Skin is part of the integumentary system, which also includes other structures such as hair sweat glands, and subcutaneous tissue.  The skin is the body's largest organ in serves many of vital functions. Perhaps the most important function of the skin is to serve as a barrier to germs, water, and temperature changes. The skin is carefully designed it to keep the good stuff in and bad stuff out.  That tissue under the skin helps connect the skin to the underlying muscle but is also very important for insulating the body from extreme temperatures in the environment.  The skin is resistant to minor injury and is able to heal itself very quickly. The skin and other parts of the integumentary system will be described below.

What are the layers of the skin? 

The two major layers of the skin are the epidermis and the dermis. 

What is the epidermis? 

The epidermis is on the outside and is the visible part of the skin it is made up of cells called keratinocytes.  The epidermis can be further subdivided in two two layers: the innermost layer is called the stratum basale (or basal layer) and the outermost layer is called the stratum corneum (from the Latin term for horn - tough like a animal's horn). The stratum basale is sometimes called the stratum germinativum, because this is where new own skin cells are germinated (from the Latin word for sprout - like a plant). It is here where new skin cells are created by division of other skin cells.  This process is called mitosis.  As new cells are produced the older cells are pushed towards the surface where they become the stratum corneum. The cells produce a tough protein called keratin that gives skin its strength and flexibility.   As skin cells move towards the surface they lose access to blood vessels, dry out, flatten, and die. Skin cells exposed to the environment will eventually be rubbed off or will simply fall off.  This process is probably most obvious on the scalp where dried skin forms flakes that fall off as dandruff.

What is the dermis? 

The dermis is the thick innermost layer of the skin which is made up mostly of fibrous connective tissue.  Here cells called fibroblasts produce collagen and elastin fibers which provide skin with eight, but flexible base. The term fibroblast comes from the Latin roots fibro- for fibrous and -blasts for builders.  The dermis also connects the outer layers of skin to muscle and inner tissues of the body.  Within the dermis are other skin structures including hair follicles, sweat glands, oil-producing glands, touch-sensing nerves, and blood vessels.

How does skin protect us from germs? 

Perhaps the most obvious way skin protects us from germs is simply to serve as a barrier between what we touch and what is allowed within the body.   Most germs are unable to penetrate the skin unless there is damage to the surface such as a scratch or cut. The living skin cells are also able to make antimicrobial proteins called defensins which helped destroy the cell walls of bacteria. The constant renewal of skin cells also helps remove bacteria attached to the skin surface as these cells flake off.  Sebaceous glands produce an oily substance which helps lubricate the skin.  This substance (called sebum) helps keep the skin from drying out but also inhibits the growth of bacteria.

What does skin have to do with vitamin D?

The body produces a form of cholesterol that when exposed to sunlight is changed to vitamin D within the skin.  Vitamin D is important for the absorption of calcium and phosphorus from food that we eat.  This vitamin also plays a role in other body processes such as maintaining normal muscle function, assisting the immune system, and more.  Children in cold climates who wear clothing that limits exposure to the sun are at a higher risk of developing a vitamin D deficiency. Fortunately, foods are now fortified with vitamin D (for example milk, orange juice, breakfast cereals).

How does skin help regulate body temperature? 

First of all, skin acts like a leather coat covering the entire body.  This helps prevent heat from escaping the body.  When the body is cold, blood vessels near the surface of the skin will contract, reducing blood flow close to the surface the body.  This will reduce the amount of heat that is lost to the environment by the blood.

When the body becomes too warm, the skin has other strategies. The skin contains sweat glands which release a salty water substance (sweat) onto the surface of skin.  Sweat releases heat as it evaporates.  In this case, small blood vessels close to the skin (capillaries) open widely allowing blood to deliver heat to the environment easily.

Why to skin a different colors in different people? 

The skin contains cells called melanocytes which produce a protein called melanin.  Melanin has a dark brown color.  The more melanin that is present, the darker the skin appears.  Melanocytes produce more melanin in response to sunlight.  In light-skinned people this leads to skin tanning.  Melanin helps protect skin cells from damaging ultraviolet light.  The amount of melanin produced by the melanocytes is determined by your genes.  Melanin also gives color to hair and the colored part of the eye called the iris.

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