurs with the passage of time and that is associated with functional loss during aging.
Evidence supporting this theory comes from disease studies showing that women who reach menopause later have less heart disease
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
and stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
, less dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...
, and less osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
, supporting the theory that the longer the HPG axis is in balance, the less likely one is to develop age-related diseases. Conversely, early surgical menopause has been demonstrated to increase the incidence of these diseases. However, the most compelling supportive evidence is from studies of Hormone Replacement Therapy
Hormone replacement therapy (menopause)
Hormone replacement therapy is a system of medical treatment for surgically menopausal, perimenopausal and to a lesser extent postmenopausal women...
(HRT). Research with women and men undertaking HRT has shown that taking sex hormones that are biologically identical to human hormones delays the onset, decreases the incidence of, and can reverse the course of age related illnesses such as heart disease, Alzheimer’s disease, osteoporosis, and some types of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. However, only biological hormones appear to have these effects. The use of non-human or synthetic hormones has been shown to increase the risk of certain of these diseases. Compellingly, 18 studies have demonstrated an increase in longevity for those women taking HRT.
Further studies in support of the theory have shown that suppressing the HPG axis, such as when organisms experience either caloric restriction, cold, or exercise stress, increases lifespan. This is thought to be an evolutionary conserved mechanism that allows organisms to suppress HPG axis signaling and reproduction, thereby conserving reproductive resources (germ cells) for a later time when the environment is better suited to raising offspring. By having the same hormones regulate both reproduction and aging, an animal is able to modulate its fertility
Fertility
Fertility is the natural capability of producing offsprings. As a measure, "fertility rate" is the number of children born per couple, person or population. Fertility differs from fecundity, which is defined as the potential for reproduction...
and its rate of aging based on environmental conditions .
External links
- The Laboratory of Aging, Endocrinology and Disease http://agingresearch.wisc.edu/atwood.htm