A capillary is a small blood vessel from 5 to 10 micrometres in diameter, having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules; these microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water and glucose. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation. During early embryonic development new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells which form vascular tubes; the term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and present endothelium which divides. Blood flows from the heart through arteries, which branch and narrow into arterioles, branch further into capillaries where nutrients and wastes are exchanged; the capillaries join and widen to become venules, which in turn widen and converge to become veins, which return blood back to the heart through the venae cavae.
Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and organs. The more metabolically active a tissue is, the more capillaries are required to supply nutrients and carry away waste products. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, metarterioles, found only in the mesenteric circulation, they are short vessels that directly connect the arterioles and venules at opposite ends of the beds. Metarterioles are found in the mesenteric microcirculation; the physiological mechanisms underlying precapillary resistance is no longer considered to be a result of precapillary sphincters outside of the mesentery organ. Lymphatic capillaries are larger in diameter than blood capillaries, have closed ends; this structure permits interstitial fluid to flow into them but not out. Lymph capillaries have a greater internal oncotic pressure than blood capillaries, due to the greater concentration of plasma proteins in the lymph.
There are three types of blood capillaries: Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, they only allow smaller molecules, such as water and ions to pass through their intercellular clefts. Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients. Continuous capillaries can be further divided into two subtypes: Those with numerous transport vesicles, which are found in skeletal muscles, fingers and skin; those with few vesicles, which are found in the central nervous system. These capillaries are a constituent of the blood–brain barrier. Fenestrated capillaries have pores in the endothelial cells that are spanned by a diaphragm of radially oriented fibrils and allow small molecules and limited amounts of protein to diffuse. In the renal glomerulus there are cells with no diaphragms, called podocyte foot processes or pedicels, which have slit pores with a function analogous to the diaphragm of the capillaries.
Both of these types of blood vessels have continuous basal laminae and are located in the endocrine glands, intestines and the glomeruli of the kidney. Sinusoid capillaries are a special type of open-pore capillary, that have larger openings in the endothelium; these types of blood vessels allow red and white blood cells and various serum proteins to pass, aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles, therefore utilize gaps present in cell junctions to permit transfer between endothelial cells, hence across the membrane. Sinusoid blood vessels are located in the bone marrow, lymph nodes, adrenal glands; some sinusoids are distinctive in. They are called discontinuous sinusoidal capillaries, are present in the liver and spleen, where greater movement of cells and materials is necessary. A capillary wall is simple squamous epithelium; the capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Molecules larger than 3 nm such as albumin and other large proteins pass through transcellular transport carried inside vesicles, a process which requires them to go through the cells that form the wall.
Molecules smaller than 3 nm such as water and gases cross the capillary wall through the space between cells in a process known as paracellular transport. These transport mechanisms allow bidirectional exchange of substances depending on osmotic gradients and can be further quantified by the Starling equation. Capillaries that form part of the blood–brain barrier however only allow for transcellular transport as tight junctions between endothelial cells seal the paracellular space. Capillary beds may control their blood flow via autoregulation; this allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response, in the kidney by tubuloglomerular feedback; when blood pressure increases, arterioles are stretched and subsequently constrict to counteract the
Extracellular fluid denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% of total body weight. About two thirds of this is intracellular fluid within cells, one third is the extracellular fluid; the main component of the extracellular fluid is the interstitial fluid. Extracellular fluid is the internal environment of all multicellular animals, in those animals with a blood circulatory system a proportion of this fluid is blood plasma. Plasma and interstitial fluid are the two components that make up at least 97% of the ECF. Lymph makes up a small percentage of the interstitial fluid; the remaining small portion of the ECF includes the transcellular fluid. The ECF can be seen as having two components – plasma and lymph as a delivery system, interstitial fluid for water and solute exchange with the cells; the extracellular fluid, in particular the interstitial fluid, constitutes the body's internal environment that bathes all of the cells in the body.
The ECF composition is therefore crucial for their normal functions, is maintained by a number of homeostatic mechanisms involving negative feedback. Homeostasis regulates, among others, the pH, sodium and calcium concentrations in the ECF; the volume of body fluid, blood glucose and carbon dioxide levels are tightly homeostatically maintained. The volume of extracellular fluid in a young adult male of 70 kg is 20% of body weight – about fourteen litres. Eleven litres is interstitial fluid and the remaining three litres is plasma; the main component of the extracellular fluid is the interstitial fluid which surrounds the cells in the body. The other major component of the ECF is the intravascular fluid of the circulatory system called blood plasma; the remaining small percentage of ECF includes the transcellular fluid. These constituents are called fluid compartments; the volume of extracellular fluid in a young adult male of 70 kg, is 20% of body weight – about fourteen litres. The interstitial fluid and the plasma make up about 97% of the ECF, a small percentage of this is lymph.
