Epidermis

The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and hypodermis.[1] The epidermis layer provides a barrier to infection from environmental pathogens[2] and regulates the amount of water released from the body into the atmosphere through transepidermal water loss.[3] The epidermis is composed of multiple layers of flattened cells[4] that overlie a base layer (stratum basale) composed of columnar cells arranged perpendicularly.

The rows of cells develop from stem cells in the basal layer. Cellular mechanisms for regulating water and sodium levels (ENaCs) are found in all layers of the epidermis.[5]

The word epidermis is derived through Latin from Ancient Greek epidermis, itself from Ancient Greek epi, meaning 'over, upon' and from Ancient Greek dermis, meaning 'skin'. Something related to or part of the epidermis is termed epidermal.

The human epidermis is a familiar example of epithelium, particularly a stratified squamous epithelium

Epidermis
Epidermis-delimited
Microscopic image of the epidermis, which constitutes the outer layer of skin, shown here by the white bar
Epidermal layers
Microscopic image showing the layers of the epidermis. The stratum corneum appears more compact in this image than above because of different sample preparation.
Details
Part ofSkin
SystemIntegumentary system
Identifiers
LatinEpidermis
MeSHD004817
TAA16.0.00.009
THH3.12.00.1.01001
FMA70596
Anatomical terms of microanatomy

Structure

Cellular components

The epidermis primarily consists of keratinocytes[4] (proliferating basal and differentiated suprabasal), which comprise 90% of its cells, but also contains melanocytes, Langerhans cells, Merkel cells,[6] and inflammatory cells. Epidermal thickenings called Rete ridges (or rete pegs) extend downward between dermal papillae.[7] Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. The epidermis itself has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air.[8]

Cell junctions

Epidermal cells are tightly interconnected to serve as a tight barrier against the exterior environment. The junctions between the epidermal cells are of the adherens junction type, formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells,[5] although the actin filaments are actually located inside the cell and run parallel to the cell membrane. Because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells.[5]

Layers

Skinlayers
Schematic image showing a section of epidermis, with epidermal layers labeled

The epidermis is composed of 4 or 5 layers, depending on the region of skin being considered.[9] Those layers in descending order are:[2]

Confocal image of the stratum corneum
Confocal image of the stratum corneum
Composed of 10 to 30 layers of polyhedral, anucleated corneocytes (final step of keratinocyte differentiation), with the palms and soles having the most layers. Corneocytes are surrounded by a protein envelope (cornified envelope proteins), filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids.[10] Most of the barrier functions of the epidermis localize to this layer.[11]
This narrow layer is found only on the palms and soles. The epidermis of these two areas is known as "thick skin" because with this extra layer, the skin has 5 epidermal layers instead of 4.
Confocal image of the stratum granulosum
Confocal image of the stratum granulosum
Keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids.[10]
Confocal image of the stratum spinosum with some basal cell clusters
Confocal image of the stratum spinosum already showing some clulsters of basal cells
Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes.[3] Langerhans cells, immunologically active cells, are located in the middle of this layer.[10]
Confocal image of the stratum basale showing some papillae
Confocal image of the stratum basale already showing some papillae
Composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation.[10]

The Malpighian layer (stratum malpighi) is both the stratum basale and stratum spinosum.[4]

The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.

Cellular kinetics

Cell division

As a stratified squamous epithelium, the epidermis is maintained by cell division within the stratum basale. Differentiating cells delaminate from the basement membrane and are displaced outward through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface (desquamation). Differentiated keratinocytes secrete keratin proteins, which contribute to the formation of an extracellular matrix that is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss,[4] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum.[2] The entire epidermis is replaced by new cell growth over a period of about 48 days.[12]

Calcium concentration

Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers.[3]

Elevation of extracellular calcium concentrations induces an increase in intracellular free calcium concentrations.[13] Part of that intracellular increase comes from calcium released from intracellular stores[14] and another part comes from transmembrane calcium influx,[15] through both calcium-sensitive chloride channels[16] and voltage-independent cation channels permeable to calcium.[17] Moreover, it has been suggested that an extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration.[18]

