Skin

In biology, skin is a flexible organ (group of tissues which perform a specific function) that serves as the outer covering of an animal. As the interface with the surroundings, skin plays an important immune system role in protecting against pathogens. Its other main functions are insulation and temperature regulation, sensation, water and fat storage, vitamin A and B synthesis, and protection against light and injury. Skin is considered one of the most important parts of the body.

Skin consists of two layers of tissues (collection of interconnected cells that perform a similar function): A thin outer layer called the epidermis and a thicker inner layer called the dermis. The epidermis is largely composed of stratified squamous epithelial cells, under which are round basal cells. The underlying dermis is made up of connective tissue, and contains sensory nerves, blood vessels, lymph channels, nerve endings, sweat glands, fat cells, hair follicles, and muscles. Individual uniqueness is also reflected by a region of the dermis that has projections which extend toward the epidermis, forming contours in the skin's surface (such as fingerprints) that are genetically determined, and thus unique to the individual.

The epidermis lacks blood vessels, but its deeper layers contain melanocytes. Melanocytes are cells that produce melanin, a pigment that absorbs some of the potentially dangerous ultraviolet radiation in sunlight and gives color to the skin. Human skin pigmentation varies among populations in a striking manner. This has sometimes led to the classification of people(s) on the basis of skin color and the establishment of cultural barriers between people.

The skin is often known as "the largest organ of the human body." This applies both to exterior surface, which appears to have the largest surface area of all the organs, as well as to its weight, as it weighs more than any single internal organ. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters, most of it is between 2-3 mm thick. The average square inch of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings. The skin accounts for about 15 percent of adult body weight.

Horns, hoofs, hair (fur), feathers, and scales are modifications of the epidermis.

Another function of skin is aesthetic. The use of natural or synthetic cosmetics to treat the appearance of the face and condition of the skin is common in many cultures. Often, however, attractiveness based on external features ("skin deep") has been overemphasized versus the importance of internal character.

Dermatology (from Greek derma, "skin") is a branch of medicine dealing with the skin and its appendages (hair, nails, sweat glands etc). A medical doctor who specializes in dermatology is a dermatologist.

Layers

Skin is composed of two primary layers, the epidermis and the dermis. The basal lamina, or basement membrane, anchors the epidermis to the dermis. Below the dermis is the hypodermis. The hypodermis is not part of the skin, but attaches the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves.

Epidermis

Epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface, and is made up of stratified squamous epithelium with an underlying basal lamina. Epithelium is a tissue that covers organs and surfaces of the bodies of animals, and is characterized by cells that are close together, with very little intercellular material, and almost always free of blood vessels. "Stratified" means there is more than one layer of cells, and "squamous" refers to cells that are flat with an irregular flattened shape. The basal lamina is the layer on which epithelium sits and which is secreted by the epithelial cells.

Since the epidermis contains no blood vessels, the cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main type of cells that make up the epidermis are keratinocytes, with melanocytes, Langerhans cells, and Merkels cells also present.

The epidermis can be further subdivided into the following strata (beginning with the outermost layer): Corneum, lucidum (only in feet), granulosum, spinosum, and basale. Cells are formed through mitosis at the basale layer. The daughter cells move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein keratin is inserted. The cells eventually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection. The outermost layer of epidermis may consist of 25 to 30 layers of dead cells.

Epidermis also contains DNA repair enzymes, which help to reverse UV damage, and people who lack the genes for these enzymes suffer high rates of skin cancer. One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly.

Damaged skin will try to heal by forming scar tissue, often giving rise to discoloration and depigmentation of the skin.

As noted, epidermis has been modified into such structures as hair (fur), scales, horns, hoofs, and feathers. Mammalian skin contains hairs, which in non-human mammals and sufficient density is called fur. Hair mainly serves to augment the insulation the skin provides, but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. A frog sitting in an anesthetic solution will quickly go to sleep.

Dermis
The distribution of the blood vessels in the skin of the sole of the foot. (Corium—TA alternate term for dermis—is labeled at upper right.)
A diagrammatic sectional view of the skin (magnified). (Dermis labeled at center right.)
Gray's	subject #234 1065
MeSH	Dermis
Dorlands/Elsevier	d_11/12289496

Dermis

The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane (basal lamina).

The dermis harbors many nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, and blood vessels. The blood vessels in the dermis provide nourishment and waste removal to its own cells as well as the Stratum basale of the epidermis.

The dermis is structurally divided into two areas: A superficial area adjacent to the epidermis, called the papillary region and a deep, thicker area known as the reticular region.

Papillary region

The papillary region is composed of loose areolar connective tissue. It is named for its finger-like projections called papillae that extend toward the epidermis. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin's surface. These are called friction ridges, because they help the hand or foot to grasp by increasing friction. Friction ridges occur in patterns (see fingerprint) that are genetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification.

Reticular region

The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility, and elasticity.

Located within the reticular region are also the roots of the hair, sebaceous glands, sweat glands, receptors, nails, and blood vessels.

Tattoo ink is injected into the dermis. Stretch marks from pregnancy are also located in the dermis.

