Human skin color
Human skin color ranges in variety from the darkest brown to the lightest hues. An individual's skin pigmentation is the result of genetics, being the product of both of the individual's biological parents' genetic makeup, exposure to sun. In evolution, skin pigmentation in human beings evolved by a process of natural selection to regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects; the actual skin color of different humans is affected by many substances, although the single most important substance is the pigment melanin. Melanin is produced within the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skinned humans; the skin color of people with light skin is determined by the bluish-white connective tissue under the dermis and by the hemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible in the face, when, as consequence of physical exercise or the stimulation of the nervous system, arterioles dilate.
Color is not uniform across an individual's skin. There is a direct correlation between the geographic distribution of ultraviolet radiation and the distribution of indigenous skin pigmentation around the world. Areas that receive higher amounts of UVR located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, reflected in lighter-skinned populations. Researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones, that such major changes in pigmentation may have happened in as little as 100 generations through selective sweeps. Natural skin color can darken as a result of tanning due to exposure to sunlight; the leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the DNA of the skin cells.
In addition, it has been observed that adult human females on average are lighter in skin pigmentation than males. Females need more calcium during lactation; the body synthesizes vitamin D from sunlight. Females evolved to have lighter skin; the social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination. Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is made within small membrane–bound packages called melanosomes; as they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the keratinocytes. Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes, they regulate melanin production and replication of melanocytes. People have different skin colors because their melanocytes produce different amount and kinds of melanin.
The genetic mechanism behind human skin color is regulated by the enzyme tyrosinase, which creates the color of the skin and hair shades. Differences in skin color are attributed to differences in size and distribution of melanosomes in the skin. Melanocytes produce two types of melanin; the most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, their reduced forms. Most are derived from the amino acid tyrosine. Eumelanin is found in hair and skin, the hair colors gray, black and brown. In humans, it is more abundant in people with dark skin. Pheomelanin, a pink to red hue is found in large quantities in red hair, the lips, glans of the penis, vagina. Both the amount and type of melanin produced is controlled by a number of genes that operate under incomplete dominance. One copy of each of the various genes is inherited from each parent; each gene can come in several alleles. Melanin controls the amount of ultraviolet radiation from the sun that penetrates the skin by absorption.
While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health. Loss of body hair in Hominini species is assumed to be related to the emergence of bipedalism some 5 to 7 million years ago. Bipedal hominin body hair may have disappeared to allow better heat dissipation through sweating; the emergence of skin pigmentation dates to at about 1.2 million years ago, under conditions of a megadrought that drove early humans into arid, open landscapes. Such conditions caused excess UV-B radiation; this favored the emergence of skin pigmentation in order to protect from folate depletion due to the increased exposure to sunlight. A theory that the pigmentation helped counter xeric stress by increasing the epidermal permeability barrier has been disproved. With the evolution of hairless skin, abundant sweat glands, skin rich in melanin, early humans could walk and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species.
By 1.2 million years ago, around the time of Homo ergaster, archaic humans had the same receptor protein as modern sub-Saharan Africans. This wa
Infrared radiation, sometimes called infrared light, is electromagnetic radiation with longer wavelengths than those of visible light, is therefore invisible to the human eye, although IR at wavelengths up to 1050 nanometers s from specially pulsed lasers can be seen by humans under certain conditions. IR wavelengths extend from the nominal red edge of the visible spectrum at 700 nanometers, to 1 millimeter. Most of the thermal radiation emitted by objects near room temperature is infrared; as with all EMR, IR carries radiant energy and behaves both like a wave and like its quantum particle, the photon. Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. More than half of the total energy from the Sun was found to arrive on Earth in the form of infrared; the balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines transmission of photons in the infrared range. Infrared radiation is used in industrial, military, law enforcement, medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, detect objects such as planets, to view red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to detect overheating of electrical apparatus. Extensive uses for military and civilian applications include target acquisition, night vision and tracking.
Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication and weather forecasting. Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 millimeter; this range of wavelengths corresponds to a frequency range of 430 THz down to 300 GHz. Below infrared is the microwave portion of the electromagnetic spectrum. Sunlight, at an effective temperature of 5,780 kelvins, is composed of near-thermal-spectrum radiation, more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, 32 watts is ultraviolet radiation. Nearly all the infrared radiation in sunlight is shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight.
However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, fires produce far more infrared than visible-light energy. In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors collect radiation only within a specific bandwidth. Thermal infrared radiation has a maximum emission wavelength, inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. Therefore, the infrared band is subdivided into smaller sections. A used sub-division scheme is: NIR and SWIR is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared". Due to the nature of the blackbody radiation curves, typical "hot" objects, such as exhaust pipes appear brighter in the MW compared to the same object viewed in the LW.