Development

Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system. In most vertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or stratum germinativum has formed.[19]

This inner layer is a germinal epithelium that gives rise to all epidermal cells. It divides to form the outer spinous layer (stratum spinosum). The cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis.[19]

The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer (stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 0.001 - 0.003 ounces of skin flakes every hour, or 0.024-0.072 ounces per day.[20]

Epidermal development is a product of several growth factors, two of which are:[19]

Function

Barrier

The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light), and chemical compounds, and provides mechanical resistance to minor injury. Most of this barrier role is played by the stratum corneum.[11]

Characteristics
  • Physical barrier: Epidermal keratinocytes are tightly linked by cell–cell junctions associated to cytoskeletal proteins, giving the epidermis its mechanical strength.[3]
  • Chemical barrier: Highly organized lipids, acids, hydrolytic enzymes, and antimicrobial peptides[3] inhibit passage of external chemicals and pathogens into the body.
  • Immunologically active barrier: The humoral and cellular constituents of the immune system[3] found in the epidermis actively combat infection.
  • Water content of the stratum corneum drops towards the surface, creating hostile conditions for pathogenic microorganism growth.[11]
  • An acidic pH (around 5.0) and low amounts of water make the epidermis hostile to many microorganic pathogens.[11]
  • Non-pathogenic microorganisms on the surface of the epidermis help defend against pathogens by competing for food, limiting its availability, and producing chemical secretions that inhibit the growth of pathogenic microbiota.[11]
Permeability

Skin hydration

The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin.[23] Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss.[24][25]

Skin color

The amount and distribution of melanin pigment in the epidermis is the main reason for variation in skin color in Homo sapiens. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes vary between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected.[26]

Clinical significance

Laboratory culture of keratinocytes to form a 3D structure (artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for drug development and testing.

