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A melanocytic nevus, or mole, is a benign, or non-cancerous, growth made up of cells known as melanocytes. Melanocytes make a pigment called melanin, which gives the skin its color, and a mole is formed when these cells cluster together. A melanocytic nevus may be present at birth or appear later in life. Melanocytic nevi tend to occur to a greater extent in certain families, in people with fair skin, and in those who have spent more time in the sun. Occasionally, a melanocytic nevus may undergo changes which lead to the formation of a cancerous growth called a melanoma.
Melanocytic nevi do not only occur in humans but in all mammals. Those which are present at birth are known as congenital melanocytic nevi, and can be larger than moles acquired later in life, with giant versions of more than 1.2 inches (around 3 cm) occurring. Melanocytes do not normally cluster together, so, while they are common, melanocytic nevi are technically abnormal collections of pigment cells. Melanocytic nevi are gradually acquired throughout life, reaching a peak in middle age, before slowly declining in old age. The decrease in formation of new moles after middle age is combined with an increased tendency for existing moles to disappear, so that, generally, older people tend to have fewer moles.
Although melanocytic nevi are examples of what are called benign neoplasms, or non-cancerous growths, they have the potential to become malignant, or cancerous. A congenital melanocytic nevus carries a higher risk of melanoma development. Prevention may be carried out by protecting the skin from the sun and by closely observing existing moles for signs of suspicious changes such as itching, bleeding or alterations in shape, color or size.
Any mole containing suspicious nevus cells may be removed and examined under a microscope to check for cancer. For people who have a large number of melanocytic nevi, a topographic chart, or mole map, may be created. This map represents the whole body and records the location and characteristics of each melanocytic nevus.
A melanocytic nevus is usually left untreated, but plastic surgery may be used to remove it if it appears unsightly, catches on clothes leading to irritation, or shows suspicious signs indicating a possible melanoma. Where it is known that a mole is not cancerous, it is sometimes possible to shave it off. Larger moles, or those which may be malignant, are usually removed completely by cutting, and the wound is then stitched closed. Once removed, a melanocytic nevus can be passed on to a pathology department, where it may be examined to determine whether cancer is present.