The skin is the body’s largest organ. It protects against heat, sunlight, injury, and infection. It helps regulate body temperature, stores water and fat, and produces vitamin D.

The skin has three layers:

• the outer epidermis
• the inner dermis
• the subcutis

The outermost part of the epidermis is called the stratum corneum, or horny layer. It is composed of dead keratinocytes that are continually shed. Below the stratum corneum are layers of living keratinocytes, also called squamous cells. These cells are flat and scale-like and form an important protein called keratin, which contributes to the skin’s ability to protect the rest of the body. The lowest part of the epidermis, under the squamous cells, is formed by basal cells. These round cells continually divide to form new keratinocytes, which replace older keratinocytes that wear off the skin’s surface. The lower part of the epidermis also contains melanocytes. These skin cells produce the protective pigment called melanin, which give a tan or brown color to the skin and helps protect the deeper layers of the skin from the harmful effects of the sun.

The middle layer, called the dermis, contains blood vessels, lymph vessels, hair follicles, nerves and glands, help in place by a protein called collagen. Some of these glands produce sweat, which helps regulate body temperature. Other glands produce sebum, an oily substance that helps keep the skin from drying out. Sweat and sebum reach the skin’s surface through tiny openings called pores.

The deepest layer beneath the skin is called the subcutis. The subcutis and the lowest part of the dermis form a network of collagen and fat cells. The subcutis conserves heat and has a shock-absorbing effect that helps protect the body’s organs from injury.

Melanocytes produce melanin, the pigment that gives skin its natural color. When skin is exposed to the sun, melanocytes produce more pigment, causing the skin to tan, or darken. Sometimes, clusters of melanocytes and surrounding tissue form noncancerous growths called moles. (Also called a nevus) Moles are very common. Most people have between 10 and 40 moles. Moles may be pink, tan, brown, or a color that is very close to the person’s normal skin tone. People who have dark skin tend to have dark moles. Moles can be flat or raised. They are usually round or oval and smaller than a pencil eraser. They may be present at birth or may appear later on—usually before age 40. They tend to fade away in older people. When moles are surgically removed, they normally do not return.

Melanoma is a type of skin cancer that begins in skin cells called melanocytes (pigment cells). Most pigment cells are in the skin; when melanoma starts in the skin, the disease is called cutaneous melanoma. Melanoma may also occur in the eye (ocular melanoma or intraocular melanoma. Rarely, melanoma may arise in the meninges, the digestive tract, lymph nodes, or other areas where melanocytes are found.

Melanoma is more aggressive than other kinds of skin cancer such as basal cell or squamous cell cancer. Melanoma is one of the most common cancers. The chance of developing it increases with age, but this disease affects people of all ages. It can occur on any skin surface. Melanoma is rare in people with dark skin.When melanoma spreads, cancer cells may show up in nearby lymph nodes. Groups of lymph nodes are found throughout the body. Lymph nodes trap bacteria, cancer cells, or other harmful substances that may be in the lymphatic system. If the cancer has reached the lymph nodes, it may mean that cancer cells have spread to other parts of the body such as the liver, lungs, or brain. In such cases, the cancer cells in the new tumor are still melanoma cells, and the disease is called metastatic melanoma.