Human skin
Encyclopedia
The human skin is the outer covering of the body. In human
s, it is the largest organ of the integumentary system
. The skin has multiple layers of ectodermal tissue
and guards the underlying muscle
s, bone
s, ligament
s and internal organ
s. Human skin is similar to that of most other mammal
s, except that it is not protected by a pelt
. Though nearly all human skin is covered with hair follicle
s, it appears hairless. There are two general types of skin, hairy and glabrous skin. 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 pathogen
s 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 at 1000 species from 19 phyla
.
ation, or melanin
provided by melanocyte
s, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight
. It also contains DNA
-repair enzyme
s that help reverse UV damage, and people who lack the gene
s 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
.
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 metres (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 1,000 nerve endings.
s of microorganism
s, including yeast
s 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 20 square feet (1.9 m²) 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 antibiotic
s 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.
The negative aspect of the oily-skin type is that oily complexions are especially susceptible to clogged pores, blackhead
s, 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 wax
es 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.
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.
A validated comprehensive grading scale has categorized the clinical findings of skin aging as laxity (sagging), rhytids (wrinkles), and the various facets of photoaging, including erythema/telangiectasia (redness), dyspigmentation (brown discoloration), solar elastosis (yellowing), keratoses (abnormal growths) and poor texture.
Cortisol
causes degradation of collagen
, accelerating skin aging.
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.
s and skin neoplasms (including skin cancer).
Dermatology
is the branch of medicine
that deals with conditions of the skin.
n ancestry (black people
) tend towards darker skin, while those of Northern Europe
an descent (Nordic people
) have paler skin. Between these extremes are individuals of Asia
n, South-East Asian, Native American
, Middle East
ern, Polynesia
n and Melanesia
n 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.
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 ones. The areas with least similarity between people in species were the spaces between finger
s, the spaces between toe
s, axillae, and umbilical cord
stump. Most similarly were beside the nostril
, nares (inside
the 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 NIH 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.
Skin is composed of three primary layers:
with an underlying basal lamina
.
The epidermis contains no blood vessel
s, 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 cell
s, keratinocyte
s, with melanocyte
s and Langerhans cell
s 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.
s, and is nourished by diffusion
from the dermis. The main type of cells which make up the epidermis are keratinocyte
s, melanocyte
s, Langerhans cell
s and Merkels cells. The epidermis helps the skin to regulate body temperature.
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.
Mnemonic
s that are good for remembering the layers of the skin (using "stratum basale" instead of "stratum germinativum"):
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.
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.
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
.
, 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.
as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblast
s, macrophage
s and adipocyte
s (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.
is the ability of foreign substances to penetrate and diffuse through the skin. Skin naturally has a low permeability, thus protects the body from particles and foreign toxins by not allowing them to penetrate through the surface. However, technologies in nanomedicine
and biology
have led to developments in techniques to increase permeability of skin for various applications. The stratum corneum is the outermost layer of skin and is an effective barrier to most inorganic nanosized particles. Nanomedicine researchers are interested in nanoparticles that can penetrate the stratum corneum and settle in the epidermis where cells primarily reproduce. If the nanoparticles are targeted to surround cancer cells, they can be used to map where the cancer is located and deliver therapeutic agents directly to the site.
s and reach the dermis layer.
The permeability of skin relative to different shapes of nanoparticles has also been studied. Research has shown that spherical particles have a better ability to penetrate the skin better than oblong
(ellipsoidal) particles because spheres are symmetric in all three spacial dimensions. One study compared the two shapes and recorded data that showed spherical particles located deep in the epidermis and dermis whereas ellipsoidal particles were mainly found in the stratum corneum and epidermal layers. Nanorods are used in experiments because of their unique fluorescent properties but have shown mediocre penetration.
