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The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organ. Skin of a different nature exists in amphibians, reptiles, birds. Human skin is not unlike that of most other mammals except that it is not protected by a pelt and appears hairless though in fact nearly all human skin is covered with hair follicles. The adjective cutaneous literally means "of the skin" (from Latin cutis, skin).

Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.

In humans, skin pigmentation varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habit for bacteria which number roughly a 1000 species from 19 phyla.

Skin components

See also: Skin layers


Skin has mesodermal cells, pigmentation, or melanin, provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight. It also contains DNA-repair enzymes that help 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. Human skin pigmentation varies among populations in a striking manner. This has led to the classification of people(s) on the basis of skin color.

Mammalian skin often contains hairs, which in sufficient density is called fur. The 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 could quickly go to sleep.

The skin is the largest organ in the human body. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters (16.1-21.5 sq ft.), most of it is between 2–3 mm (0.10 inch) thick. The average square inch (6.5 cm²) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings.

Functions

Skin performs the following functions:

  1. Protection: an anatomical barrier from pathogens and damage 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 nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury; see somatosensory system and haptics.
  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 heatloss, 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 semi-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 by action of UV on certain parts of the skin.
  7. Excretion: sweat contains urea, however its concentration is 1/130th that of urine, hence 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, some animals using their skin for their sole respiration organ (contrary to popular belief, however, humans do not absorb oxygen through the skin). In addition, medicine can be administered through the skin, by ointments or by means of adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.
  9. Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body.


Hygiene and skin care

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 (6.5 square cm) of human skin at 50 million, though this figure varies greatly over the average of human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per square inch (6.5 cm²). Despite these vast quantities, all of the bacteria found on the skin's surface would fit into a volume the size of a pea. In general, the microorganisms 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.

Proper skin hygiene is important because unclean skin favors the development of pathogenic organisms. The dead cells that continually slough off the epidermis mix with the secretions of the sweat and sebaceous glands and the dust found on the skin 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 on the skin 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.

Oily Skin

Oily skin is caused by over-active sebaceous glands, that produce a substance called sebum, a naturally healthy skin lubricant. When the skin produces excessive sebum, it becomes heavy and thick in texture. Oily skin is typified by shininess, blemishes and pimples. The oily-skin type is not necessarily bad, since such skin is less prone to wrinkling, or other signs of aging, because the oil helps to keep needed moisture locked into the epidermis (outermost layer of skin).

The negative aspect of the oily-skin type is that oily complexions are especially susceptible to clogged pores, blackheads, and buildup of dead skin cells on the surface of the skin. Oily skin can be sallow and rough in texture and tends to have large, clearly visible pores everywhere, except around the eyes and neck.

The goal of treating oily skin is to remove excess surface sebum without complete removal of skin lipids. Severe degreasing treatment can foster an actual worsening of sebum secretion, which defeats the aim of the cleansing. A method of cleansing oily skin is to cleanse with a natural face cleanser formulated especially for oily skin. The cleansers pH should be 4.5 - 5.5, since the skin's pH value is approximately 5.4. Gel cleansers work best on oily skin. (see: surfactant) Oily skin products should contain very little natural oils. They should not contain waxes or other synthetic lipid agents that could aggravate the oily condition of the skin. A toning lotion should also be natural and have a pH of 4.5-5.5 and formulated especially to help balance and hydrate oily skin. Some cleansing products have lower concentrations of hydroxy acids, which remove dead cells from the upper levels of the stratum corneum. Those products should be used on a regular basis to work adequately.

In cases of excessive output of sebum, there have been anecdotal reports of successful control using dietary supplementation of Niacin (Vitamin B3) at a dosage of 500 mg to 1000 mg a day

Aging

A typical rash
As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself as a person ages.

Among other things, skin aging is noted by a decrease in volume and elasticity. There are many internal and external causes to skin aging. For example: Aging skin receives less blood flow and lower glandular activity.

Cortisol causes degradation of collagen, accelerating skin aging.

