A nevus is a a birthmark or mole. In broad usage, a nevus (or naevus) is any nonmalignant mark, spot, or growth in the skin believed to be of essentially congenital or developmental origin. In more restricted usage, a nevus is a lesion that primarily contains an accumulation of cells derived from the embryonic neural crest- namely, melanocytes (melanin-pigment-forming cells) and nevus cells or from nevus cells alone, as is the case in common brown hairy moles. The relative number of nevus cells and melanocytes in common pigmented or papular moles is widely variable. The greater the number of melanocytes, the darker brown is the color of the lesion. Where the melanocytes are deeply situated below the skin surface, the lesions assume a dark bluish hue.
Caucasian adults have an average of about 20 pigmented nevi on the skin, with wide individual variations both in the size and number of such lesions. Although the great majority of pigmented nevi are less than 1 cm in diameter, on very rare occasions large areas of the skin surface may be entirely covered by such lesions. Less than 13 of pigmented nevi are evident at birth. The great majority appear during childhood and early adult life, often as tiny frecklelike spots that grow to little dome-shaped elevations. Malignant neoplasms arise scarcely more frequently from small common pigmented nevi than from equivalent areas of normal skin.
Vascular nevi, or birthmarks, are maldevelop-ments of vascular elements in the skin. They consist mostly of richly anastomosing, variably-sized and shaped conglomerations of oversized capillary, venous, or lymphatic channels. Such maldevelopments are known as angiomas, and angiomas consisting mostly of blood vessels are known as hemangiomas. The common, elevated, strawberry type of superficial hemangioma that appears and grows only during the first few months of infancy often shrinks spontaneously. Angiomatous nevi acquired in adult life, nonpal-pable so-called port-wine stains, and deeply situated spongy or cavernous angiomas are generally permanent.
Various epidermal and connective-tissue elements of the skin may also show benign, basically congenital overgrowth or maldevelopment to give rise to a great variety of persistent wartlike or other disfiguring marks on the skin. Occasionally, epidermal nevi are displayed in bizarre linear distribution patterns.