Nanoparticles of different materials have shown skin’s permeability limitations. In many experiments, gold nanoparticles 40 nm in diameter or smaller are used and have shown to penetrate to the epidermis. Titanium oxide
(TiO2), zinc oxide
(ZnO), and silver nanoparticles
are ineffective in penetrating the skin past the stratum corneum. Cadmium selenide
(CdSe) quantum dots have proven to penetrate very effectively when they have certain properties. Because CdSe is toxic to living organisms, the particle must be covered in a surface group. An experiment comparing the permeability of quantum dots coated in polyethylene glycol
(PEG), PEG-amine
, and carboxylic acid
concluded the PEG and PEG-amine surface groups allowed for the greatest penetration of particles. The carboxylic acid coated particles did not penetrate past the stratum corneum.
(TEWL), though it may take 3–5 days for the TEWL to reach its peak value. When the TEWL reaches its highest value, the maximum density of nanoparticles is able to permeate the skin. Studies confirm that UVR damaged skin significantly increases the permeability. The effects of increased permeability after UVR exposure can lead to an increase in the number of particles that permeate the skin. However, the specific permeability of skin after UVR exposure relative to particles of different sizes and materials has not been determined.
Other skin damaging methods used to increase nanoparticle penetration include tape stripping, skin abrasion, and chemical enhancement. Tape stripping is the process in which tape is applied to skin then lifted to remove the top layer of skin. Skin abrasion is done by shaving the top 5-10 micrometers off the surface of the skin. Chemical enhancement is the process in which chemicals such as polyvinylpyrrolidone (PVP), dimethyl sulfoxide
(DMSO), and oleic acid
are applied to the surface of the skin to increase permeability.
Electroporation
is the application of short pulses of electric fields on skin and has proven to increase skin permeability. The pulses are high voltage and on the order of milliseconds when applied. Charged molecules
penetrate the skin more frequently than neutral molecules after the skin has been exposed to electric field pulses. Results have shown molecules on the order of 100 micrometers to easily permeate electroporated skin.
because of the possibility of a painless application of therapeutic agents with very few side effects. Transdermal patches have been limited to administer a small number of drugs, such as nicotine
, because of the limitations in permeability of the skin. Development of techniques that increase skin permeability has led to more drugs that can be applied via transdermal patches and more options for patients.
Increasing the permeability of skin allows nanoparticles to penetrate and target cancer
cells. Nanoparticles along with multi-modal imaging
techniques have been used as a way to diagnose cancer non-invasively. Skin with high permeability allowed quantum dots with an antibody
attached to the surface for active targeting to successfully penetrate and identify cancerous tumors in mice. Tumor targeting is beneficial because the particles can be excited using fluorescence microscopy and emit light energy and heat that will destroy cancer cells.
Sunblock
Sunblock is opaque and is stronger than sunscreen since it is able to block majority of the UVA/UVB rays and radiation from the sun, thus not having to be reapplied several times a day. Titanium dioxide and zinc oxide are two of the important ingredients in sunblock.
Sunscreen
Sunscreen is more transparent once applied to the skin and also has the ability to protect against UVA/UVB rays as well, although the sunscreen's ingredients have the ability to break down at a faster rate once exposed to sunlight, and some of the radiation is able to penetrate to the skin. In order for sunscreen to be more effective it is necessary to consistently reapply and use a higher spf.
, also known as retinoids, benefits the skin by normalizing keratinization, downregulating sebum production which contributes to acne
, and reversing and treating photodamage, striae, and cellulite
.
Vitamin D
and analogs are used to downregulate the cutaneous immune system and epithelial proliferation while promoting differentiation.
Vitamin C
is an antioxidant
that regulates collagen synthesis, forms barrier lipids, regenerates vitamin E, and provides photoprotection.
Vitamin E
is a membrane antioxidant that protects against oxidative damage and also provides protection against harmful UV
rays.
Human
Humans are the only living species in the Homo genus...
s, it is the largest organ of the integumentary system
Integumentary system
The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...
. The skin has multiple layers of ectodermal tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
and guards the underlying muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s, ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
s and internal organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s. Human skin is similar to that of most other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, except that it is not protected by a pelt
Fur
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...
. Though nearly all human skin is covered with hair follicle
Hair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s, it appears hairless. There are two general types of skin, hairy and glabrous skin. 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 pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s and excessive water loss. Its other functions are insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
regulation, sensation, synthesis of vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue
Scar
Scars are areas of fibrous tissue that replace normal skin after injury. A scar results from the biological process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound results in...