Hypodermis

The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages, and adipocytes. Adipocytes are the cells that primarily compose adipose tissue, specialized in storing energy as fat. The hypodermis contains 50 percent of human body fat. Fat serves as padding and insulation for the body.

Microorganisms like Staphylococcus epidermidis colonize the skin surface. The density of skin flora depends on the region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut. and urogenital openings.

Functions

Common functions of the skin are:

1. Protection: An anatomical barrier between the internal and external environment in bodily defense; Langerhans cells in the skin are part of the adaptive immune system
2. Sensation: Contains a variety of sensory nerve endings that react to heat, cold, touch, pressure, vibration, and tissue injury.
3. Heat regulation: The skin contains a blood supply far greater than its requirements, which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles are significant in animals.
4. Control of evaporation: The skin provides a relatively dry and impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in burns.
5. Aesthetics and communication: Others see our skin and can assess our mood, physical state, and attractiveness.
6. Storage and synthesis: Acts as a storage center for lipids and water, as well as a means of synthesis of vitamin D and B by action of UV on certain parts of the skin. This synthesis is linked to pigmentation, with darker skin producing more vitamin B than D, and vice versa.
7. Excretion: The concentration of urea is 1/130th that of urine. Excretion by sweating is at most a secondary function to temperature regulation.
8. Absorption: Oxygen, nitrogen, and carbon dioxide can diffuse into the epidermis in small amounts, and some animals using their skin for their sole respiration organ. In addition, medicine can be administered through the skin, by ointments or by means of an adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.

Hygiene

The skin must be regularly cleaned; unless enough care is taken, it will become cracked or inflamed. Unclean skin favors the development of pathogenic organisms. The dead cells that continually slough off of the epidermis mix with the secretions of the sweat and sebaceous glands and the dust found on the skin to form a filthy layer on its surface. If not washed away, the slurry of sweat and sebaceous secretions mixed with dirt and dead skin is decomposed by bacterial flora, producing a foul smell. Functions of the skin are disturbed when it is excessively dirty; it becomes more easily damaged, the release of antibacterial compounds decreases, and dirty skin is more prone to develop infections. Cosmetics should be used carefully because these may cause allergic reactions. Each season requires suitable clothing in order to facilitate the evaporation of the sweat. Sunlight, water, and air play an important role in keeping the skin healthy.

The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning. Estimates place the number of individual bacteria on the surface of one square inch of human skin at 50 million, although this figure varies greatly over the average 20 feet² of human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite these vast quantities, all of the bacteria found on the skin's surface would fit into a volume the size of a pea (Rosebury 1969).

In general, microorganisms on the skin keep one another in check and are part of a healthy skin. When the balance is disturbed, there may be an overgrowth and infection, such as when antibiotics kill microbes, resulting in an overgrowth of yeast. The skin is continuous with the inner epithelial lining of the body at the orifices, each of which supports its own complement of microbes.

Variability in skin tone

Individuals with ancestors from different parts of the world have highly visible differences in skin pigmentation. Individuals with African ancestry tend towards darker skin, while those of Northern European descent have paler skin. Between these extremes are individuals of Asian, South-East Asian, Native American, Middle Eastern, Polynesian, and Melanesian descent.

The skin of black people has more variation in color from one part of the body to another than does the skin of other racial groups, particularly the palms of the hands and soles of the feet. Part of this is the result of the variations in the thickness of the skin on different parts of the human body. The thicker the skin, the more layers of cells with melanin in them, and the darker the color (Smith and Burns 1999). In addition, these parts of the body do not have melanin-producing cells.

Historically, efforts have been made to designate various human populations as distinct "races" based on skin color, along with such other observable physical traits as hair type, facial features, and body proportions. However, today many scientists from diverse fields believe that the concept of distinct human races is unscientific and that there are no distinct races as previously claimed. (See section on races in the article "human being".) The recognition of different races, along with preferences toward particular groups, or exploitation or domination of other groups, is sometimes identified with the term racism. From a biological point of view, in which species are recognized as actually or potentially interbreeding natural populations, racism could be defined fundamentally as not being willing to marry, or have one's children marry, a person of another race.

Disease

In medicine, the branch concerned with the skin is called dermatology. The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, including:

Tumors:

• Benign tumors of the skin such as Squamous cell papilloma
• Skin cancer

Others:

• Rashes
• Blisters
• Acne
• Keratosis pilaris
• Fungal infections such as athlete's foot
• Microbial infections.
• Calcinosis cutis
• Sunburn
• Keloid
• Scabies

As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself. Skin sagging is caused by the fall in elasticity. Skin also receives less blood flow and lower gland activity.

Animal skin products

Skins and hides from different animals are used for clothing, bags, and other consumer products, usually in the form of leather, but also furs, rawhide, snakeskin, and hagfish. Skin can also be used to make products such as gelatin and glue.

References

ISBN links support NWE through referral fees

• Cannell, J.J., et al. 2006. Epidemic influenza and vitamin D. Epidemiology and Infection 136(December):1129-1140.
• Rosebury, T. 1969. Life on Man. Secker & Warburg. ISBN 0670427934
• Smith, W., and C. Burns. 1999. Managing the hair and skin of African American pediatric patients. Journal of Pediatric Health Care 13(2):72-8.