The skin

The skin has many purposes. It:

  • acts as a barrier to protect the body from injury
  • keeps necessary fluids and proteins in the body
  • protects the body from the harmful effects of ultraviolet (UV) light
  • helps control the body’s temperature.

The skin is divided into two main layers. The outer layer is known as the epidermis. The layer underneath is known as the dermis.

The epidermis contains three types of cell. On the surface are flat cells, known as squamous cells. Under these are rounder cells called basal cells. In between the basal cells are melanocytes.

The dermis contains nerve endings, blood vessels, and oil and sweat glands. It’s held together by a protein called collagen.


Melanocytes are cells that produce a pigment called melanin. Melanin is responsible for the natural colour of our skin. It also protects skin from the harmful effects of the sun.

When our skin is exposed to sunlight, our melanocytes increase the amount of melanin. This is to absorb more potentially harmful ultraviolet (UV) rays. This makes the skin darker and gives it a suntanned appearance. A suntan is a sign that the skin is trying to protect itself.

If you have naturally dark (brown or black) skin, you have the same number of melanocytes as people with white skin, but they make more melanin. This means you have more natural protection from UV rays.

Moles (sometimes called naevi) are a group or cluster of melanocytes that are close together. Most people with white skin have about 10–50 moles on their skin. Some people can have as many as 100.

Structure of the skin
Structure of the skin

View a large version

Read a description of this image

Back to Understanding melanoma

What is melanoma?

Melanoma is a type of skin cancer. It develops from cells in the skin called melanocytes.

Cancer and cell types

Cancers are grouped into types. Types of cancer often behave and respond to treatments in different ways.

How is it treated?

There are five main types of cancer treatment. You may receive one, or a combination of treatments, depending on your cancer type.

Why do cancers come back?

Sometimes, tiny cancer cells are left behind after cancer treatment. These can divide to form a new tumour.