. This is often discolored and depigmented.
In humans, skin pigmentation
Human skin color
Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...
varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habit for bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
which number roughly at 1000 species from 19 phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
.
Skin components
Skin has mesodermal cells, pigmentPigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
ation, or melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
provided by melanocyte
Melanocyte
-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...
s, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
. It also contains DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
-repair enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s that help reverse UV damage, and people who lack the gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s for these enzymes suffer high rates of skin cancer
Skin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...
. One form predominantly produced by UV light, malignant
Malignant
Malignancy is the tendency of a medical condition, especially tumors, to become progressively worse and to potentially result in death. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis...
melanoma
Melanoma
Melanoma is a malignant tumor of melanocytes. Melanocytes are cells that produce the dark pigment, melanin, which is responsible for the color of skin. They predominantly occur in skin, but are also found in other parts of the body, including the bowel and the eye...
, 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
Human skin color
Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...
.
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 metres (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 1,000 nerve endings.
Functions
Skin performs the following functions:- Protection: an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense; Langerhans cellLangerhans cellLangerhans cells are dendritic cells of the skin and mucosa, and contain large granules called Birbeck granules. They are present in all layers of the epidermis, but are most prominant in the stratum spinosum. They also occur in the papillary dermis, particularly around blood vessels, as well as...
s in the skin are part of the adaptive immune systemAdaptive immune systemThe adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic growth. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate...
. - Sensation: contains a variety of nerve endings that react to heat and coldThermoreceptorA thermoreceptor is a sensory receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range...
, touch, pressure, vibration, and tissue injury; see somatosensory systemSomatosensory systemThe somatosensory system is a diverse sensory system composed of the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception , and nociception . The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal...
and hapticsHapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
. - 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.
- 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 burnsBurn (injury)A burn is a type of injury to flesh caused by heat, electricity, chemicals, light, radiation or friction. Most burns affect only the skin . Rarely, deeper tissues, such as muscle, bone, and blood vessels can also be injured...
. - Aesthetics and communication: others see our skin and can assess our mood, physical state and attractiveness.
- Storage and synthesis: acts as a storage center for lipids and water, as well as a means of synthesis of vitamin DVitamin DVitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
by action of UVUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
on certain parts of the skin. - Excretion: sweatSWEATSWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004.Each of the 13 half-hour episodes of SWEAT features a different outdoor sport: kayaking, mountain biking, ice hockey, beach volleyball, soccer, windsurfing, rowing, Ultimate, triathlon, wakeboarding, snowboarding, telemark...
contains ureaUreaUrea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, however its concentration is 1/130th that of urineUrineUrine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, hence excretionExcretionExcretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
by sweating is at most a secondary function to temperature regulation. - Absorption: the cells comprising the outermost 0.25–0.40 mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total respiration is negligible". In addition, medicine can be administered through the skin, by ointments or by means of adhesive patchTransdermal patchA transdermal patch is a medicated adhesive patch that is placed on the skin to deliver a specific dose of medication through the skin and into the bloodstream. Often, this promotes healing to an injured area of the body. An advantage of a transdermal drug delivery route over other types of...
, such as the nicotine patchNicotine patchA nicotine patch is a transdermal patch that releases nicotine into the body through the skin. It is used as an aid in nicotine replacement therapy , a process for smoking cessation. The first published study of the pharmacokinetics of a transdermal nicotine patch in humans was authored by Jed E....
or iontophoresisIontophoresisIontophoresis is a technique using a small electric charge to deliver a medicine or other chemical through the skin. It is basically an injection without the needle...
. The skin is an important site of transport in many other organisms. - Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body.
Pigments
There are at least five different pigments that determine the color of the skin. These pigments are present at different levels and places.- MelaninMelaninMelanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
: It is brown in color and present in the germinative zone of the epidermis. - Melanoid: It resembles melanin but is present diffusely throughout the epidermis.
- CaroteneCaroteneThe term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...