Additional images

Normal Epidermis and Dermis with Intradermal Nevus 10x

Epidermis and dermis of human skin

Skin

Cross-section of all skin layers

Blausen 0353 Epidermis

Illustration of epidermal layers

See also

References

  1. ^ Young, Barbara (2014). Wheater's functional histology a text and colour atlas. Elsevier. pp. 160 & 175. ISBN 9780702047473.
  2. ^ a b c Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology (4th ed.). Elsevier. pp. 1–7. ISBN 978-1-4160-3185-7.
  3. ^ a b c d e f Proksch, E.; Brandner, J.; Jensen, J.M. (2008). "The skin: an indispensable barrier". Experimental Dermatology. 17 (12): 1063–1072. doi:10.1111/j.1600-0625.2008.00786.x. PMID 19043850.
  4. ^ a b c d McGrath, J.A.; Eady, R.A.; Pope, F.M. (2004). Rook's Textbook of Dermatology (7th ed.). Blackwell Publishing. pp. 3.1–3.6. ISBN 978-0-632-06429-8.
  5. ^ a b c Hanukoglu I, Boggula VR, Vaknine H, Sharma S, Kleyman T, Hanukoglu A (January 2017). "Expression of epithelial sodium channel (ENaC) and CFTR in the human epidermis and epidermal appendages". Histochemistry and Cell Biology. 147 (6): 733–748. doi:10.1007/s00418-016-1535-3. PMID 28130590.
  6. ^ James, William; Berger, Timothy; Elston, Dirk (2005) Andrews' Diseases of the Skin: Clinical Dermatology (10th ed.). Saunders. pp. 2–3. ISBN 0-7216-2921-0.
  7. ^ TheFreeDictionary > rete ridge Citing: The American Heritage Medical Dictionary Copyright 2007, 2004
  8. ^ Stücker, M; Struk, A; Altmeyer, P; Herde, M; Baumgärtl, H; Lübbers, DW (2002). "The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis". The Journal of Physiology. 538 (3): 985–994. doi:10.1113/jphysiol.2001.013067. PMC 2290093. PMID 11826181.
  9. ^ The Ageing Skin - Structure
  10. ^ a b c d "Please update" (PDF). Archived from the original (PDF) on 2010-12-14. Retrieved 2015-01-07.
  11. ^ a b c d e Elias, P.M. (2007). "The skin barrier as an innate immune element". Seminars in Immunopathology. 29 (1): 3–14. doi:10.1007/s00281-007-0060-9. PMID 17621950.
  12. ^ Iizuka, Hajime (1994). "Epidermal turnover time". Journal of Dermatological Science. 8 (3): 215–217. doi:10.1016/0923-1811(94)90057-4. PMID 7865480.
  13. ^ Hennings, H; Kruszewski, FH; Yuspa, SH; Tucker, RW (1989). "Intracellular calcium alterations in response to increased external calcium in normal and neoplastic keratinocytes". Carcinogenesis. 10 (4): 777–80. doi:10.1093/carcin/10.4.777. PMID 2702726.
  14. ^ Pillai, S; Bikle, DD (1991). "Role of intracellular-free calcium in the cornified envelope formation of keratinocytes: Differences in the mode of action of extracellular calcium and 1,25 dihydroxyvitamin D3". Journal of Cellular Physiology. 146 (1): 94–100. doi:10.1002/jcp.1041460113. PMID 1990023.
  15. ^ Reiss, M; Lipsey, LR; Zhou, ZL (1991). "Extracellular calcium-dependent regulation of transmembrane calcium fluxes in murine keratinocytes". Journal of Cellular Physiology. 147 (2): 281–91. doi:10.1002/jcp.1041470213. PMID 1645742.
  16. ^ Mauro, TM; Pappone, PA; Isseroff, RR (1990). "Extracellular calcium affects the membrane currents of cultured human keratinocytes". Journal of Cellular Physiology. 143 (1): 13–20. doi:10.1002/jcp.1041430103. PMID 1690740.
  17. ^ Mauro, TM; Isseroff, RR; Lasarow, R; Pappone, PA (1993). "Ion channels are linked to differentiation in keratinocytes". The Journal of Membrane Biology. 132 (3): 201–9. doi:10.1007/BF00235738. PMID 7684087.
  18. ^ Tu, CL; Oda, Y; Bikle, DD (1999). "Effects of a calcium receptor activator on the cellular response to calcium in human keratinocytes". The Journal of Investigative Dermatology. 113 (3): 340–5. doi:10.1046/j.1523-1747.1999.00698.x. PMID 10469331.
  19. ^ a b c Gilbert, Scott F (2000). "The Epidermis and the Origin of Cutaneous Structures". Developmental Biology. Sinauer Associates. ISBN 978-0-87893-243-6.
  20. ^ Weschler, Charles J. (April 8, 2011). "Squalene and Cholesterol in Dust from Danish Homes and Daycare Centers". Environ. Sci. Technol. 45 (9): 3872–3879. Bibcode:2011EnST...45.3872W. doi:10.1021/es103894r. PMID 21476540.
  21. ^ Denda, M.; Tsuchiya, T.; Elias, P.M.; Feingold, K.R. (2000). "Stress alters cutaneous permeability barrier homeostasis". Am J Physiol Regul Integr Comp Physiol. 278 (2): R367–372. doi:10.1152/ajpregu.2000.278.2.R367. PMID 10666137.
  22. ^ Tsai, Jui-Chen; Guy, Richard H.; Thornfeldt, Carl R.; Gao, Wen Ni; Feingold, Kenneth R.; Elias, Peter M. (1996). "Metabolic Approaches To Enhance Transdermal Drug Delivery. 1. Effect of Lipid Synthesis Inhibitors". Journal of Pharmaceutical Sciences. 85 (6): 643–648. doi:10.1021/js950219p. PMID 8773963.
  23. ^ Blank, IH (1952). "Factors which influence the water content of the stratum corneum". The Journal of Investigative Dermatology. 18 (6): 433–40. doi:10.1038/jid.1952.52. PMID 14938659.
  24. ^ Downing, DT; Stewart, ME; Wertz, PW; Colton, SW; Abraham, W; Strauss, JS (1987). "Skin lipids: An update". The Journal of Investigative Dermatology. 88 (3 Suppl): 2s–6s. doi:10.1111/1523-1747.ep12468850. PMID 2950180.
  25. ^ Bonté, F; Saunois, A; Pinguet, P; Meybeck, A (1997). "Existence of a lipid gradient in the upper stratum corneum and its possible biological significance". Archives of Dermatological Research. 289 (2): 78–82. doi:10.1007/s004030050158. PMID 9049040.
  26. ^ Montagna, William; Prota, Giuseppe; Kenney, John A. (1993). Black skin: structure and function. Gulf Professional Publishing. p. 69. ISBN 978-0-12-505260-3.
Abrasion (medical)

