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Structure and Function of the Skin

By Mr Ghaz, January 10, 2011

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Structure and Function of the Skin

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The skin is made up of two different layers of tissue: the dermis and epidermis. Both layers contain nerve endings which transmit sensations of pain, pressure, heat and cold. The sweat glands are vital in regulating the body's temperature, while the sebaceous glands lubricate the skin and hair. The apocrine glands develop at puberty and are a sexual characteristic. The pigment-producing cells, called melanocytes, can cause freckles.

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The skin, or integument, is much more than a simple wrapping around our bodies. It is an active and versatile organ which is waterproof so that we do not dry up in the heat or melt in the rain, and it protects us from the damaging radiation of sunlight. It is tough enough to act as a shield against injury, yet supple enough to permit movement. It conserves heat or cools the body as required, thus keeping our internal temperature constant.

Structure of the Skin

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The skin is made of two main parts. The outermost part-the epidermis-consists of several layers of cells, the lowest of which are called the mother cells. Here the cells are constantly dividing and moving up to the surface, where they flatten, die and are transformed into a material called keratin which is finally shed as tiny, barely visible scales. It takes three to four weeks for a cell in the lowest layer to reach the skin surface.

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This outer protective layer is firmly attached to an underlying layer called the dermis. Tiny, finger-like bulges from the dermis fit into sockets in the epidermis, and this waviness at the junction of the two layers of skin gives rise to ridges, which are most obvious at the fingertips and give us our fingerprints. The dermis is made up of bundles of collagen and elastin fibers. Embedded in the dermis are sweat, sebaceous and apocrine glands, hair follicles, blood vessels and nerves. The nerves penetrate the epidermis but the blood vessels are confined to the dermis. The hairs and ducts from the glands pass through the epidermis to the surface.

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Each sweat gland is formed of a coiled tube of epidermal cells which leads into the sweat duct to open out on the skin surface. The sweat glands are controlled by the nervous system and are stimulated to secrete either by emotion or by the body’s need to lose heat.

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The sebaceous glands open into the hair follicles and are made up of specialized epidermal cells which produce grease, or sebum. They are most numerous on the head, face, chest and back. Their function is to lubricate the hair shaft and surrounding skin and they are controlled by sex hormones.

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The apocrine glands develop at puberty and are found in the armpits, breasts and near the genitals. They are odor-producing and are a sexual characteristic. When they begin to function they secrete a thick milky substance.

There is a fine network of nerve endings in both layers of skin, and they are particularly numerous at the fingertips. They transmit pleasurable sensations of warmth and touch, as well as cold, pressure, itching and pain which may evoke protective reflexes.

Hair and Nails

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Hair and nails are both specialized forms of keratin. Although nails are produced by living skin cells the nail itself is dead and will not hurt or bleed if it is damaged. The visible part of the nail is called the nail body and its shape is partly determined by genetic factors. The bottom part of the nail, which is implanted in a groove in the skin, is called the root. Overlapping this root are the cuticles (eponychia). These outer layers of skin cover the white crescent, or lunula, found towards the base of the nail. The lunula, which is usually most clearly visible on the thumb, is slightly thicker than the rest of the nail and looks white because it obscures the blood beneath.

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The lowest layer of cells in the skin composing the nail folds is known as the general matrix. The cells of the matrix divide and the upper ones become thickened and toughened with keratin. When the cells die they become part of the nail itself. If the matrix is seriously damaged the whole nail is lost.

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Hair is formed by cells in the hair follicles and there are two types: fine, downy hair which is found over most of the body except the palms of the hands and soles of the feet, and thick, pigmented hair which is present on the scalp, eyebrows, beard and genital areas.

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The visible part of a hair is called the shaft: it is formed from keratin and is composed of dead tissue. The shaft is rooted in a tube-like depression in the skin called the follicle. The hair develops from a root, the dermal papilla, which is at the bottom of the follicle, and is nourished by the blood-stream. If the root is damaged, hair growth stops and it may never regrow.

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The follicle also contains a sebaceous gland, and arrector pili muscles. When a person is cold, afraid or alarmed, these muscles contract, making the hair stand on end and bunching the skin around the shaft to form what are known as goose pimples.

Adult have about 120,000 hairs on their head: redheads have fewer, blondes more. Hair type varies according to structure: there are fine, soft baby hairs which grow on portions of the body; long hairs which grow on the scalp; and short, stiff hairs which compose the eyebrows. Blonde hair is the finest; black the coarsest.

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The type of hair shaft determines whether hair is straight or curly. A cylindrical hair shaft produces straight hair, and an oval shaft produces curly or wavy hair, and a flattened or kidney-shaped shaft produces wooly hair.

The cells that make keratin for hair are among the most rapidly dividing of the body. Scalp hair grows an average of 1.25 cm ( ½ in) a month. Hair growth is not continuous, and every five or six months the hair goes into a resting phase, during which no growth takes place. The roots of resting hair become club-shaped – hence their name, club hairs – and lose their normal pigmentation. Up to ten per cent of our scalp hairs are in the resting phase at any one time. It is the club hairs that seem to come out in handfuls when we wash our hair. No damage is done to the follicles, and when the root has finished its rest, normal hair growth begins again.

Skin Color

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Skin color is due to the black pigment melanin. Melanin is also found in the hair and in the iris of the eye. It is formed in melanin-making cells, called melanocytes, situated in the basal layer of the skin.

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Irrespective of racial type, the same numbers of melanocytes are found in the skin of every human being. The cells, however, varies greatly. In dark-skinned races, the melanocytes are larger and produce more pigment. Melanin’s function is to protect the skin from the harmful rays of the sun; the darker the skin the less likely it is to suffer from sunburn.

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The complex chemical process of the body which converts the amino acid, tyrosine, into melanin, takes place on the outer part of each melanocyte. Once formed, the pigment moves to the center of the cell to cloud over, and thereby protect, the highly sensitive nucleus. Exposure to ultraviolet light, either from artificial source or sunlight, stimulates melanin production by the normal process of tanning. Melanin is formed, the cells expand and the skin darkens in color. Response varies from individual to individual, but all persons except albinos can eventually become pigmented when exposed to enough sunlight.

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Other factors contributing to skin color are the blood in the blood vessels of the skin and the natural yellowish tinge of the skin tissue. The state of the blood within the blood vessels can greatly change skin color. Thus we become ‘white’ with fear when small vessels close off, ‘red’ with anger due to an increased blood flow, and ‘blue’ with cold when most of the oxygen in the blood moves out to the tissues as the flow slows down.

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