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What is melanoma?

Melanoma is a malignant tumour that starts in melanocytes. Melanocytes are a type of cell that make melanin, which is the pigment that gives your skin and eyes their colour. Malignant means that it can spread, or metastasize, to other parts of the body.

The skin is the body’s largest organ. It covers your whole body and protects it from injury, infection and ultraviolet (UV) light from the sun. The skin helps control your body temperature and gets rid of waste materials through the sweat glands. It also makes vitamin D and stores water and fat.

The skin has 2 main layers. The top layer, on the surface of the body, is called the epidermis. The dermis is below the epidermis. It has nerves, blood vessels, sweat glands, oil (sebaceous) glands and hair follicles. The epidermis is made up of 3 types of cells:

  • Squamous cells are flat, thin cells on the surface of the skin.
  • Basal cells are round cells that lie under the squamous cells.
  • Melanocytes are found in between the basal cells.

Cells in the skin sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous, or benign, tumours such as dermatofibromas, epidermal cysts or moles (also called nevi).

Changes to cells in the skin can also cause cancer. Different types of skin cells cause different types of skin cancers. When cancer starts in melanocytes, it is called melanoma. When skin cancer starts in squamous cells or basal cells, it is called non-melanoma skin cancer. Find out more about non-melanoma skin cancer.

When melanocytes change and become abnormal, they can cause precancerous conditions. This means that the cells are not yet cancer but there is a higher chance these abnormal changes may become cancer. An atypical mole, or dysplastic nevus, is a precancerous condition of the skin that is more likely than ordinary moles to develop into melanoma. Another precancerous condition caused by changes to melanocytes is called congenital melanocytic nevi. In some cases, congenital melanocytic nevi may develop into melanoma.

Melanoma usually starts on the skin. The most common type of melanoma is called superficial spreading melanoma. The second most common type of melanoma skin cancer is nodular melanoma. Other types of melanoma are lentigo maligna melanoma and acral lentiginous melanoma.

Melanoma can also start in other parts of the body where melanocytes are found, but these types of melanoma are rare. They include mucosal lentiginous melanoma, which can develop in mucous tissue anywhere in the body such as internal organs, the mouth or the vagina. Intraocular melanoma is another rare type of melanoma. It can develop in the eyeball.

Diagram of cross-section of the skin


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