: This pigment is yellow to orange in color. It is present in the stratum corneum and fat cells of dermis and superficial fascia. - HemoglobinHemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
(also spelled haemoglobin): It is found in blood and is not a pigment of the skin but develops a purple color. - Oxyhemoglobin: It is also found in blood and is not a pigment of the skin. It develops a red color.
Hygiene and skin care
The skin supports its own ecosystemEcosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s of microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s, including yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
s 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 20 square feet (1.9 m²) 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 antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s 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, blackhead
Blackhead
A blackhead is a yellow or blackish bump or plug on the skin. A blackhead is a type of acne vulgaris. Contrary to the common belief that it is caused by poor hygiene, blackheads are caused by excess oils that have accumulated in the sebaceous gland's duct...
s, 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
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
) Oily skin products should contain very little natural oils. They should not contain wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
es 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
Stratum corneum
The stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress...
. Those products should be used on a regular basis to work adequately.
Aging
Among other things, skin aging is noted by a decrease in volume and elasticity. There are many internal and external causes
Intrinsic and extrinsic aging
Intrinsic aging and Extrinsic aging are terms used to describe cutaneous aging of the skin and other parts of the integumentary system, which while having epidermal concomitants, seems to primarily involve the dermis. Intrinsic aging is influenced by internal physiological factors alone, and...
to skin aging. For example: Aging skin receives less blood flow and lower glandular activity.
A validated comprehensive grading scale has categorized the clinical findings of skin aging as laxity (sagging), rhytids (wrinkles), and the various facets of photoaging, including erythema/telangiectasia (redness), dyspigmentation (brown discoloration), solar elastosis (yellowing), keratoses (abnormal growths) and poor texture.
Cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
causes degradation of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, accelerating skin aging.
Photoaging
Photoaging has two main concerns: an increased risk for skin cancerSkin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...
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
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
layer being damaged.
Disease
Diseases of the skin include skin infectionSkin infection
A skin infection is an infection of the skin. Infection of the skin is distinguished from dermatitis, which is inflammation of the skin, but a skin infection can result in skin inflammation...
s and skin neoplasms (including skin cancer).
Dermatology
Dermatology
Dermatology is the branch of medicine dealing with the skin and its diseases, a unique specialty with both medical and surgical aspects. A dermatologist takes care of diseases, in the widest sense, and some cosmetic problems of the skin, scalp, hair, and nails....
is the branch of medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
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 AfricaSub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
n ancestry (black people
Black people
The term black people is used in systems of racial classification for humans of a dark skinned phenotype, relative to other racial groups.Different societies apply different criteria regarding who is classified as "black", and often social variables such as class, socio-economic status also plays a...
) tend towards darker skin, while those of Northern Europe
Northern Europe
Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...
an descent (Nordic people
Nordic countries
The Nordic countries make up a region in Northern Europe and the North Atlantic which consists of Denmark, Finland, Iceland, Norway and Sweden and their associated territories, the Faroe Islands, Greenland and Åland...
) have paler skin. Between these extremes are individuals of Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
n, South-East Asian, Native American
Native Americans in the United States
Native Americans in the United States are the indigenous peoples in North America within the boundaries of the present-day continental United States, parts of Alaska, and the island state of Hawaii. They are composed of numerous, distinct tribes, states, and ethnic groups, many of which survive as...
, Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
ern, Polynesia
Polynesia
Polynesia is a subregion of Oceania, made up of over 1,000 islands scattered over the central and southern Pacific Ocean. The indigenous people who inhabit the islands of Polynesia are termed Polynesians and they share many similar traits including language, culture and beliefs...
n and Melanesia
Melanesia
Melanesia is a subregion of Oceania extending from the western end of the Pacific Ocean to the Arafura Sea, and eastward to Fiji. The region comprises most of the islands immediately north and northeast of Australia...
n 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.
| 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 bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
from 19 bacterial phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
have been found. Most come from only four phyla: Actinobacteria
Actinobacteria
Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
(51.8%), Firmicutes
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
(24.4%), Proteobacteria
Proteobacteria
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
(16.5%), and Bacteroidetes
Bacteroidetes
The phylum Bacteroidetes is composed of three large classes of bacteria that are widely distributed in the environment, including in soil, in sediments, sea water and in the guts and on the skin of animals....