The International Commission on Illumination recommended the division of infrared radiation into the following three bands: ISO 20473 specifies the following scheme: Astronomers divide the infrared spectrum as follows: These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, hence different environments in space; the most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used. These letters are understood in reference to atmospheric windows and appear, for instance, in the titles of many papers. A third scheme divides up the band based on the response of various detectors: Near-infrared: from 0.7 to 1.0 µm. Short-wave infrared: 1.0 to 3 µm. InGaAs covers to about 1.8 µm. Mid-wave infrared: 3 to 5 µm (defined by the atmospheric window and covered by indium antimonide and mercury cadmium telluride and by lead
A space heater is a device used to heat a single, small area. Space heaters are powered by electricity or a burnable fuel, such as natural gas, fuel oil, or wood pellets. Portable space heaters are electric, because a permanent exhaust is needed for heaters which burn fuel. Space heaters are powered by the combustion of flammable fuel. Electric space heaters fall into three main categories: Convection heaters pass electricity through a heating element, causing the element to become hot; the elements are either metal or ceramic, the process is known as joule heating. Heat is transferred to the air in the room by convection; some heaters have a fan to increase air circulation. Infrared heaters pass electricity through a conductive wire, heating it. Most of the heat is radiant heating, rather than convection; the hot wire emits infrared rays, which transfer heat to a solid surface rather than the surrounding air. Heat pumps in reverse. While convective and infrared heaters make heat from electricity, heat pumps move the location of heat.
Heat is moved from inside a refrigerator to the room. Combustion space heaters burn flammable fuel, such as natural gas, propane, or wood. Many residential space heaters use convective heating, they can be divided into two categories: those with a fan, those without a fan. Convective heaters provide diffuse heat to well-insulated rooms; some convective heaters use a fan to help circulate warm air throughout a room. Their heating elements are metal or ceramic and are in direct contact with room air, allowing fan heaters to warm a room quickly. In convective heaters without a fan, the heating element is surrounded by water; these heaters warm a room more because the liquid must be heated before the heat can reach the surrounding air. They produce more heat after being turned off, because of the hot liquid inside the heater; the risk of fire is sometimes less with oil-filled heaters than those with fans, but some fan-assisted heaters have a lower risk of fire than other oil-filled heaters. The main advantage of radiant heaters is that the infrared radiation they produce is absorbed directly by clothing and skin, without first heating the air in a space.
This makes them suitable for warming people in poorly insulated rooms or outdoors, allows more distance between people and the heater. Some of the earliest electric heaters were radiant, consisting of nichrome heating wires held by ceramic or mica insulation at the focal point of a polished metal reflector; the cost was low since nothing else, not a switch, was needed. Models included a wire guard to prevent accidental contact with the heating wires or the hot ceramic; the metal reflectors needed to be thick, however. Inexpensive mid-20th century heaters were radiant, with the heating wires stretched closely across a larger, metal reflector separated from a thin metal housing. A small fan blew just enough air between the housing and the reflector to cool them, the main output to the room was radiant heat. Stretching the heating wires across a larger area required fewer ceramic insulators, a small fan was cheaper than a larger housing. Quartz heaters are radiant heaters which are more efficient in the amount and direction of heat, with coiled heating wire inside unsealed quartz tubing.
The wires could be thinner than ceramic-supported wires. If the heating elements are at a higher temperature, proportionally more energy is radiated than open-wire heaters. Halogen heaters have tungsten filaments in sealed quartz envelopes, mounted in front of a metal reflector in a plastic case, they operate at a higher temperature than nichrome-wire heaters but not as high as incandescent light bulbs, radiating in the infrared spectrum. They convert up to 86 percent of their input power to radiant energy, losing the remainder to conductive and convective heat; the halogen cycle reduces darkening of the quartz envelope. Many space heaters are plugged into an electric power source, most a two-prong – for older models – or three-prong outlet. Appliance power is measured in kilowatts, which permits simple estimation of operating cost per hour. Fire and carbon monoxide poisoning are the main risks of space heaters. About 25,000 fires are caused by space heaters in the United States each year, resulting in about 300 deaths.
