A congenital nevus is a common non-cancerous tumour made up of melanocytes. A congenital nevus looks dark because it is made up of melanocytes that are producing a lot of melanin. Congenital nevi (nevi is the plural of nevus) are more common in people with light coloured skin. They can be found anywhere on the body, but the most common locations are the trunk and limbs.
Skin 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.
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.
Congenital nevi range in size from very small to very large. A congenital nevus is called a giant congenital nevus when it is larger than 20 cm. Many congenital nevi have an irregular shape and they can range in colour from light brown to black. It is not uncommon to find a hair growing out of a congenital nevus.
The diagnosis of congenital nevus is usually made after the tumour has been surgically removed and sent to a pathologist for examination. When examined under the microscope, the tumour is made up of melanocytes. Dark melanin pigment is often seen throughout the tumour.
The tumour cells grow in the layer of skin just below the surface called the dermis. As they grow, they can wrap around different normal structures like hair follicles, sebaceous glands, nerves, and blood vessels. Sometimes they can grow into the fat (subcutaneous tissue) below the dermis.
In general, when your doctor is looking at a nevus on your skin, they are looking for some basic features known as ABCDE.
A nevus may be removed by your doctor if there are any changes in ABCDE, or for cosmetic reasons. Some nevi will grow back after being removed and this is known as a recurrent or persistent nevus. If you see any colour in a scar after nevus is removed, you should see your family doctor.
While a congenital nevus is a non-cancerous skin tumour, it can change over time into a type of skin cancer called melanoma. You should let your doctor know if your nevus suddenly changes in size or colour or if it starts to bleed.