The skin

The skin has many functions. It:

• protects the body from heat, cold, sunlight, injury and infection
• controls body temperature
• stores water and fat
• prevents the loss of fluid and electrolytes
• removes waste products from the body through sweat
• provides the sense of touch, including temperature, pain and pressure
• helps make vitamin D

The epidermis and dermis are the 2 main layers of the skin. They lie on top of a third layer called the subcutis.

The epidermis is the top or outer layer of the skin that you can see. It is a thin, tough layer of skin that protects the body, gives skin its colour and makes new skin. The epidermis is made of several different types of cells and has 2 layers.

Squamous cells are flat cells that make up one of the outer cell layers of the epidermis (called the squamous cell layer).

Squamous cells start from keratinocytes, which are cells that contain a tough, protective protein called keratin. As keratinocytes move toward the surface of the skin, they start to change. The keratinocytes get bigger and flatter and stick together (becoming squamous cells), then eventually die. The dead squamous cells on the surface of the skin are constantly shed from the skin and replaced by new cells.

Basal cells are round cells in the deepest part of the epidermis (called the basal cell layer).

Basal cells continually divide, make new cells and push the older cells toward the surface of the skin. The older basal cells eventually become squamous cells and then mature keratinocytes.

Melanocytes are cells found in the deepest part of the epidermis along with basal cells. They are also found in hair follicles and the retina of the eye. Melanocytes make melanin, which is the substance that gives skin its colour and helps protect the body from some of the harmful effects of the sun and ultraviolet radiation (UVR). When your skin is exposed to UVR from the sun, the melanocytes make more melanin. This causes the skin to darken or become tanned. The extra melanin is transferred to other skin cells (keratinocytes) to help protect the skin and the DNA in the cells from the damaging UVR.

The amount of melanin made is also affected by your genes. People with white skin make less melanin and are at a higher risk of sun damage than people with other skin types. People with brown and black skin have more protection from the sun because their melanocytes are more active and make more melanin.

Other types of specialized cells found in the epidermis include:

• Langerhans cells that support the immune system by attaching to and helping destroy foreign substances invading the skin (called antigens)
• Merkel cells that help give the skin its sense of touch

The dermis is the thickest layer of skin under the epidermis. Collagen and elastin are proteins in the dermis that give skin strength, sturdiness, stretch and flexibility.

The dermis contains many structures including:

• blood vessels that carry nutrients and oxygen to the skin
• lymph vessels that help fight infection
• hair follicles, which are small sacs where hair starts growing
• sweat glands that make sweat to control body temperature
• sebaceous glands that make oil to keep the skin and hair smooth
• nerve endings that help provide the sense of touch

The subcutis is the innermost layer of the skin, under the dermis, and is mainly made up of fat tissue. It helps keep the body warm and protect the internal organs and delicate tissues from injury. The subcutis is also called the hypodermis or subcutaneous tissue.