6,000 hospital emergency department visits annually in the US are caused by space heaters from burns. Improper use can increase the risk of fire and burns. Safe operation includes: Plugging space heaters directly into a wall outlet or heavy-duty extension cord. Light-duty extension cords can cause fires. Plugs and cords should be checked periodically for cracks or damage, replaced if needed. Flammable materials, such as curtains and bedding, should be kept at least 3 feet from the heater. Turn off the heater when the last adult leaves the room or goes to sleep. Children and pets should be kept three feet from the heater. Heaters should be placed on a flat, nonflammable surface. Avoid using heaters near flammable materials such as paint or gasoline. Smoke alarms
Hot water bottle
A hot-water bottle is a bottle filled with hot water and sealed with a stopper, used to provide warmth while in bed, but for the application of heat to a specific part of the body. Containers for warmth in bed were in use as early as the 16th century; the earliest versions contained hot coals from the dying embers of the fire, these bed warmers were used to warm the bed before getting into it. Containers using hot water were soon used, with the advantage that they could remain in the bed with the sleeper. Prior to the invention of rubber that could withstand sufficient heat, these early hot-water bottles were made of a variety of materials, such as zinc, brass, earthenware or wood. To prevent burning, the metal hot water flasks were wrapped in a soft cloth bag. "India rubber" hot-water bottles were in use in Britain at least by 1875. Modern conventional hot-water bottles were patented in 1903 and are manufactured in natural rubber or PVC, to a design patented by the Croatian inventor Eduard Penkala.
They are now covered in fabric, sometimes with a novelty design. By the late 20th century, the use of hot-water bottles had markedly declined around most of the world. Not only were homes better heated, but newer items such as electric blankets were competing with hot-water bottles as a source of night-time heat; however the hot-water bottle remains a popular alternative in Australia, United Kingdom, developing countries and rural areas. For example, it is used in Chile, where it is called a "guatero". There has been a recent surge in popularity in Japan where it is seen as an ecologically friendly and thrifty way to keep warm; some newer products use a polymer gel or wax in a heat pad. The pads can be heated in a microwave oven, they are marketed as safer than liquid-filled bottles or electrically-heated devices; some newer bottles now use a silicone-based material instead of rubber, which resists hot water better, does not deteriorate as much as rubber. Although the stopper size in Ireland and the UK has been standard for many decades, the newer bottles use a wider mouth, easier to fill.
While used for keeping warm, conventional hot-water bottles can be used to some effect for the local application of heat as a medical treatment, for example for pain relief, but newer items such as purpose-designed heating pads are used now. Hot-water bottles are meant to contain hot fluids and supposed to be in contact with human skin, it is therefore of the utmost importance to ensure through standards and regulations, that the closing and welding is stable enough to prevent burns, but to make sure that the bottle’s chemical components are not dangerous for human health. More it is crucial to certify and assure that hot-water bottles, whether manufactured, sold or imported, are safe. For instance, the United Kingdom defined British Standards for hot-water bottles to regulate their manufacture and sale as well as to ensure their compliance with all safety standards; the British Standards BS 1970 and BS 1970:2012 define, for instance, the bottles’ filling characteristics, safety instructions, allowed materials and components as well as testing methods such as tensile tests for PVC bottles.
Most regulations applied to a country are harmonized in order to be applied and applicable in a larger area, such as a trade zone. There have been problems with premature failure of rubber hot-water bottles due to faulty manufacture; the rubber may fail strength or fitness tests, or become brittle if manufacturing is not controlled closely. Natural rubber filled with calcium carbonate is the most common material used, but is susceptible to oxidation and polymer degradation at the high temperatures used in shaping the product. Though the brittle cracks may not be visible externally, the bottle can fracture after filling with hot water, can scald the user—sometimes requiring hospitalization for severe burn cases. Boiling water is not recommended for use in hot-water bottles; this is due to risks of the rubber being degraded from high-temperature water, the risk of injury in case of breakage. Alfred, the cantankerous hot-water bottle, is a character from Johnson and Friends, a popular Australian children's television series from the 1990s.
This character has gained a cult following in recent years among those who grew up with the series, due to the odd character choice. Alfred is believed to be the only anthropomorphised hot-water bottle in existence. Electric blanket Bed warmer Heating pad Hot water bottle blowing Media related to Hot-water bottles at Wikimedia Commons
Electronics comprises the physics, engineering and applications that deal with the emission and control of electrons in vacuum and matter. The identification of the electron in 1897, along with the invention of the vacuum tube, which could amplify and rectify small electrical signals, inaugurated the field of electronics and the electron age. Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes, diodes, integrated circuits and sensors, associated passive electrical components, interconnection technologies. Electronic devices contain circuitry consisting or of active semiconductors supplemented with passive elements; the nonlinear behaviour of active components and their ability to control electron flows makes amplification of weak signals possible. Electronics is used in information processing, telecommunication, signal processing; the ability of electronic devices to act as switches makes digital information-processing possible. Interconnection technologies such as circuit boards, electronics packaging technology, other varied forms of communication infrastructure complete circuit functionality and transform the mixed electronic components into a regular working system, called an electronic system.