An abrasion is a partial thickness wound caused by damage to the skin and can be superficial involving only the epidermis to deep, involving the deep dermis. Abrasions usually involve minimal bleeding. Mild abrasions, also known as grazes or scrapes, do not scar or bleed because the dermis is left intact, but deep abrasions that disrupt the normal dermal structures may lead to the formation of scar tissue. A more traumatic abrasion that removes all layers of skin is called an avulsion.

Abrasion injuries most commonly occur when exposed skin comes into moving contact with a rough surface, causing a grinding or rubbing away of the upper layers of the epidermis.

Cork cambium

Cork cambium (pl. cambia or cambiums) is a tissue found in many vascular plants as part of the epidermis. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems. It is found in woody and many herbaceous dicots, gymnosperms and some monocots (monocots usually lack secondary growth). It is one of the plant's meristems – the series of tissues consisting of embryonic disk (incompletely differentiated) cells from which the plant grows. It is one of the many layers of bark, between the cork and primary phloem. The function of cork cambium is to produce the cork, a tough protective material.Synonyms for cork cambium are bark cambium, pericambium and phellogen. Phellogen is defined as the meristematic cell layer responsible for the development of the periderm. Cells that grow inwards from there are termed phelloderm, and cells that develop outwards are termed phellem or cork (note similarity with vascular cambium). The periderm thus consists of three different layers:

phelloderm – inside of cork cambium; composed of living parenchyma cells

phellogen (cork cambium) – meristem that gives rise to periderm

phellem (cork) – dead at maturity; air-filled protective tissue on the outsideGrowth and development of cork cambium is very variable between different species, and is also highly dependent on age and growth conditions, as can be observed from the different surfaces of bark, which may be smooth, fissured, tesselated, scaly, or flaking off.

Dermis

The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided into two layers, the superficial area adjacent to the epidermis called the papillary region and a deep thicker area known as the reticular dermis. The dermis is tightly connected to the epidermis through a basement membrane. Structural components of the dermis are collagen, elastic fibers, and extrafibrillar matrix. It also contains mechanoreceptors that provide the sense of touch and thermoreceptors that provide the sense of heat. In addition, hair follicles, sweat glands, sebaceous glands (oil glands), apocrine glands, lymphatic vessels , nerves and blood vessels are present in the dermis. Those blood vessels provide nourishment and waste removal for both dermal and epidermal cells.

Epidermis (botany)

The epidermis (from the Greek ἐπιδερμίς, meaning "over-skin") is a single layer of cells that covers the leaves, flowers, roots and stems of plants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulates gas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such as potato tubers produce a secondary covering called the periderm that replaces the epidermis as the protective covering.

Epidermis (zoology)

In zoology, the epidermis is an epithelium (sheet of cells) that covers the body of a eumetazoan (animal more complex than a sponge). Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis. The epidermis of a more complex invertebrate is just one layer deep, and may be protected by a non-cellular cuticle. The epidermis of a higher vertebrate has many layers, and the outer layers are reinforced with keratin and then die.

Human skin

The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue and guards the underlying muscles, bones, ligaments and internal organs. Human skin is similar to most of the other

mammals skin, and human skin is very similar to pig skin. Though nearly all human skin is covered with hair follicles, it can appear hairless. There are two general types of skin, hairy and glabrous skin (hairless). The adjective cutaneous literally means "of the skin" (from Latin cutis, skin).

Because it interfaces with the environment, skin plays an important immunity 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 habitat for bacteria that number roughly 1000 species from 19 phyla, present on the human skin.