Interstitial fluid is fluid that surrounds cells, providing them with nutrients and removing their waste products. Eleven litres of the ECF is interstitial fluid and the remaining three litres is plasma. Plasma and interstitial fluid are similar because water and small solutes are continuously exchanged between them across the walls of capillaries, through pores and capillary clefts. Interstitial fluid consists of a water solvent containing sugars, fatty acids, amino acids, hormones, neurotransmitters, white blood cells and cell waste-products; this solution accounts for 26% of the water in the human body. The composition of interstitial fluid depends upon the exchanges between the cells in the biological tissue and the blood; this means that tissue fluid has a different composition in different tissues and in different areas of the body. The plasma that filters through the blood capillaries into the interstitial fluid does not contain red blood cells or platelets as they are too large to pass through but can contain some white blood cells to help the immune system.
Once the extracellular fluid collects into small vessels it is considered to be lymph, the vessels that carry it back to the blood are called the lymphatic vessels. The lymphatic system returns protein and excess interstitial fluid to the circulation; the ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs–Donnan effect. This causes a slight difference in the concentration of cations and anions between the two fluid compartments. Transcellular fluid is formed from the transport activities of cells, is the smallest component of extracellular fluid; these fluids are contained within epithelial lined spaces. Examples of this fluid are cerebrospinal fluid, aqueous humor in the eye, serous fluid in the serous membranes lining body cavities and endolymph in the inner ear, joint fluid. Due to the varying locations of transcellular fluid, the composition changes dramatically; some of the electrolytes present in the transcellular fluid are sodium ions, chloride ions, bicarbonate ions.
The extracellular fluid provides the medium for the exchange of substances between the ECF and the cells, this can take place through dissolving and transporting in the fluid medium. Substances in the ECF include dissolved gases and electrolytes, all needed to maintain life; the ECF contains materials secreted from cells in soluble form, but which coalesces into fibres or precipitates out into a solid or semisolid form. These and many other substances occur in association with various proteoglycans to form the extracellular matrix or the "filler" substance between the cells throughout the body; these substances occur in the extracellular space, are therefore all bathed or soaked in ECF, without being part of the ECF. The internal environment is stabilised in the process of homeostasis. Complex homeostatic mechanisms operate to keep the composition of the ECF stable. Individual cells can regulate their internal composition by various mechanisms. There is a significant difference between the concentrations of sodium and potassium ions inside and outside the cell.
The concentration of sodium ions is higher in the extracellular fluid than in the intracellular fluid. The converse is true outside the cell; these differences c
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a higher resolving power than light microscopes and can reveal the structure of smaller objects. A scanning transmission electron microscope has achieved better than 50 pm resolution in annular dark-field imaging mode and magnifications of up to about 10,000,000x whereas most light microscopes are limited by diffraction to about 200 nm resolution and useful magnifications below 2000x. Electron microscopes have electron optical lens systems that are analogous to the glass lenses of an optical light microscope. Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, large molecules, biopsy samples and crystals. Industrially, electron microscopes are used for quality control and failure analysis.
Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grabbers to capture the images. In 1926 Hans Busch developed the electromagnetic lens. According to Dennis Gabor, the physicist Leó Szilárd tried in 1928 to convince him to build an electron microscope, for which he had filed a patent; the first prototype electron microscope, capable of four-hundred-power magnification, was developed in 1931 by the physicist Ernst Ruska and the electrical engineer Max Knoll. The apparatus was the first practical demonstration of the principles of electron microscopy. In May of the same year, Reinhold Rudenberg, the scientific director of Siemens-Schuckertwerke, obtained a patent for an electron microscope. In 1932, Ernst Lubcke of Siemens & Halske built and obtained images from a prototype electron microscope, applying the concepts described in Rudenberg's patent. In the following year, 1933, Ruska built the first electron microscope that exceeded the resolution attainable with an optical microscope.
Four years in 1937, Siemens financed the work of Ernst Ruska and Bodo von Borries, employed Helmut Ruska, Ernst's brother, to develop applications for the microscope with biological specimens. In 1937, Manfred von Ardenne pioneered the scanning electron microscope. Siemens produced the first commercial electron microscope in 1938; the first North American electron microscope was constructed in 1938, at the University of Toronto, by Eli Franklin Burton and students Cecil Hall, James Hillier, Albert Prebus. Siemens produced a transmission electron microscope in 1939. Although current transmission electron microscopes are capable of two million-power magnification, as scientific instruments, they remain based upon Ruska’s prototype; the original form of the electron microscope, the transmission electron microscope, uses a high voltage electron beam to illuminate the specimen and create an image. The electron beam is produced by an electron gun fitted with a tungsten filament cathode as the electron source.
The electron beam is accelerated by an anode at +100 keV with respect to the cathode, focused by electrostatic and electromagnetic lenses, transmitted through the specimen, in part transparent to electrons and in part scatters them out of the beam. When it emerges from the specimen, the electron beam carries information about the structure of the specimen, magnified by the objective lens system of the microscope; the spatial variation in this information may be viewed by projecting the magnified electron image onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide. Alternatively, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution phosphor may be coupled by means of a lens optical system or a fibre optic light-guide to the sensor of a digital camera; the image detected by the digital camera may be displayed on a computer. The resolution of TEMs is limited by spherical aberration, but a new generation of hardware correctors can reduce spherical aberration to increase the resolution in high-resolution transmission electron microscopy to below 0.5 angstrom, enabling magnifications above 50 million times.
