The human body, like most living organisms, is assembled from millions and millions of individual cells. The cells are the building bricks of the body, making up the skin, blood and every functioning part. We start life as a single cell, which is brought about by the fertilization of a single egg by a single sperm. Each cell multiplies by dividing into two cells (called 'daughter' cells), and the resulting collection of cells develops into the unborn child. Growth into adult life requires further huge multiplication of cells, a process known as cellular proliferation. Once we reach adult life certain parts of our bodies require continuous repair and renewal. We are continuously shedding our skin, the lining of our intestines and the lining of our lungs and bladders; and, as these are shed, the cells in these organs are replaced by further proliferation. Cellular proliferation is therefore a basic process for maintaining life and health.
The proliferation of cells is linked to a process by which cells in different parts of the body develop different functions. Heart cells have to contract to push blood around the body, liver cells become chemical factories altering the content of our diet to turn it into the essential nutrition for all parts of the body. Cells which line the gut or cover the skin are developed specially for these purposes. The process by which cells develop different functions is called differentiation.
Cancer is a disease in which cellular proliferation and differentiation become disordered. The problem with cancer cells is that they continue to proliferate when they should not. This means that too many cells accumulate and this is how tumours are formed (the word tumour comes from the Latin word for a swelling); the accumulation of too many cells as a consequence of proliferation results in tumour masses in the affected part of the body. In addition to exhibiting disordered proliferation, the cancer cell usually fails to behave in the correct way for a cell in a particular location. It lacks correct differentiation. This means that it will look abnormal through a microscope and will behave abnormally, often manufacturing the wrong substances or failing to manufacture the substances that it should be making.
So if cancer is the result of a disorder of the way cells multiply and the way they function, how does it cause the extensive damage that we associate with this disease? Every medical student is taught that there are three essential characteristics of a cancer. These are growth, invasion and spread.
The process of growth of a cancer is easy to understand. We have already explained that the controls that normally act on the multiplication of cells are deranged and that excessive numbers of cells accumulate. Sometimes, the multiplication of cells occurs very rapidly but, equally often, the process may not be particularly rapid but may simply continue when it should be switched off. Either way, the result will be an uncontrolled growth of cells within the organ affected. With lung cancer, such a growth will be seen as a shadow on a chest X-ray. For breast cancer, a lump will appear in the breast. For a cancer occurring in the intestines, the lump will not be visible unless special X-rays or scans are used, but it will usually alter the function of the bowel or tend to obstruct it, a process which can produce pain. Sometimes these lumps may bleed, which is why bleeding is a common symptom leading to the diagnosis of cancer in many sites. The process of growth of the cancer is in itself dangerous. For instance, large cancers within the lung will interfere with the function of the lungs and sometimes, when cancers occur in particularly vital situations like the brain, the presence of quite a small growth is capable of causing catastrophic damage. However, growths can often be removed and it is the other features of the cancer which represent the greatest challenges to effective treatment.
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Cancer
