What is skin?
| March 1, 2007
What is skin?
The skin is the body's largest organ, weighing about nine pounds. It carries out a number of functions that help maintain health. Skin is a complex fabric of tissues working together to form a basic control system (see Figure 1). Skin helps control your body temperature by sweating and dilating its blood vessels to cool you down. When you're cold, those blood vessels constrict to conserve heat deep inside your body.
Figure 1: More than skin deepStructures
Functions Protective barrier Temperature control Vitamin D manufacture Fights infection Sensory organ Skin is more than just a cosmetic covering for the body. Its blood vessels and sweat glands regulate body temperature. Its immune cells ward off infection. Tiny nerve cells detect pressure and temperature, and other skin cells manufacture vitamin D. |
The skin is also a sensory organ. Nerve endings on its surface pick up and relay information about the surrounding environment to your brain. Your brain then translates these nerve impulses into the sensations of heat and cold, as well as touch, pressure, and pain.
In addition, the skin helps ward off infection by way of its Langerhans cells, part of the immune system that fights off foreign invaders such as bacteria and viruses. The skin is also a manufacturing plant, using the sun's energy to make vitamin D, essential to making bones strong.
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.
Review Date: 2007-03-01
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