(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
Proteobacteria
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
and Flavobacteriales
Flavobacteriales
The order Flavobacteriales is composed of three families of environmental bacteria....
. Ecologically, sebaceous areas had greater species richness than moist and dry ones. The areas with least similarity between people in species were the spaces between finger
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
s, the spaces between toe
Toe
Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being digitigrade. Humans, and other animals that walk on the soles of their feet, are described as being plantigrade; unguligrade animals are those that walk on hooves at the tips of...
s, axillae, and umbilical cord
Umbilical cord
In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
stump. Most similarly were beside the nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...
, nares (inside
the nostril), and on the back.
Reflecting upon the diversity of the human skin researchers on the human skin microbiome
Microbiome
A microbiome is the totality of microbes, their genetic elements , and environmental interactions in a defined environment. A defined environment could, for example, be the gut of a human being or a soil sample. Thus, microbiome usually includes microbiota and their complete genetic elements...
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 NIH has been launched the Human Microbiome Project
Human microbiome project
The Human Microbiome Project is a United States National Institutes of Health initiative with the goal of identifying and characterizing the microorganisms which are found in association with both healthy and diseased humans . Launched in 2008, it is a five-year project, best characterized as a...
to characterize the human microbiota which includes that on the skin and the role of this microbiome in health and disease.
Skin layers
Skin is composed of three primary layers:
- the epidermis, which provides waterproofing and serves as a barrier to infection;
- the dermisDermisThe dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
, which serves as a location for the appendages of skin; and - the hypodermisHypodermisThe hypodermis, also called the hypoderm, subcutaneous tissue, or superficial fascia is the lowermost layer of the integumentary system in vertebrates. Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages...
(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 epitheliumEpithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
with an underlying basal lamina
Basal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
.
The epidermis contains no blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s, 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 cell
Merkel cell
Merkel cells or Merkel-Ranvier cells are oval receptor cells found in the skin of vertebrates that have synaptic contacts with somatosensory afferents. They are associated with the sense of light touch discrimination of shapes and textures. They can turn malignant and form the skin tumor known as...
s, keratinocyte
Keratinocyte
Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"...
s, with melanocyte
Melanocyte
-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...
s and Langerhans cell
Langerhans cell
Langerhans cells are dendritic cells of the skin and mucosa, and contain large granules called Birbeck granules. They are present in all layers of the epidermis, but are most prominant in the stratum spinosum. They also occur in the papillary dermis, particularly around blood vessels, as well as...
s 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
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
at the basale layer. The daughter cells (see cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
) 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
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
is inserted. They eventually reach the corneum and slough off (desquamation
Desquamation
Desquamation , also called skin peeling, is the shedding of the outermost membrane or layer of a tissue, such as the skin.-Skin:Normal, nonpathologic desquamation of the skin occurs when keratinocytes, after moving apically over about 14 days, are individually shed unnoticeably...
). 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.
Components
The epidermis contains no blood vesselBlood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s, and is nourished by diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
from the dermis. The main type of cells which make up the epidermis are keratinocyte
Keratinocyte
Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"...
s, melanocyte
Melanocyte
-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...
s, Langerhans cell
Langerhans cell
Langerhans cells are dendritic cells of the skin and mucosa, and contain large granules called Birbeck granules. They are present in all layers of the epidermis, but are most prominant in the stratum spinosum. They also occur in the papillary dermis, particularly around blood vessels, as well as...
s and Merkels cells. The epidermis helps the skin to regulate body temperature.
Layers
Epidermis is divided into several layers where cells are formed through mitosisMitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
. 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:- Stratum corneumStratum corneumThe stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress...
- Stratum lucidumStratum lucidumThe stratum lucidum is a thin, clear layer of dead skin cells in the epidermis named for its translucent appearance under a microscope...
- Stratum granulosumStratum granulosumThe stratum granulosum is a thin layer of cells in the epidermis. Keratinocytes migrating from the underlying stratum spinosum become known as granular cells in this layer...