Hydroid (zoology)

Hydroids are a life stage for most animals of the class Hydrozoa, small predators related to jellyfish. Some hydroids such as the freshwater Hydra are solitary, with the polyp attached directly to the substrate. When these produce buds, they become detached and grow on as new individuals.

The majority of hydroids are colonial. The original polyp is anchored to a solid substrate and forms a bud which remains attached to its parent. This in turn buds and in this way a stem is formed. The arrangement of polyps and the branching of the stem is characteristic of the species.

Some species have the polyps budding directly off the stolon which roots the colony. The polyps are connected by epidermis which surrounds a gastrovascular cavity. The epidermis secretes a chitinous skeleton which supports the stem and in some hydroids, the skeleton extends into a cup shape surrounding the polyp.

Most of the polyps are gastrozooids or feeding polyps, but some are specialised reproductive structures known as gonozooids. In some species, further specialised zooids are formed.

Integumentary system

The integumentary system comprises the skin and its appendages acting to protect the body from various kinds of damage, such as loss of water or damages from outside. The integumentary system includes hair, scales, feathers, hooves, and nails. It has a variety of additional functions; it may serve to waterproof, and protect the deeper tissues, excrete wastes, and regulate body temperature, and is the attachment site for sensory receptors to detect pain, sensation, pressure, and temperature. In most land vertebrates with significant exposure to sunlight, the integumentary system also provides for vitamin D synthesis.

Keratinocyte

A keratinocyte is the predominant cell type in the epidermis, the outermost layer of the skin, constituting 90% of the cells found there. Those keratinocytes found in the basal layer (stratum basale) of the skin are sometimes referred to as "basal cells" or "basal keratinocytes".

Microsporangia

Microsporangia are sporangia that produce microspores and give rise to male gametes. Microsporangia occur in all plants that have heterosporic life cycles, such as spike mosses. In gymnosperms and angiosperm anthers, the microsporangia produce the microsporocytes, the microspore mother cells, which then produce four microspores through the process of meiosis. In the microsporocyte of Arabidopsis thaliana, meiosis depends on the expression of genes that facilitate DNA repair and homologous recombination.

The microspores divide by mitosis to produce pollen grains.

In angiosperms, a very young anther (the part of the stamen that contains the pollen) consists of actively dividing meristematic cells surrounded by a layer of epidermis. It then becomes two-lobed. Each anther lobe develops two pollen sacs. Then, a two-lobed anther develops four pollen sacs that situate at four corners of the anther. Development of pollen sacs begins with the differentiation of archesporial cells in the hypodermal region below epidermis at four corners of the young anther. The archesporial cells divide by periclinal division to give a subepidermal primary parietal layer and a primary sporogenous layer. The cells of the primary parietal layer divide by successive periclinal and anticlinal divisions to form concentric layers of pollen sac wall. The wall layers from periphery to center consist of:

A single layer of epidermis between, which becomes stretched and shrivels off at maturity

A single layer of endothecium. The cells of endothecium have fibrous thickenings.

One to three middle layers. Cells of these layers generally disintegrate in the mature anther

A single layer of tapetum. The tapetal cells may be uni-, bi- or multinucleate and possess dense cytoplasm. The cells of the primary sporogenous layer divide further and give rise to diploid sporogenous tissue.

Plant cell

Plant cells are eukaryotic cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.

Skin

Skin is the soft outer tissue covering of vertebrates with three main functions: protection, regulation, and sensation.Other animal coverings, such as the arthropod exoskeleton, have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis, skin). In mammals, the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals like whales, dolphins, and porpoises which appear to be hairless.

The skin interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar tissue. This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is 4 mm thick and is the thickest skin on the body. The speed and quality of wound healing in skin is promoted by the reception of estrogen.Fur is dense hair. Primarily, fur augments 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, especially regarding the passage of chemicals via skin and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly, as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.

Skin condition

A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment.Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying causes and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), cause (skin conditions resulting from physical factors), and so on.Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). The diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data. The introduction of cutaneous ultrasound has allowed the detection of cutaneous tumors, inflammatory processes, nail disorders and hair diseases.

