- Build up of age pigments (lipofuscin). The presence of these fat/protein granules (found in nerve and muscle cells) is a result of the loss of the ability to normalize cross-linkage of such molecules following free radical activity.
- Enzymes which have changed in their sensitivity to heat and their functional ability to act as catalysts.
- Enzymes which behave poorly in their defensive roles as part of immune function.
- Plaque and tangles of tissue found in aged brain tissue (e.g. in Alzheimer's disease)
As well as these accumulating deposits, there seems with ageing to be a tendency for both the quality and rate of protein synthesis to become increasingly disturbed. Whether these alterations are the result of a gradual loss of efficiency in dealing with the hazards of life, or whether they are the result of a built-in (genetically encoded and therefore programmed) decline feature, remains a main question for research. The results of such research will probably show that they are both operating and that it is the hazards of life which we can most easily influence, although doubtless genetic engineering will eventually allow for tinkering with the coded DNA messages which set the current human limit of life at around 120 years.
The genetic argument
Some scientists believe that as the processes (listed above) get underway - whether due to free radical activity, enzyme inefficiency or anything else - a genetic process of ageing is triggered. It is as though there are encoded messages in our DNA just waiting for this stimulus in order to start acting, and of course the time of life at which this begins will relate directly to how efficient the individual's body is in dealing with free radicals and with producing active enzymes, and generally coping with the vicissitudes of life.
In simplistic terms, then, this argument says that the healthier you are, the longer it will be before your genetic ageing factors, are triggered, and therefore the longer you will live. It is a clear example of survival of the fittest, with those of us with the most problems (whether genetically inherited or acquired through inadequate lifestyle and nutritional imbalances) dying younger than those with 'good' genes for life expectancy and more health promoting lifestyles and diets. It seems highly probable that those animals which live longer on dietary restriction programmes are not exceeding their allotted genetic life spans. Rather it is those who fail to live as long as these dietary restriction animals which are over-eating themselves into early old-age.
In human experience there is evidence for a genetic ageing gene, and this is seen in the condition known as Werner's syndrome. This occurs in about one in a million children and causes rapid ageing, so that by the early advanced ageing is seen, and few live to reach 40 years of age. All the diseases associated with age, degenerative and malignant conditions are common in Werner's syndrome. When the cells of these people are examined they are found to continue to divide in laboratory dishes no more than between 10 and 20 times before dying, unlike normal human cells which continue dividing up to 50 times. Identifying which genes are involved in this condition is one quest of those scientists who see life expectancy enhancement to come from manipulation of inherited characteristics.
The search for evidence of a genetic control of ageing has led to experiments involving insects such as flies. At the University of California, Irvine, flies have been bred which are themselves not allowed to breed until they are at an advanced (for them) age of 70 days. After between six and ten generations the offspring of these long-lived flies are found to be living 40 per cent longer than their normal expected span. These flies are also found to be healthier and more vigorous, as well as being better able to cope with stress factors than 'normal' flies. The genes so far identified as being involved relate to the insects' metabolic rate (how fast the organism 'burns' fuel) and this, as is explained in the next chapter, is a key area of interest for those examining life extension.