- Stratum spinosumStratum spinosumThe stratum spinosum is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is also referred to as the "spinous" or "prickle-cell" layer. This appearance is due to desmosomal connections of adjacent cells. Keratinization begins in the stratum spinosum....
- Stratum germinativumStratum germinativumThe stratum basale is the deepest layer of the five layers of the epidermis, which is the outer covering of skin in mammals. The stratum basale is a continuous layer of cells...
(also called "stratum basale")
Mnemonic
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
s that are good for remembering the layers of the skin (using "stratum basale" instead of "stratum germinativum"):
- "Can Long Get Some Burgers" (from superficial to deep)
- "Cher Likes Getting Skin Botoxed" (from superficial to deep)
- "Before Signing, Get Legal Counsel" (from deep to superficial)
- "Bare Skin Grows Like Corn" (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 tissueConnective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
. 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
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
. Friction ridges occur in patterns (see: fingerprint
Fingerprint
A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
) 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
Forensic identification
Forensic identification is the application of forensic science, or "forensics", and technology to identify specific objects from the trace evidence they leave, often at a crime scene or the scene of an accident. Forensic means "for the courts"....
.
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, elasticElasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
, and reticular fibers that weave throughout it. These protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
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
Cutaneous receptor
A cutaneous receptor is a type of sensory receptor found in the dermis or epidermis. They are a part of the somatosensory system. Cutaneous receptors include e.g...
, nails
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....
, 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 muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblast
Fibroblast
A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...
s, macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s and adipocyte
Adipocyte
However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...
s (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body.
Microorganisms like Staphylococcus
Staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....
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.
Permeability
Human skin permeabilityPermeation
Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...
is the ability of foreign substances to penetrate and diffuse through the skin. Skin naturally has a low permeability, thus protects the body from particles and foreign toxins by not allowing them to penetrate through the surface. However, technologies in nanomedicine
Nanomedicine
Nanomedicine is the medical application of nanotechnology. Nanomedicine ranges from the medical applications of nanomaterials, to nanoelectronic biosensors, and even possible future applications of molecular nanotechnology. Current problems for nanomedicine involve understanding the issues related...
and biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
have led to developments in techniques to increase permeability of skin for various applications. The stratum corneum is the outermost layer of skin and is an effective barrier to most inorganic nanosized particles. Nanomedicine researchers are interested in nanoparticles that can penetrate the stratum corneum and settle in the epidermis where cells primarily reproduce. If the nanoparticles are targeted to surround cancer cells, they can be used to map where the cancer is located and deliver therapeutic agents directly to the site.
Nanoparticles
Nanoparticles 40 nm in diameter and smaller have been successful in penetrating the skin. Research confirms that nanoparticles larger than 40 nm do not penetrate the skin past the stratum corneum. Most particles that do penetrate will diffuse through skin cells, but some will travel down hair follicleHair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s and reach the dermis layer.
The permeability of skin relative to different shapes of nanoparticles has also been studied. Research has shown that spherical particles have a better ability to penetrate the skin better than oblong
Rectangle
In Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. The term "oblong" is occasionally used to refer to a non-square rectangle...
(ellipsoidal) particles because spheres are symmetric in all three spacial dimensions. One study compared the two shapes and recorded data that showed spherical particles located deep in the epidermis and dermis whereas ellipsoidal particles were mainly found in the stratum corneum and epidermal layers. Nanorods are used in experiments because of their unique fluorescent properties but have shown mediocre penetration.
Nanoparticles of different materials have shown skin’s permeability limitations. In many experiments, gold nanoparticles 40 nm in diameter or smaller are used and have shown to penetrate to the epidermis. Titanium oxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
(TiO2), zinc oxide
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
(ZnO), and silver nanoparticles
Silver nanoparticles
Silver nanoparticles are nanoparticles of silver, i.e. silver particles of between 1 nm and 100 nm in size. While frequently described as being 'silver' some are composed of a large percentage of silver oxide due to their large ratio of surface-to-bulk silver atoms.-Synthesis:There are...
are ineffective in penetrating the skin past the stratum corneum. Cadmium selenide
Cadmium selenide
Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
(CdSe) quantum dots have proven to penetrate very effectively when they have certain properties. Because CdSe is toxic to living organisms, the particle must be covered in a surface group. An experiment comparing the permeability of quantum dots coated in polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
(PEG), PEG-amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
, and carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
concluded the PEG and PEG-amine surface groups allowed for the greatest penetration of particles. The carboxylic acid coated particles did not penetrate past the stratum corneum.
