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| Theories of Ageing |
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| Many theories have been proposed to explain the process of ageing. The important ones have been listed. |
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| The theories of Ageing have been grouped into two categories. |
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| 1) Programmed theories |
| This theory brings in the concept of internal biological clock to explain the process of ageing from childhood. |
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| This theory has three sub-categories: |
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 Endocrine Theory |
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 Programmed Senescence Theory |
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 Immunological Theory |
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| 2) Damage or Error theories |
| This theory blames the external or environmental forces that gradually damages the internal cells and organs leading to ageing. |
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| This theory includes |
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 Living theory |
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 Free radius theory |
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 Cross linking theory |
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 Wear and tear theory |
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 Theory of error catastrophe |
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 Somatic mutation theory |
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| 1. Endocrine theory |
| According to this theory, biological clocks act through hormones, which are the secretions of the endocrine glands, to control the pace of ageing. |
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| It has been observed that the production of some of the hormones declines with age. |
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 Human growth hormone (GH) levels decrease with age. |
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 Sex hormones like oestrogen and testosterone levels also fall off. |
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| Decreased levels of estrogen lead to bone thinning leading to disability in women. |
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 Hormones like melatoxin and thyroxin also decline with age. |
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| 2. Programmed senescence theory |
| According to this theory, ageing is the result of the sequential switching on and off of certain given, with senescence. |
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| T- lymphocytes or B - lymphocytes have the receptors for self-antigens by which they undergo programmed cell death or apoptosis, thus reducing their number. |
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| 3. Immunological theory |
| According to this theory, programmed decline in the functioning of the immune system leads to increased vulnerability to infectious diseases thus causing ageing and death. |
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| 4. Living theory |
| According to this theory, ageing is the by-product of metabolism. Therefore greater is the organisms rate of metabolism, shorter is its life span and vice versa. |
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| 5. Free Radicals theory |
| According to this theory, accumulated damage caused by oxygen radicals causes cells to stop functioning and eventually organs also stop functioning. |
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| An oxygen free radical is a by-product of normal metabolism produced when cells turn food and oxygen into energy. This free radical, in need of a mate, takes an electron from another molecule, which in turn becomes unstable. This chain reaction produces a series of compounds, some of which are harmful. They damage proteins, membranes and nucleic acids, particularly DNA, organelles, etc. All these damages within the body caused by oxygen free radicals causes ageing. |
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| 6. Cross linking theory |
| According to this theory, the accumulation of cross linked proteins damages cells and tissues and slows down bodily processes resulting in ageing. |
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| In a process called non-enzymatic glycosylation, glucose molecules attach themselves to proteins resulting in a chain of chemical reactions. This causes protein cross-linking, thus altering their biological and structural roles. Cross links (known as advanced glycosylation and products AGEs) toughens the tissues and may result in stiffening of connective tissue, hardened arteries, clouded ages, loss of nerve function and less efficient kidneys. |
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| 7. Wear and tear theory |
| According to this theory, cells and tissues have vital parts that wear out resulting in ageing. |
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| DNA undergoes continual damage throughout life, due to various reasons. DNA damage leads to malfunctioning of genes, proteins, cells and deterioration of tissues and organs. |
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| An animals ability to repair certain types of DNA damage is directly related to the life span of its species. |
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| 8. Theory of error catastrophe |
| According to this theory, damage to mechanisms that synthesise proteins, results in faulty proteins, which accumulate to a level that causes catastrophic damage to cells, tissues and organs. |
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| 9. Somatic mutation theory |
| According to this theory genetic mutations occur and accumulate with increasing age, causing cells to deteriorate and malfunction. |
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| Ageing and death |
| Biological ageing affects everybody. It is a deleterious process, which affects the functioning of the cells, tissues, organs and finally the organism itself. |
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| Like ageing, death is also a biological event. It occurs due to breakdown in body functioning. Causes of death in humans are many. A few of them are: |
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| i) Tissues of vital organs become weakened due to ageing disrupting various physiological processes causing death. |
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| ii) Malfunctioning of the bodys immune system reduces the resistance of the body to different antigens. As a result the individual is more prone to diseases, which may ultimately cause his death. |
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