Dermal nevus

by Allison Osmond MD FRCPC and Archan Kakadekar MD
November 14, 2024

A dermal nevus is a non-cancerous skin tumour made up of specialized cells called melanocytes. Melanocytes produce melanin, the pigment that gives skin its colour. The term “dermal” is used because the melanocytes in a dermal nevus are located in the dermis, a layer of skin below the surface. This type of growth is commonly called a “mole,” a term used for any growth made up of melanocytes.

Dermal nevi (the plural of nevus) are typically found in individuals with lighter skin tones and can appear anywhere on the body. They are usually light in colour, but their appearance can vary depending on factors like skin type and the age of the nevus.

What causes a dermal nevus?

A dermal nevus is thought to result from prolonged exposure to ultraviolet (UV) light, usually from the sun, and genetic factors. UV light exposure can encourage the growth of melanocytes, while genetic susceptibility may make certain individuals more likely to develop nevi.

Can a dermal nevus turn into melanoma?

Melanoma is a type of skin cancer that also originates from melanocytes. Research suggests that about one-third of melanomas may arise from pre-existing melanocytic nevi. However, because dermal nevi are common and generally stable, the actual risk of a dermal nevus developing into melanoma is very low. Regular self-exams and monitoring for any changes in the size, shape, or colour of a dermal nevus can help detect any early signs of transformation.

What is the difference between a congenital nevus and an acquired nevus?

A congenital nevus is a type of dermal nevus present at or shortly after birth. Congenital nevi are usually larger than acquired nevi and may have a slightly higher risk of turning into melanoma, especially if they are large.

Acquired nevi develop later in life, typically during childhood or adulthood. They are more common than congenital nevi, and they are often smaller and have a very low risk of turning into melanoma.

How does a dermal nevus form?

Most dermal nevi begin as other types of nevi that evolve over time.

1. Junctional nevus: A junctional nevus forms first, with melanocytes located in the epidermis, the outer layer of skin.
2. Compound nevus: Over time, the melanocytes move deeper, entering both the epidermis and the dermis, forming a compound nevus.
3. Dermal nevus: As the nevus matures, all melanocytes eventually settle in the dermis, resulting in a dermal nevus.

This progression from junctional to compound to dermal nevus is a normal process and does not indicate any increased risk of cancer.

What does a dermal nevus look like?

Dermal nevi can vary in appearance but typically have the following features:

• Shape: They can be flat or raised, with a smooth surface that may contain small hairs.
• Colour: Most dermal nevi are light brown or flesh-coloured, although they may lose colour over time.
• Border: The border between the nevus and surrounding skin is usually well-defined. This distinction can be subtle if the colour of the nevus is similar to the surrounding skin.

Changes in appearance, such as rapid growth, irregular borders, or a shift in colour, should be assessed by a healthcare provider.

How is this diagnosis made?

The diagnosis is confirmed by examining a tissue sample under a microscope. This process, called a biopsy, typically involves removing the entire nevus and a small margin of normal skin. A pathologist examines the sample to confirm the diagnosis and rule out other types of skin lesions, including melanoma.

What are the microscopic features of a dermal nevus?

Under the microscope, a dermal nevus is made up of melanocytes that are found only within the dermis. The melanocytes typically form small groups called nests, though some single cells may also be seen. The appearance of melanocytes varies depending on their location in the dermis:

• Near the top: Melanocytes are usually larger and round.
• Near the bottom: Melanocytes tend to be smaller and thinner.

This change in size and shape, called maturation, is a normal finding in a dermal nevus. Maturation helps distinguish a benign dermal nevus from other, more concerning lesions where melanocytes may not exhibit this pattern.