A nevus (KNEE-vus) is a benign overgrowth of skin pigment forming cells called melanocytes (melanin, the skin pigment, and cyte, cell) on the skin surface, present at birth or appearing early in life. They fall into two broad categories, congenital and acquired.
Acquired nevi (develop after birth) are the pigmented moles with which everyone is familar. They are classified into three types, junctional, compound, or intradermal. By late adolescence, most people have at least 5 to 20 such nevi.
Junctional nevi are flat, brown to black in color, slightly elevated. They can appear anywhere but are more common on sun-exposed surfaces. They are small, 1-5 mm in diameter.
Compound nevi are also brown to black, and slightly elevated. they may develop corarse hairs within them.
Intradermal nevi are dome shaped or pedunculated (hangs by a stalk). They may eventually become fleshy colored, and also may develop coarse hairs within them.
It is not necessary to have these moles removed under ordinary circumstances, but any mole that ulcerates, becomes painful or itchy, or changes rapidly in size, shape or color should be removed and checked for malignancy. "Rapidly" means it grows faster than the child.
Other acquired melanocytic nevi include:
The Spitz nevus is a brown to pink dome-shaped nodule that arises commonly in children on the head, neck or arm region. It may mimic melanoma in outward appearance but is benign.
The blue nevus is a benign dome-shaped blue to black nodule up to 1/2 inch in diameter, usually occurring in childhood or adolescence. Blue nevi of typical appearance do not need to be removed.