Dermal nevus

What is a dermal nevus?

A dermal nevus is a non-cancerous type of growth made up of specialized cells called melanocytes. Dermal nevi (more than one nevus are called nevi) are usually seen in individuals of lighter skin complexion and can be found anywhere on the body. Most dermal nevi are called acquired because they develop in children or young adults. A nevus that develops shortly after birth is called a congenital nevus. Another name for this type of growth is a mole. Mole is a common term used to describe any kind of growth made up of melanocytes.

The skin

Your skin is the largest organ in your body. It is made up of three layers: epidermis, dermis, and subcutaneous fat. The surface and the part you can see when you look at your skin is called the epidermis. The cells that make up the epidermis include squamous cells, basal cells, melanocytes, Merkel cells, and cells of the immune system. The dermis is directly below the epidermis. The dermis is separated from the epidermis by a thin layer of tissue called the basement membrane. The dermis contains blood vessels and nerves. Below the dermis is a layer of fat called subcutaneous adipose tissue.

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Melanocytes

Melanocytes are specialized cells that are normally found at the very bottom of the epidermis. They are responsible for producing a dark pigment called melanin that helps protect our skin from the sun’s ultraviolet light. The amount of melanin in a person’s skin determines their skin colour – people with light skin produce little melanin and people with darker skin produce more melanin.

What does a dermal nevus look like when examined without a microscope?

A compound nevus can be flat or raised. The surface of the nevus is usually smooth and may contain hair. Most dermal nevi are light in colour and the colour may be lost slowly over time. The border between the nevus and the surrounding normal skin is usually well defined and easy to see although it may be more difficult to see if the colour of the nevus is similar to the surrounding skin.

How do pathologists make the diagnosis of dermal nevus?

This diagnosis can only be made after a tissue sample is removed and examined under the microscope by a pathologist. This usually involves removing the entire nevus in one piece along with a small amount of surrounding normal-appearing skin.

When examined under the microscope, dermal nevi are made up of melanocytes. The melanocytes in a dermal nevus are found only in the dermis. The melanocytes form groups called nests although single cells may also be seen. The melanocytes near the top of the dermis are usually large and round while those near the bottom are often smaller and thinner. This change in size and shape is called maturation and is a normal finding in a dermal nevus.

Most dermal nevi develop from junctional and compound nevi. In a junctional nevus, the melanocytes are only found in the epidermis. Over time, the melanocytes in a junctional nevus spread down towards the dermis forming a compound nevus. Eventually, all of the melanocytes move into the dermis creating a dermal nevus.

by Allison Osmond MD FRCPC and Archan Kakadekar MD (updated July 2, 2021)
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