Photoaging

Photoaging has two main concerns: an increased risk for skin cancer and the appearance of damaged skin. In younger skin, sun damage will heal faster since the cells in the epidermis have a faster turnover rate, while in the older population the skin becomes thinner and the epidermis turnover rate for cell repair is lower which may result in the dermis layer being damaged.

Disease

Dermatology is the branch of medicine that deals with conditions of the skin.

Variability in skin tone

Individuals with ancestors from different parts of the world can have highly visible differences in skin pigmentation. Individuals with sub-Saharan African ancestry (black people) tend towards darker skin, while those of Northern European descent (white people) 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 body. The thicker the skin, the more layers of cells with melanin in them, and the darker the color. In conclusion, these parts of the body have melanin-producing cells.

Darker skin hinders UVA rays from penetrating. Because UVA degrades folate (a B vitamin) and is required for vitamin D synthesis, people with darker skin tones are more susceptible to deficiencies of these vitamins.

Skin types

Skin can be classified based on its reaction to ultraviolet radiation:
Type Definition Description
I Always burns, never tans Pale, Fair, Freckles
II Usually burns, sometimes tans Fair
III May burn, usually tans Light Brown
IV Rarely burns, always tans Olive brown
V Moderate constitutional pigmentation Brown
VI Marked constitutional pigmentation Black


Skin flora

The human skin is a rich environment for microbes. Around 1000 species of bacteria from 19 bacterial phyla have been found. Most come from only four phyla: Actinobacteria (51.8%), Firmicutes (24.4%), Proteobacteria (16.5%), and Bacteroidetes (6.3%). Propionibacteria and Staphylococci species were the main species in sebaceous areas. There are three main ecological areas: moist, dry and sebaceous. In moist places on the body Corynebacteria together with Staphylococci dominate. In dry areas, there is a mixture of species but dominated by b-Proteobacteria and Flavobacteriales. Ecologically, sebaceous areas had greater species richness than moist and dry one. The areas with least similarity between people in species were the spaces between fingers, the spaces between toes, axillae, and umbilical cord stump. Most similarly were beside the nostril, nares (insidethe nostril), and on the back.

Reflecting upon the diversity of the human skin researchers on the human skin microbiome have observed: "hairy, moist underarms lie a short distance from smooth dry forearms, but these two niches are likely as ecologically dissimilar as rainforests are to deserts."

The NIHmarker has been launched the Human Microbiome Project to characterize the human microbiota which includes that on the skin and the role of this microbiome in health and disease.

Animal skin products

The term skin refers to the covering of a small animal, such as a sheep, goat (goatskin), pig, snake (snakeskin) etc or the young of a large animal.

The term hides or rawhide refers to the covering of a large adult animal such as a cow, buffalo, horse etc.

Skins and hides from different animals are used for clothing, bags and other consumer products, usually in the form of leather, but also furs.

Skin can also be cooked to make pork rind or cracklin. The skin on roasted chicken and turkey is another coveted delicacy.

Skin layers

Skin is composed of three primary layers:
  • the epidermis, which provides waterproofing and serves as a barrier to infection;
  • the dermis, which serves as a location for the appendages of skin; and
  • the hypodermis (subcutaneous adipose layer).


Epidermis

Epidermis, "epi" coming from the Greek meaning "over" or "upon", 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.

The epidermis contains no blood vessels, and 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 which make up the epidermis are Merkel cells, keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through mitosis at the basale layer. The daughter cells (see cell division) 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. They eventually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 27 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.

[[Image:HautFingerspitzeOCT nonanimated.gif|thumb |209px| [also see:  image rotating ]

Optical Coherence Tomography tomogram of fingertip, depicting stratum corneum (~500 µm thick) with stratum disjunctum on top and stratum lucidum (connection to stratum spinosum) in the middle. At the bottom superficial parts of the dermis. Sweatducts are clearly visible.]]

Components

The epidermis contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, melanocytes, Langerhans cells and Merkels cells. The epidermis helps the skin to regulate body temperature.