Increasing permeability
Scientists believed that the skin was an effective barrier to all inorganic particles and alterations to the skin using mechanical stressors causing damage to the skin would be the only way to increase permeability. However, simpler and more effective methods have been developed. For example, ultraviolet radiation (UVR) has been used to slightly damage the surface of skin, causing a time-dependent defect allowing easier penetration of nanoparticles. The UVR’s high energy causes a restructuring of cells, weakening the boundary between the stratum corneum and the epidermal layer. The damage of the skin is typically measured by the transepidermal water lossTransepidermal water loss
Transepidermal Water Loss or is a term associated with dermatology and connected sciences.It is defined as the measurement of the quantity of water that passes from inside a body through the epidermal layer to the surrounding atmosphere via diffusion and evaporation processes.Transepidermal...
(TEWL), though it may take 3–5 days for the TEWL to reach its peak value. When the TEWL reaches its highest value, the maximum density of nanoparticles is able to permeate the skin. Studies confirm that UVR damaged skin significantly increases the permeability. The effects of increased permeability after UVR exposure can lead to an increase in the number of particles that permeate the skin. However, the specific permeability of skin after UVR exposure relative to particles of different sizes and materials has not been determined.
Other skin damaging methods used to increase nanoparticle penetration include tape stripping, skin abrasion, and chemical enhancement. Tape stripping is the process in which tape is applied to skin then lifted to remove the top layer of skin. Skin abrasion is done by shaving the top 5-10 micrometers off the surface of the skin. Chemical enhancement is the process in which chemicals such as polyvinylpyrrolidone (PVP), dimethyl sulfoxide
Dimethyl sulfoxide
Dimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
(DMSO), and oleic acid
Oleic acid
Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...
are applied to the surface of the skin to increase permeability.
Electroporation
Electroporation
Electroporation, or electropermeabilization, is a significant increase in the electrical conductivity and permeability of the cell plasma membrane caused by an externally applied electrical field...
is the application of short pulses of electric fields on skin and has proven to increase skin permeability. The pulses are high voltage and on the order of milliseconds when applied. Charged molecules
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
penetrate the skin more frequently than neutral molecules after the skin has been exposed to electric field pulses. Results have shown molecules on the order of 100 micrometers to easily permeate electroporated skin.
Applications
A large area of interest in nanomedicine is the transdermal patchTransdermal patch
A transdermal patch is a medicated adhesive patch that is placed on the skin to deliver a specific dose of medication through the skin and into the bloodstream. Often, this promotes healing to an injured area of the body. An advantage of a transdermal drug delivery route over other types of...
because of the possibility of a painless application of therapeutic agents with very few side effects. Transdermal patches have been limited to administer a small number of drugs, such as nicotine
Nicotine
Nicotine is an alkaloid found in the nightshade family of plants that constitutes approximately 0.6–3.0% of the dry weight of tobacco, with biosynthesis taking place in the roots and accumulation occurring in the leaves...
, because of the limitations in permeability of the skin. Development of techniques that increase skin permeability has led to more drugs that can be applied via transdermal patches and more options for patients.
Increasing the permeability of skin allows nanoparticles to penetrate and target cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
cells. Nanoparticles along with multi-modal imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
techniques have been used as a way to diagnose cancer non-invasively. Skin with high permeability allowed quantum dots with an antibody
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
attached to the surface for active targeting to successfully penetrate and identify cancerous tumors in mice. Tumor targeting is beneficial because the particles can be excited using fluorescence microscopy and emit light energy and heat that will destroy cancer cells.
