Melanocytic Nevus

A melanocytic nevus, more commonly known as a mole, is one of the most frequently occurring benign skin lesions formed due to a local proliferation of melanocytes. Since a brown or black melanocytic nevus contains the pigment melanin, it is often referred to as a pigmented nevus.

A melanocytic nevus is generally classified as either a congenital melanocytic nevus or an acquired nevus. A congenital melanocytic nevus is present at birth, while an acquired nevus appears later in life.

Melanocytic nevi are found on virtually all individuals. Approximately 1% of the world's population is born with at least one congenital melanocytic nevus. Individuals with fair skin tend to have more melanocytic nevi in comparison with those who have a darker skin type.

Moles that appear between 2 and 10 years of age are the most prominent and persistent throughout life. Melanocytic nevi formed in later childhood and adulthood are usually caused by sun exposure and may eventually disappear.

The exact reason why local proliferation of nevus cells occurs is unknown. However, it has been determined that genetic factors, immune status, and sun exposure all contribute to their formation. Individuals who have multiple melanocytic nevi often have family members with similar skin lesions. Somatic mutations in RAS genes are linked with congenital melanocytic nevi. New melanocytic nevi may also arise as a result of taking BRAF inhibitor drugs such as dabrafenib and vemurafenib.

Melanocytic nevi vary between patients in terms of their clinical, dermatoscopic, and histological appearances. They can affect any part of the body, and their appearance depends on the body site at which they are found. Melanocytic nevi can be flat or protruding, vary in colour (from pink or flesh tones to dark brown and black), and range in diameter from a few millimetres to a couple of centimetres. Most melanocytic nevi have a round or oval shape, though irregular shapes are sometimes observed.

Melanocytic nevi are often associated with melanoma, and untreated melanoma can result in death from skin cancer. Melanoma and a harmless melanocytic nevus may appear alike; however, melanoma eventually changes in structure and increases in size. Patients who have a large number of nevi are at greater risk of developing melanoma. It is important to note that melanocytic nevi may enlarge, disappear, or regress for reasons other than melanoma, such as pregnancy and sun exposure.

Melanocytic nevi are usually diagnosed clinically by their appearance. If abnormalities are detected, further examinations will be performed.

The majority of moles are harmless and are left untreated. Removal may be considered for cosmetic reasons, if a mole causes irritation from a razor or clothing, or to exclude the possibility of cancer.