Skin 101: The Epidermis, Melanin, and More

The skin is the largest organ of the human body, and its primary purpose is to cover and protect the rest of the body — such as your muscles, bones, and organs — from light, heat, injury, and infection. Skin health is important because your skin also serves to:

  • Keep your body temperature regulated
  • Store supplies of water, fat, and vitamin D for the rest of the body
  • Produce sensory input to the brain through your sense of touch

Your skin weighs between six and nine pounds. Stretched out flat, the skin of a 150-pound man would cover about two square yards. The density of your skin varies across the body; it is at its thinnest on your eyelids and its thickest on the soles of your feet.

Your hair and nails are modified types of skin that serve specific protective purposes. Body hair provides protection for the skin's outer layer and, depending on location, serves specialized functions, such as a filter for the nose and ears and as a means of regulating balance in the inner ear. Nails protect and support the tips of your fingers and toes.

A Closer Look at Skin Layers

The skin is made up of three main layers: the epidermis, the dermis, and the subcutaneous tissue. Skin health depends on each of these layers both performing their own functions as well as supporting each of the other layers.

The epidermis. This is the outermost layer of your skin. There are three sub- layers within the epidermis itself: the outer stratum corneum, the middle squamous cell layer, and the bottom basal cell layer. The stratum corneum is the visible part of the epidermis and is actually a layer of dead skin cells immediately on the skin's surface. This layer uses a protein called keratin to form a tough barrier between the outside world and the more vulnerable cells inside the skin and body.

The following types of cells make up the epidermis:

  • Basal cells are at the bottom of the epidermis and continually reproduce to form new keratinocytes.
  • Keratinocytes or squamous cells are in the middle layer of the epidermis and produce keratin, the protein that forms the protective outer layer. Keratin also is used to produce hair and nails.
  • Melanocytes make melanin, the pigment that provides color to the skin. People with darker skin have melanocytes that produce more melanin. Exposure to sunlight also can increase melanin production, causing freckles or a suntan.
  • Langerhans cells are part of the body's immune system and help fight off infection.

About 95 percent of the skin cells in the epidermis are devoted to creating new skin cells in the lower two levels of the epidermis, which then cycle to the top layer to help form the stratum corneum. Eventually the dead cells of the stratum corneum flake off as new keratinocytes move up, and the cycle repeats itself. Healthy skin care habits help remove these dead cells.

The dermis. Found underneath the epidermis, the dermis is held together by a protein called collagen. The dermis contains many structures that help keep the epidermis healthy and growing as well as structures that aid the body in other ways:

  • Nerve endings transmit sensory input to the brain. These nerves allow you to sense texture, pressure, heat, and pain through the skin, all part of your overall sense of touch.
  • Blood vessels deliver nutrients and oxygen to the epidermis and dermis and cart away waste products.
  • Oil glands keep the skin lubricated and prevent hair follicles from becoming brittle.
  • Sweat glands keep the body's temperature regulated. The sweat comes up through pores that travel through the epidermis and open onto the outer skin.

The subcutaneous tissue. This is a layer of collagen and fat cells at the base of your skin. This layer helps hold in body heat and absorb the impact of blows and other injuries.

Your Hair and Nails

Hair starts in the dermis, where hair follicles provide a base for the hair that grows up through the epidermis. Several structures form around the hair follicles:

  • Basket cells surround the base of each hair follicle and help sense pressure placed on the hair.
  • Erector pili muscles connect the hair follicle to the dermis. These muscles can contract and are what cause your hair to stand on end when you receive a fright or are cold.

Human hair grows from the root, a bulb that rests in the follicle. Cells multiply and produce keratin, which forms a hair shaft and pushes up through the skin's surface. The shaft is formed by dead cells held together by hardened keratin.

Nails grow in much the same way as hair does, but they begin in deep folds in the skin of the fingers and toes. Growth starts in the nail root, where epidermal cells become flattened and are pressed tightly together to form the nail. Like hair, your fingernails and toenails consist of dead cells held together by hardened keratin. As the nail cells at the root accumulate, they push the nail outward.

Healthy skin, hair, and nails are important not just for your appearance, but also for the health of your body, from head to toe. Take care of them well.

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