The Outermost Layer - What Is Skin: Skin Health

Content provided by the Faculty of the Harvard Medical School

The outermost layer

The outermost layer

The epidermis is a protective, physical barrier. The outermost layer of skin, the epidermis is about as thick as a piece of paper. The very top portion of the epidermis is known as the stratum corneum. It's composed of cells called keratinocytes that produce a tough protein called keratin, forming a flexible outer shield. The keratinocytes die as younger living cells from the lower part of the epidermis rise to the surface from below. Finally, the older cells are rubbed off or fall off. This continuous cycle completely renews the skin about once a month.

This outermost layer of skin plays a key role in protecting you from the sun's radiation. In particular, pigmented cells called melanocytes are located at the bottom of the epidermis. These cells produce the melanin, or pigment, that colors skin and helps protect against ultraviolet radiation. When exposed to sunlight, the melanocytes churn out more melanin, and the skin darkens to help shield against further damage. If the melanocytes become cancerous, the condition is termed melanoma.

The middle layer

The dermis lies directly beneath the epidermis. It is a thicker layer that contains collagen, blood and lymph vessels, nerves, hair follicles, and glands that produce sweat and oil. Blood vessels in the dermis expand or contract to maintain a constant body temperature. White blood cells patrol the dermis to fight infectious microbes that manage to break through the epidermis. Cells called fibroblasts secrete collagen, which gives the skin its strength and firmness. Elastin fibers made of protein in the dermis give skin its elasticity.

The deepest layer

The subcutaneous tissue, which consists of connective tissue and fat, lies between the dermis and underlying muscles or bones. It, too, contains blood vessels and infection-fighting white blood cells, but not to the same extent as in the dermis. Fat in the subcutaneous layer stores nutrients and insulates and cushions muscles and bones.

Nails, skin, and hair

Your nails are skin, too. They're a thickened, hardened form of epidermis. Nail cells originate from the base of the nail bed. They die quickly, but unlike the keratinocytes, they aren't sloughed off. They're also composed of a much stronger form of keratin. Thus, a nail is simply a much harder and thicker sheet of keratin than the topmost layer of skin. Hair, however, is a thin fiber made of many overlapping layers of keratin, which is produced in the hair root.

Last updated:	July 20, 2007