Definition of Nevus

Nevus is a discolored lesion on the skin. Nevi are very common and may take various shapes and colors. Most nevi contain primarily melanocytes and may differ in texture from the surrounding skin.

A nevus may be smooth, distinguishable only by its color, or rough and segmented. Some nevi contain coarse hair. The most common form of nevus is a mole, a small lesion that can be smooth or raised and is usually darker in color than the surrounding skin. A nevus may be congenital (present at birth) or acquired (develops at any point in the lifespan after birth). Nearly everyone has some nevi by early adulthood.

Congenital Nevus

Giant congenital nevus, a rare presentation of congenital nevus, may cover a large area of the skin’s surface. In another uncommon genetic disorder, neurocutaneous melanosis, nevi develop within the structures of the brain and spinal cord. The presence of melanocytes in some nervous system tissue is normal and functional.

Melanocytes populate the substantia nigra, for example, a structure of the midbrain. The pigmented cells of substantia nigra produce dopamine, a neurotransmitter essential for the brain’s coordination of muscle function throughout the body. However, melanocytes in other parts of the nervous system can generate overgrowths—nevi—just as they do on the skin. Such nevi cause pressure as they grow, resulting in neurologic symptoms.


Acquired nevi begin to appear during childhood in most people, with the most intense activity occurring in middle adulthood (ages 30 to 50). Though sun exposure plays a role in their development, genetic encoding seems to regulate characteristics such as size, color, shape, and numbers. In the typical structure of the dermis, the layer of skin where melanocytes reside in greatest concentration, melanocytes are frequent but are not in contact with each other. Researchers believe this distribution pattern results from “contact inhibition” genetic encoding.

A nevus can form when the contact inhibition lapses, allowing the melanocytes to drift into contact with one another. Sometimes nevi appear after extensive injury to the skin, such as occurs with conditions that cause widespread blistering. This suggests that such injuries disrupt contact inhibition in some way, though the mechanisms through which this occurs remain unknown.

Cancerous Nevus

A nevus should have regular borders, consistent coloration, and a symmetrical (balanced) appearance. A doctor should evaluate a nevus that has or develops irregular borders, variable coloration, or an asymmetrical appearance as these characteristics may indicate a nevus that is becoming cancerous.

Removal Nevus and Risks

Though nevi are themselves benign (noncancerous), they can become cancerous over time and particularly with repeated, unprotected sun exposure. Congenital nevi in particular carry an increased risk for malignant melanoma, a serious type of skin cancer. Most often, the only health concerns with nevi are the increased risk for malignancy and cosmetic appearance.

The dermatologist may choose to remove nevi that receive frequent irritation, such as those that form in areas where clothing rubs them. The resulting scar may require plastic surgery for the desired cosmetic appearance, depending on the size, nature, and location of the nevus. Small nevi heal without noticeable scarring. Nevi do tend to recur after removal.


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