One major difference between the human body and a machine, however fantastic the latter may be, is that a machine cannot renew itself, replace damaged components, and continue to function relatively efficiently for nearly a century. No machine can do that.

One way by which the body can achieve this is through a constant replacement of its parts. Among the most efficient organs in this respect is the bone marrow, which is constantly producing a stream of cells that float into the bloodstream and the lining of the intestinal tract and the skin, which both shed their lining every few days. Without such a self-replicating mechanism, our body would wear down and become irreversibly damaged within weeks.

The liver is in a class of its own. It does not feel the need to replace itself at such a rapid rate, but still retains the capacity to do so if and when the need arises. When a surgeon removes a lobe from the liver due to a tumour or for whatever other reason, the remaining liver bursts into activity, its cells start dividing, and within several weeks, a new liver is produced, capable of functioning in precisely the same way as before.

What is so wonderful about this whole process is that the body knows precisely the amount of tissue it requires. It produces exactly the required amount, neither more nor less. The regenerating liver does not keep on growing indefinitely but stops as soon as the original size is reached.

If you are inclined to think that this is a curious but irrelevant biological detail, just bear in mind that this is precisely the distinction between normal tissue and cancer: the latter has lost this capacity of being inhibited by the normal bodily mechanisms.

On the other hand, there are organs which seem to have lost their capacity for self-replacement – or at least this is what medical scientists used to think for a long time. Such organs include the brain and the heart.

In the case of the brain cells (neurones), it has long been thought that once lost (through old age, trauma, reduced blood supply, and so forth), they can never come back, and the loss of neural function is permanent. We have become wiser now: we know that the brain retains its ‘plasticity’ and still has a considerable reserve of function that can be directed and developed according to needs.

The heart is another such organ. It used to be thought that the heart muscle can never reproduce new muscle cells, and once damage has occurred (after, for example, a heart attack), the muscle is replaced by scar tissue which is permanent and nothing can be done to improve the situation.

This fatalistic approach to common disorders is slowly changing. The heart also changes itself, albeit slowly, at the rate of about one per cent per year, so that by the age of 80 or so, the vast majority of the muscle cells have been replaced.

For several years now, stem cells – cells which still have a capacity to reproduce themselves – have been harvested from bone marrow or foetal tissues and coaxed into transforming into cells which can replace damaged tissue. While this technique is still largely experimental, enormous progress has been made, and many patients are currently leading a more productive life through the use of such techniques.

Currently a lot of research effort is being directed into producing whole organs, such as kidneys and hearts, from stem cells which are made to grow on a connective tissue skeleton which will eventually be made available for transplantation.

Curiously, stem cells have recently been harvested from the heart itself – an organ long thought to be devoid of such cells. Several experimenters have shown that muscle tissue taken from hearts of patients who suffered a heart attack contains specific stem cells which can be made to multiply in the laboratory. When these cells were transferred back into the heart by injection into the coronary arteries, a definite beneficial effect was achieved, with reduction in the scar tissue, and increased function of the heart.

There is one danger in all this – that of assuming that such a procedure is a panacea for all and every disorder under the sun. It requires years of experience to achieve what has been achieved. The efficacy of any new medical procedure has to be tested extensively and only controlled trials can detect small, beneficial effects.