An electronic system may be a component of a standalone device. Electrical and electromechanical science and technology deals with the generation, switching and conversion of electrical energy to and from other energy forms; this distinction started around 1906 with the invention by Lee De Forest of the triode, which made electrical amplification of weak radio signals and audio signals possible with a non-mechanical device. Until 1950 this field was called "radio technology" because its principal application was the design and theory of radio transmitters and vacuum tubes; as of 2018 most electronic devices use semiconductor components to perform electron control. The study of semiconductor devices and related technology is considered a branch of solid-state physics, whereas the design and construction of electronic circuits to solve practical problems come under electronics engineering; this article focuses on engineering aspects of electronics. Digital electronics Analogue electronics Microelectronics Circuit design Integrated circuits Power electronics Optoelectronics Semiconductor devices Embedded systems An electronic component is any physical entity in an electronic system used to affect the electrons or their associated fields in a manner consistent with the intended function of the electronic system.
Components are intended to be connected together by being soldered to a printed circuit board, to create an electronic circuit with a particular function. Components may be packaged singly, or in more complex groups as integrated circuits; some common electronic components are capacitors, resistors, transistors, etc. Components are categorized as active or passive. Vacuum tubes were among the earliest electronic components, they were solely responsible for the electronics revolution of the first half of the twentieth century. They allowed for vastly more complicated systems and gave us radio, phonographs, long-distance telephony and much more, they played a leading role in the field of microwave and high power transmission as well as television receivers until the middle of the 1980s. Since that time, solid-state devices have all but taken over. Vacuum tubes are still used in some specialist applications such as high power RF amplifiers, cathode ray tubes, specialist audio equipment, guitar amplifiers and some microwave devices.
In April 1955, the IBM 608 was the first IBM product to use transistor circuits without any vacuum tubes and is believed to be the first all-transistorized calculator to be manufactured for the commercial market. The 608 contained more than 3,000 germanium transistors. Thomas J. Watson Jr. ordered all future IBM products to use transistors in their design. From that time on transistors were exclusively used for computer logic and peripherals. Circuits and components can be divided into two groups: digital. A particular device may consist of circuitry that has a mix of the two types. Most analog electronic appliances, such as radio receivers, are constructed from combinations of a few types of basic circuits. Analog circuits use a continuous range of voltage or current as opposed to discrete levels as in digital circuits; the number of different analog circuits so far devised is huge because a'circuit' can be defined as anything from a single component, to systems containing thousands of components.
Analog circuits are sometimes called linear circuits although many non-linear effects are used in analog circuits such as mixers, etc. Good examples of analog circuits include vacuum tube and transistor amplifiers, operational amplifiers and oscillators. One finds modern circuits that are analog; these days analog circuitry may use digital or microprocessor techniques to improve performance. This type of circuit is called "mixed signal" rather than analog or digital. Sometimes it may be difficult to differentiate between analog and digital circuits as they have elements of both linear and non-linear
A central heating system provides warmth to the whole interior of a building from one point to multiple rooms. When combined with other systems in order to control the building climate, the whole system may be an HVAC system. Central heating differs from space heating in that the heat generation occurs in one place, such as a furnace room or basement in a house or a mechanical room in a large building; the heat is distributed throughout the building by forced-air through ductwork, by water circulating through pipes, or by steam fed through pipes. The most common method of heat generation involves the combustion of fossil fuel in a furnace or boiler. In much of the temperate climate zone, most detached housing has had central heating installed since before the Second World War. Where coal was available coal-fired steam or hot water systems were common. In more recent times, these have been updated to use fuel oil or gas as the source of combustion, eliminating the need for a large coal storage bin near the boiler and the need to remove and discard ashes after the coal is burned.
Coal-fired systems are now reserved for larger buildings. A cheaper alternative to hot water or steam heat is forced hot air. A furnace burns fuel oil, which heats air in a heat exchanger, blower fans circulate the warmed air through a network of ducts to the rooms in the building; this system is cheaper because the air moves through a series of ducts instead of pipes, does not require a pipe fitter to install. The space between floor joists can be boxed in and used as some of the ductwork, further lowering costs. Electrical heating systems occur less and are practical only with low-cost electricity or when ground source heat pumps are used. Considering the combined system of thermal power station and electric resistance heating, the overall efficiency will be less than for direct use of fossil fuel for space heating. More and more buildings utilize solar-powered heat sources, in which case the distribution system uses water circulation. Alternatives to such systems are district heating. District heating uses the waste heat from an industrial process or electrical generating plant to provide heat for neighboring buildings.
