What do moles, birthmarks and other nevi look like?

Pigmented nevi are made of cells that produce and store the dark-colored pigment melanin, and sometimes also release it into the surrounding cells. These melanin-producing cells are called melanocytes. If a lot of them build up in one place or some are especially active, the color of the skin there changes.

Pigmented nevi are more common on lighter skin than on darker skin. They are also more noticeable on lighter skin because of the brown color. There are different types of pigmented nevi:

• Simple lentigines (Lentigo simplex) are brown to dark brown in color, and usually just a few millimeters in size. They are in the top layer of the skin or mucous membrane. Simple lentigines usually develop in childhood or the teenage years.

• Café au lait spots can be anywhere between the size of a fingernail and palm of the hand. They have a clear border. On lighter skin they're light brown in color – quite like the color of coffee with milk ("café au lait" in French). On dark skin they look a little darker than the surrounding skin. Café au lait spots are in the top layer of skin. They develop in children and teenagers. More than five of these spots on your body could be a sign of a genetic disorder such as neurofibromatosis (Von Recklinghausen's disease).

• Nevus spilus is quite rare. It is a mixture of a café au lait spot and scattered clusters of darker pigment cells.

• Blue nevi (Nevus coeruleus) are similar to simple lentigines, but they're more bluish, purple, or gray-brown in color because they're deeper in the skin. They can often be felt as a small “lump.” If at all, people usually only have one blue nevus (or very few) and it only appears in the course of their life. Blue nevi typically develop on the back of the hand or the forearm.

• Melanocytic nevi appear as small spots or larger patches, on their own or in groups. These common moles can be any shade of brown or even look almost black. They are often hairier than the surrounding skin. Some melanocytic nevi are already visible at birth. But they often only develop in childhood, and then disappear again in later life. They can change over that time and, for example, move from the upper layer of skin to lower layers. They then usually get lighter in color and become a little more raised, which can make them look like light warts.

• Dysplastic or atypical nevi are special types of melanocytic nevi that look different to other pigmented nevi: They are usually bigger than five millimeters and have an irregular edge. A dysplastic nevus can have a mixture of different colors in it, including pink, light brown, dark brown and black. The term “dysplastic” refers to the properties of the cells in the nevus. The risk of developing melanoma skin cancer is believed to be higher in people who have a lot of dysplastic nevi. This kind of cancer can develop from a dysplastic nevus, but it sometimes also develops on skin that used to be "normal."

• Slate gray nevi (previously known as Mongolian spots) are already there at birth. They can be seen as a gray-blue patch on the lower back, tailbone area or buttocks. These patches mainly develop in children with darker skin and those of Asian origin. They usually go away again on their own after puberty.

• A nevus of Ota is a relatively large, gray-blue patch on one side of the face, usually spreading over the temple and cheek. The eye is sometimes affected too, but that doesn’t impair your vision. These nevi are common in people of Asian descent. Some are already there from birth, and others only develop in puberty. They usually don't go away again.

• Halo nevi are surrounded by a light, halo-like ring – probably because an autoimmune reaction in the body breaks down the pigment around the mole.

• Spitz nevi tend to look like a reddish bump. They develop in children and teenagers. They are harmless, but can be easily confused with other, more serious skin changes.

Nevi can also develop from other skin cells, or from connective or fatty tissue. Even without the typical pigment cells, some are darker than the surrounding skin. If they develop from skin cells, they often form in groups and look as if they're arranged along a string.

Dilated (wide) and unusually arranged blood vessels may also be visible as nevi – often just as red-purplish dots that appear with increasing age. But there are other types, too:

• Nevus flammeus is known as a port-wine stain because of its bright reddish-pink color. It is sometimes a pale pink or purple-bluish color instead. This kind of birthmark is often less noticeable on dark skin than on light skin. Some are as small as a lentil, while others cover larger areas of skin, such as one side of the face. They are usually present at birth or appear in very early childhood. When they develop on the back of the neck, they're called stork bites. Those on the forehead or back of the neck may get paler over time. They usually stay the same in other places, though. Port-wine stains may be a sign of a genetic disease.
  
  It can be difficult to tell the difference between port-wine stains and hemangiomas (non-cancerous growths of blood vessels).

• A spider nevus looks like a red dot surrounded by thin red lines extending from it, like spider’s legs. Spider nevi can develop in children or suddenly during pregnancy. They are more common in people with liver disease.