Nevus

by Jason Wasserman MD PhD FRCPC
November 18, 2024

A nevus (plural: nevi) is a medical term used to describe a growth or spot on the skin that develops from melanocytes. Melanocytes are the cells in the skin that produce melanin, the pigment responsible for skin and hair colour. Most nevi are harmless and commonly called moles, though the term nevus includes various pigmented and non-pigmented growths.

What is the difference between a nevus and a mole?

The terms “nevus” and “mole” are often used interchangeably, but they have slightly different meanings. A nevus is a broad medical term that includes all types of melanocytic growths, including those present at birth or developing later in life. A mole is a common term for a melanocytic nevus, typically referring to the types that appear as small, pigmented spots on the skin. All moles are nevi, but not all are called moles.

Types of melanocytic nevi

There are many types of nevi, each with distinct characteristics. Below are the most common types, along with a brief description of each:

Junctional, compound, and dermal nevi

• Junctional nevus: This type of nevus grows at the junction between the epidermis (outer skin layer) and dermis (inner skin layer). It appears as a flat or slightly raised, evenly pigmented spot.
• Compound nevus: This nevus grows in both the junction and the dermis. It is typically raised and pigmented, with a smooth or slightly bumpy surface.
• Dermal nevus: This nevus grows entirely within the dermis. It often appears as a raised, dome-shaped growth and may lose pigmentation over time.

Simple lentigo and lentiginous melanocytic nevus

• Simple lentigo: A small, flat spot caused by an increased number of melanocytes in the skin. It is often evenly pigmented and unrelated to sun exposure.
• Lentiginous melanocytic nevus: A nevus with melanocytes spread out in a linear pattern along the base of the epidermis. This type often resembles a lentigo but has slightly different features under the microscope.

Dysplastic nevus

A dysplastic nevus has irregular borders, uneven pigmentation, and sometimes a larger size than common moles. Though most dysplastic nevi do not become cancerous, they may carry a slightly increased risk of developing melanoma, a type of skin cancer.

Nevus spilus

A nevus spilus is a light brown patch on the skin with darker spots or moles scattered throughout. It is often present at birth or develops early in life.

Special-site nevi

These nevi develop in specific areas, such as the breast, axilla (armpit), scalp, and ear. They often have unique characteristics that reflect the location and sometimes look unusual under the microscope.

Halo nevus

A halo nevus is a mole surrounded by a ring of lighter or white skin. This occurs when the immune system attacks the mole’s melanocytes, leading to depigmentation.

Meyerson nevus

A Meyerson nevus is a nevus surrounded by a red, scaly rash. This inflammatory reaction can appear similar to eczema and usually resolves on its own.

Recurrent nevus

A recurrent nevus is a mole that regrows in an area after being removed, usually by shaving or incomplete excision. The regrowth may appear irregular, but it is typically benign.

Combined nevus

A combined nevus comprises two or more types of melanocytic nevi in the same lesion. For example, a blue nevus and a compound nevus might appear together.

Spitz nevi

A Spitz nevus is a benign mole that often appears pink, red, or tan and may resemble melanoma. It is more common in children and young adults.

Acral nevi

These nevi are found on the palms, soles, or under the nails. Due to the unique characteristics of acral skin, they often look different from moles on other parts of the body.

Mucosal and genital nevi

These nevi occur on mucous membranes, such as inside the mouth or on the genitals. Due to the higher concentration of melanin in these areas, they can look darker than other nevi.

Blue nevi

A blue nevus appears blue or grey due to the depth of the melanocytes in the dermis. These are usually small, round, and benign.

Congenital nevi

Congenital nevi are present at birth or develop within the first year of life. They can vary in size and colour and may carry a small risk of developing melanoma, particularly if they are large (giant congenital nevi).

Microscopic features

Under the microscope, a nevus is made up of melanocytes, the pigment-producing cells in the skin. These melanocytes are usually organized into clusters called nests, which can be found at different levels of the skin depending on the type of nevus:

• Junctional nevi: The melanocytes are located at the junction of the epidermis and dermis.
• Compound nevi: The melanocytes are found at the junction and extend deeper into the dermis.
• Dermal nevi: The melanocytes are entirely within the dermis.

Other common microscopic features include:

• Pigmentation: Many nevi contain melanin, the pigment that gives them their colour, which may be visible within the melanocytes or in the surrounding skin cells.
• Symmetry: Nevi are usually symmetrical under the microscope, meaning they look the same on both sides.
• Maturation: As melanocytes in a nevus grow deeper into the dermis, they typically become smaller, a process called maturation. This feature helps pathologists distinguish benign nevi from melanoma.
• Special structures: In congenital nevi, melanocytes may grow around normal skin structures like hair follicles or sebaceous glands. In blue nevi, melanocytes are located deep in the dermis, giving the nevus its characteristic colour.

Each type of nevus has unique microscopic features that help pathologists identify it. These findings are key to making an accurate diagnosis and ruling out melanoma or other skin conditions.

Can a nevus turn into melanoma over time?

Most nevi do not turn into melanoma, a type of skin cancer. However, certain types of nevi, such as dysplastic nevi or large congenital nevi, may have a slightly higher risk of developing melanoma. Monitoring nevi for changes in size, shape, colour, or symptoms like itching or bleeding is important. If a nevus changes, a doctor may recommend further evaluation or removal.

Does a nevus need to be removed?

A nevus does not usually need to be removed unless it is causing symptoms, such as pain or irritation, or if it shows worrisome features that could indicate melanoma. Some people choose to have nevi removed for cosmetic reasons. If your doctor is concerned about a nevus, they may recommend removal and examination under a microscope to ensure it is benign.

What are the worrisome features of a nevus?

Specific changes in a nevus may be signs of melanoma and should be evaluated by a doctor. These include:

• Asymmetry: One half of the nevus looks different from the other half.
• Border irregularity: The edges of the nevus are uneven, jagged, or blurred.
• Colour changes: The nevus has multiple colours or uneven pigmentation.
• Diameter: The nevus is larger than 6 millimetres (about the size of a pencil eraser).
• Evolution: The nevus changes in size, shape, colour, or symptoms like itching or bleeding over time.

These features are often summarized using the “ABCDE” rule for melanoma detection. If you notice any of these changes, consult your doctor for further evaluation.