Sunblock and sunscreen
Although some believe that sunblock and sunscreen are both the same, they are not, although they have similar properties and are both important in caring of the skin.Sunblock
Sunblock is opaque and is stronger than sunscreen since it is able to block majority of the UVA/UVB rays and radiation from the sun, thus not having to be reapplied several times a day. Titanium dioxide and zinc oxide are two of the important ingredients in sunblock.
Sunscreen
Sunscreen is more transparent once applied to the skin and also has the ability to protect against UVA/UVB rays as well, although the sunscreen's ingredients have the ability to break down at a faster rate once exposed to sunlight, and some of the radiation is able to penetrate to the skin. In order for sunscreen to be more effective it is necessary to consistently reapply and use a higher spf.
Nutrition for healthy skin
Vitamin AVitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
, also known as retinoids, benefits the skin by normalizing keratinization, downregulating sebum production which contributes to acne
Acne
Acne is a general term used for acneiform eruptions. It is usually used as a synonym for acne vulgaris, but may also refer to:*Acne aestivalis*Acne conglobata*Acne cosmetica*Acne fulminans*Acne keloidalis nuchae*Acne mechanica...
, and reversing and treating photodamage, striae, and cellulite
Cellulite
Cellulite is a topographic skin change that occurs in most postpubertal females. It presents as a modification of skin topography evident by skin dimpling and nodularity that occurs mainly in women on the pelvic region, lower limbs, and abdomen, and is caused by the herniation of subcutaneous fat...
.
Vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
and analogs are used to downregulate the cutaneous immune system and epithelial proliferation while promoting differentiation.
Vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...
is an antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
that regulates collagen synthesis, forms barrier lipids, regenerates vitamin E, and provides photoprotection.
Vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...
is a membrane antioxidant that protects against oxidative damage and also provides protection against harmful UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
rays.
See also
- Acid mantleAcid mantleThe acid mantle is a very fine, slightly acidic film on the surface of the skin acting as a barrier to bacteria, viruses and other potential contaminants that might penetrate the skin. The pH of the skin is between 4.5 and 6.2, so it is acidic...
- Anthropodermic bibliopegyAnthropodermic bibliopegyAnthropodermic bibliopegy is the practice of binding books in human skin. Though extremely uncommon in modern times, the technique dates back to at least the 17th century...
- Artificial skinArtificial skinArtificial skin can refer to skin grown in a laboratory that can be used as skin replacement for people who have suffered skin trauma such as severe burns or skin diseases.Alternatively, it can also refer to skin synthetically produced for other purposes....
- CallusCallusA callus is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses...
, thick area of skin - List of cutaneous conditions
- Cutaneous structure developmentCutaneous structure developmentCutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails.During embryogenesis, the epidermis splits into two layers: the periderm and the basal layer. The basal layer is a stem cell layer and through asymmetrical divisions, becomes...
- FingerprintFingerprintA fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
, skin on fingertips - HyperpigmentationHyperpigmentationIn dermatology, hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.-Causes:Hyperpigmentation may be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris...
, about excess skin color - IntertriginousIntertriginousIntertriginous is a medical term used to define an area where two skin areas may touch or rub together. Examples of intertriginous areas are the axilla of the arm, the anogenital region, nares, skin folds of the breasts, and between digits...
- Meissner's corpuscleMeissner's corpuscleMeissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
- Pacinian corpusclePacinian corpuscleLamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs...
- Polyphenol antioxidantPolyphenol antioxidantA polyphenol antioxidant is a type of antioxidant containing a polyphenolic substructure. Numbering over 4,000 distinct species, many of these compounds have antioxidant activity in vitro but are unlikely to have antioxidant roles in vivo...
- Skin lesion
- Skin repairSkin repairProtection from mechanical injury, chemical hazards, and bacterial invasion is provided by the skin because the epidermis is relatively thick and covered with keratin. Secretions from sebaceous glands and sweat glands also benefit this protective barrier. In the event of an injury that damages the...
- Superficial fasciaSuperficial fasciaSuperficial fascia is found in the subcutis in most regions of the body, blending with the reticular layer of the dermis. It is present on the face, over the upper portion of the sternocleidomastoid, at the nape of the neck, and overlying the sternum. It is mainly loose areolar connective tissue...
- Baumann skin types