Similar to cogeneration, this requires underground piping to circulate hot steam. The ancient Greeks developed central heating; the temple of Ephesus was heated by flues planted in the ground and circulating the heat, generated by fire. Some buildings in the Roman Empire used central heating systems, conducting air heated by furnaces through empty spaces under the floors and out of pipes in the walls—a system known as a hypocaust; the Roman hypocaust continued to be used on a smaller scale during late Antiquity and by the Umayyad caliphate, while Muslim builders employed a simpler system of underfloor pipes. After the collapse of the Roman Empire, overwhelmingly across Europe, heating reverted to more primitive fireplaces for a thousand years. In the early medieval Alpine upland, a simpler central heating system where heat travelled through underfloor channels from the furnace room replaced the Roman hypocaust at some places. In Reichenau Abbey a network of interconnected underfloor channels heated the 300 m² large assembly room of the monks during the winter months.
The degree of efficiency of the system has been calculated at 90%. In the 13th century, the Cistercian monks revived central heating in Christian Europe using river diversions combined with indoor wood-fired furnaces; the well-preserved Royal Monastery of Our Lady of the Wheel on the Ebro River in the Aragon region of Spain provides an excellent example of such an application. The three main methods of central heating were developed in the late 18th to mid-19th centuries. William Strutt designed a new mill building in Derby with a central hot air furnace in 1793, although the idea had been proposed by John Evelyn a hundred years earlier. Strutt's design consisted of a large stove that heated air brought from the outside by a large underground passage; the air was ventilated through the building by large central ducts. In 1807, he collaborated with another eminent engineer, Charles Sylvester, on the construction of a new building to house Derby's Royal Infirmary. Sylvester was instrumental in applying Strutt's novel heating system for the new hospital.
He published his ideas in The Philosophy of Domestic Economy. Sylvester documented the new ways of heating hospitals that were included in the design, the healthier features such as self-cleaning and air-refreshing toilets; the infirmary's novel heating system allowed the patients to breathe fresh heated air whilst old air was channeled up to a glass and iron dome at the centre. Their designs proved influential, they were copied in the new mills of the Midlands and were improved, reaching maturity with the work of de Chabannes on the ventilation of the House of Commons in the 1810s. This system remained the standard for heating small buildings for the rest of the century; the English writer Hugh Plat proposed a steam-based central heating system for a greenhouse in 1594, although this was an isolated occurrence and was not followed up until the 18th century. Colonel Coke devised a system of pipes that would carry steam around the house from a central boiler, but it was James Watt the Scottish inventor, the first to build a working system in his house.
A central boiler supplied h
The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water released from the body into the atmosphere through transepidermal water loss; the epidermis is composed of multiple layers of flattened cells that overlie a base layer 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 are found in all layers of the epidermis; 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 a stratified squamous epithelium The epidermis consists of keratinocytes, which comprise 90% of its cells, but contains melanocytes, Langerhans cells, Merkel cells, inflammatory cells.
Epidermal thickenings called. Blood capillaries are found beneath the epidermis, are linked to an arteriole and a venule; the epidermis itself has no blood supply and is nourished exclusively by diffused oxygen from the surrounding air. Epidermal cells are 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, although the actin filaments are 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. The epidermis is composed depending on the region of skin being considered; those layers in descending order are: cornified layer Composed of 10 to 30 layers of polyhedral, anucleated corneocytes, with the palms and soles having the most layers.
Corneocytes contain a protein envelope underneath the plasma membrane, are filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids. Most of the barrier functions of the epidermis localize to this layer.clear/translucent layer 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.granular layer 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 converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids.spinous layer Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols and catabolic enzymes.
Langerhans cells, immunologically active cells, are located in the middle of this layer.basal/germinal layer. Composed 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 found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips, they are associated with cutaneous nerves and seem to be involved in light touch sensation. The Malpighian layer is both stratum spinosum; the epidermis is separated from its underlying tissue, by a basement membrane. 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 shed from the surface.
Differentiated keratinocytes secrete keratin proteins, which contribute to the formation of an extracellular matrix, an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss, taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, an additional four weeks to cross the stratum corneum; the entire epidermis is replaced by new cell growth over a period of about 48 days. 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, decreasing in the stratum corneum. Calcium concentration in the stratum corneum is low in part because those 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. El