Staphylococcus epidermidis

Staphylococcus epidermidis is a Gram-positive bacterium, and one of over 40 species belonging to the genus Staphylococcus. It is part of the normal human flora, typically the skin flora, and less commonly the mucosal flora. It is a facultative anaerobic bacteria. Although S. epidermidis is not usually pathogenic, patients with compromised immune systems are at risk of developing infection. These infections are generally hospital-acquired. S. epidermidis is a particular concern for people with catheters or other surgical implants because it is known to form biofilms that grow on these devices. Being part of the normal skin flora, S. epidermidis is a frequent contaminant of specimens sent to the diagnostic laboratory.

Stoma

In botany, a stoma (plural "stomata"), also called a stomate (plural "stomates") (from Greek στόμα, "mouth"), is a pore, found in the epidermis of leaves, stems, and other organs, that facilitates gas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening.

The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture. Air enters the plant through these openings by gaseous diffusion, and contains carbon dioxide and oxygen, which are used in photosynthesis and respiration, respectively. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Also, water vapor diffuses through the stomata into the atmosphere in a process called transpiration.

Stomata are present in the sporophyte generation of all land plant groups except liverworts. In vascular plants the number, size and distribution of stomata varies widely. Dicotyledons usually have more stomata on the lower surface of the leaves than the upper surface. Monocotyledons such as onion, oat and maize may have about the same number of stomata on both leaf surfaces. In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface. Leaves with stomata on both the upper and lower leaf are called amphistomatous leaves; leaves with stomata only on the lower surface are hypostomatous, and leaves with stomata only on the upper surface are epistomatous or hyperstomatous. Size varies across species, with end-to-end lengths ranging from 10 to 80 µm and width ranging from a few to 50 µm.

Stratum basale

The stratum basale (basal layer, sometimes referred to as stratum germinativum) is the deepest layer of the five layers of the epidermis, the outer covering of skin in mammals.

The stratum basale is a single layer of columnar or cuboidal cells. The cells are attached to each other and to the overlying stratum spinosum cells by desmosomes and hemidesmosomes. The nucleus is large, ovoid and occupies most of the cell. Some basal cells can act like stem cells with ability to divide and produced new cells , whereas others serve to anchor the epidermis glabrous skin (hairless), and hyper-proliferative epidermis (from a skin disease).The stratum basale is primarily made up of basal keratinocyte stem cells, which can be considered the stem cells of the epidermis. They divide to form the keratinocytes of the stratum spinosum, which migrate superficially. Other types of cells found within the stratum basale are melanocytes (pigment-producing cells), Langerhans cells (immune cells), and Merkel cells (touch receptors).

Stratum corneum

The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis, consisting of dead cells (corneocytes). This layer is composed of 15–20 layers of flattened cells with no nuclei and cell organelles. Their cytoplasm shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids.The stratum corneum functions to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress. Desquamation, the process of cell shedding from the surface of the stratum corneum, balances proliferating keratinocytes that form in the stratum basale. These cells migrate through the epidermis towards the surface in a journey that takes approximately fourteen days.

Subcutaneous tissue

The subcutaneous tissue (from Latin subcutaneous, meaning 'beneath the skin'), also called the hypodermis, hypoderm (from Greek, Modern, meaning 'beneath the skin'), subcutis, or superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the hypodermis are fibroblasts, adipose cells, and macrophages. The hypodermis is derived from the mesoderm, but unlike the dermis, it is not derived from the dermatome region of the mesoderm. In arthropods, the hypodermis is an epidermal layer of cells that secretes the chitinous cuticle. The term also refers to a layer of cells lying immediately below the epidermis of plants.

The hypodermis is beneath dermis which is beneath epidermis. It is used mainly for fat storage.

A layer of tissue lies immediately below the dermis of vertebrate skin. It is often referred to as subcutaneous tissue though this is a less precise and anatomically inaccurate term. The hypodermis consists primarily of loose connective tissue and lobules of fat. It contains larger blood vessels and nerves than those found in the dermis.

Tissue (biology)

In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

The English word is derived from the French tissu, meaning something that is woven, from the verb tisser, "to weave".

The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant anatomy. The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.

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