Layers

Epidermis is divided into several layers where cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the top layer called stratum corneum and are sloughed off, or desquamated. This process is called keratinization and takes place within weeks. The outermost layer of the epidermis consists of 25 to 30 layers of dead cells.

Sublayers

Epidermis is divided into the following 5 sublayers or strata:



Mnemonics that are good for remembering the layers of the skin (using "stratum basale" instead of "stratum germinativum"):
  • "Cher Likes Getting Skin Botoxed" (from superficial to deep)
  • "Before Signing, Get Legal Counsel" (from deep to superficial)


Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.

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. It also harbors many Mechanoreceptor/nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from 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 fingerlike 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 and epigenetically 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.

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

Tattoo ink is held in 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 (the hypodermis contains 50% of 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 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.

In other vertebrates

The epidermis of fish and of most amphibians consists entirely of live cells, with only minimal quantities of keratin in the cells of the superficial layer. It is generally permeable, and, in the case of many amphibians, may actually be a major respiratory organ. The epidermis of birds and reptiles is closer to that of humans, with a layer of dead keratin-filled cells at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial amphibians, such as toads. However, in all of these animals there is no clear differentiation of the epidermis into distinct layers, as occurs in humans, with the change in cell type being relatively gradual. The mammalian epidermis always possesses at least a stratum germinativum and stratum corneum, but the other intermediate layers found in humans are not always distinguishable.

The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. Instead, in most species, it is largely replaced by solid, protective bony scale. Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins. Cartilaginous fish have numerous tooth-like denticles embedded in their skin, in place of true scales. Hair is a distinctive feature of mammalian skin, while feathers are (at least among living species) similarly unique to birds.

Although melanin is found in the skin of many species, in reptiles, amphibians, and fish, the epidermis is often relatively colourless. Instead, the colour of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as chameleons and flounders may be able to change the colour of their skin by adjusting the relative size of their chromatophores.

Sweat glands and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates. Fish typically have a numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have poison glands, photophores, or cells that produce a more watery, serous fluid. In amphibians, the mucus cells are gathered together to form sac-like glands. Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds.

Birds and reptiles have relatively few skin glands, although there may be a few structures for specific purposes, such as pheromone-secreting cells in some reptiles, or the uropygial gland of most birds.

See also



References

  1. "Skin care" (analysis), Health-Cares.net, 2007, webpage: HCcare.
  2. Alibardi L. (2003). Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes. J Exp Zoolog B Mol Dev Evol. 298(1):12-41. PMID 12949767
  3. Proksch E, Brandner JM, Jensen JM. (2008).The skin: an indispensable barrier. Exp Dermatol. 17(12):1063-72. PMID 19043850
  4. Madison KC. (2003). Barrier function of the skin: "la raison d'être" of the epidermis. J Invest Dermatol. 121(2):231-41. PMID 12880413
  5. Grice EA, Kong HH, Conlan S. (2009). Topographical and Temporal Diversity of the Human Skin Microbiome, Science, 324: 1190 - 1192.
  6. Pappas S. (2009). Your Body Is a Wonderland ... of Bacteria. ScienceNOW Daily News
  7. Connor, Steven: The book of skin, Cornell University Press, 2003, pg. 176
  8. Theodor Rosebury. Life on Man: Secker & Warburg, 1969 ISBN 0-670-42793-4
  9. http://www3.interscience.wiley.com/journal/107640112/abstract
  10. http://www.ingentaconnect.com/content/bsc/ics/2004/00000026/00000002/art00010
  11. Gilchrest BA: Skin aging and photoaging. Dermatology Nursing 1990 Apr; 2 (2): 79-82. ISSN: 1060-3441 PMID: 2141531 CINAHL AN: 1990116127
  12. Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology (4th ed.). Elsevier Inc. ISBN 1-4160-3185-5.
  13. Smith, Wilma and Burns, Catherine. (1999) "Managing the hair and skin of African American pediatric patients." Journal of Pediatric Health Care 13(2):72-8.
  14. NIH Human Microbiome Project.



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