Compound nevus

What is a compound nevus?

A compound nevus is a non-cancerous type of growth made up of melanocytes. Compound nevi are usually seen in individuals of lighter skin complexion and can be found anywhere on the body. Most compound 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 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 compound nevus look like when examined without a microscope?

Most compound nevi are slightly raised and round to oval in shape. The border between the nevus and the surrounding normal skin is usually well defined and easy to see. Without a microscope, these growths can look pink, brown, black, or blue with most only showing a single colour.

How do pathologists make the diagnosis of compound 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.

compound nevusWhen examined under the microscope, compound nevi are made up of large round melanocytes. The melanocytes in a compound nevus are found in both the epidermis and the dermis below. The melanocytes form groups called nests although single cells may also be seen.

Most compound nevi develop from a very similar growth called a junctional nevus. 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. When the melanocytes are found only in the dermis, the growth is called a dermal (or intradermal) nevus.

by Allison Osmond MD FRCPC and Archan Kakadekar MD, updated March 2, 2021
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