The ability of HRTEM to determine the positions of atoms within materials is useful for nano-technologies research and development. Transmission electron microscopes are used in electron diffraction mode; the advantages of electron diffraction over X-ray crystallography are that the specimen need not be a single crystal or a polycrystalline powder, that the Fourier transform reconstruction of the object's magnified structure occurs physically and thus avoids the need for solving the phase problem faced by the X-ray crystallographers after obtaining their X-ray diffraction patterns. One major disadvantage of the transmission electron microscope is the need for thin sections of the specimens about 100 nanometers. Creating these thin sections for biological and materials specimens is technically challenging. Semiconductor thin sections can be made using a focused ion beam. Biological tissue specimens are chemically fixed and embedded in a polymer resin to stabilize them sufficiently to allow ultrathin sectioning.
Sections of biological specimens, organic polymers, similar materials may require staining with heavy atom labels in order to achieve the required image contrast. One application of TEM is serial-section electron microscopy, for example in analyzing the connectivity in volumetric samples of brain tissue by imaging many thin sections in sequence; the SEM produces imag
A carbohydrate is a biomolecule consisting of carbon and oxygen atoms with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cmn. This formula holds true for monosaccharides; some exceptions exist. The carbohydrates are technically hydrates of carbon; the term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides and polysaccharides. Monosaccharides and disaccharides, the smallest carbohydrates, are referred to as sugars; the word saccharide comes from the Greek word σάκχαρον, meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides often end in the suffix -ose, as in the monosaccharides fructose and glucose and the disaccharides sucrose and lactose. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as structural components; the 5-carbon monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule known as RNA.
The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, preventing pathogenesis, blood clotting, development, they are found in a wide variety of processed foods. Starch is a polysaccharide, it is abundant in cereals and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet as table sugar, lactose and fructose, both of which occur in honey, many fruits, some vegetables. Table sugar, milk, or honey are added to drinks and many prepared foods such as jam and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, are metabolized by these bacteria to yield short-chain fatty acids.
In scientific literature, the term "carbohydrate" has many synonyms, like "sugar", "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are used with other meanings. In food science and in many informal contexts, the term "carbohydrate" means any food, rich in the complex carbohydrate starch or simple carbohydrates, such as sugar. In lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" is used for everything other than water, fat and ethanol; this includes chemical compounds such as acetic or lactic acid, which are not considered carbohydrates. It includes dietary fiber, a carbohydrate but which does not contribute much in the way of food energy though it is included in the calculation of total food energy just as though it were a sugar. In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.
The name "carbohydrate" was used in chemistry for any compound with the formula Cm n. Following this definition, some chemists considered formaldehyde to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the known carbohydrates and abundant carbohydrates deviate from this. For example, carbohydrates display chemical groups such as: N-acetyl, carboxylic acid and deoxy modifications. Natural saccharides are built of simple carbohydrates called monosaccharides with general formula n where n is three or more. A typical monosaccharide has the structure H–x–y–H, that is, an aldehyde or ketone with many hydroxyl groups added one on each carbon atom, not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose and glyceraldehydes.
However, some biological substances called "monosaccharides" do not conform to this formula and there are many chemicals that do conform to this formula but are not considered to be monosaccharides. The open-chain form of a monosaccharide coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon and hydroxyl group react forming a hemiacetal with a new C–O–C bridge. Monosaccharides can be linked togeth
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device. Transmission electron microscopes are capable of imaging at a higher resolution than light microscopes, owing to the smaller de Broglie wavelength of electrons; this enables the instrument to capture fine detail—even as small as a single column of atoms, thousands of times smaller than a resolvable object seen in a light microscope. Transmission electron microscopy is a major analytical method in the physical and biological sciences. TEMs find application in cancer research and materials science as well as pollution and semiconductor research.
TEM instruments boast an enormous array of operating modes including conventional imaging, scanning TEM imaging, diffraction and combinations of these. Within conventional imaging, there are many fundamentally different ways that contrast is produced, called "image contrast mechanisms." Contrast can arise from position-to-position differences in the thickness or density, atomic number, crystal structure or orientation, the slight quantum-mechanical phase shifts that individual atoms produce in electrons that pass through them, the energy lost by electrons on passing through the sample and more. Each mechanism tells the user a different kind of information, depending not only on the contrast mechanism but on how the microscope is used—the settings of lenses and detectors. What this means is that a TEM is capable of returning an extraordinary variety of nanometer- and atomic-resolution information, in ideal cases revealing not only where all the atoms are but what kinds of atoms they are and how they are bonded to each other.
For this reason TEM is regarded as an essential tool for nanoscience in both biological and materials fields. The first TEM was demonstrated by Max Knoll and Ernst Ruska in 1931, with this group developing the first TEM with resolution greater than that of light in 1933 and the first commercial TEM in 1939. In 1986, Ruska was awarded the Nobel Prize in physics for the development of transmission electron microscopy. In 1873, Ernst Abbe proposed that the ability to resolve detail in an object was limited by the wavelength of the light used in imaging or a few hundred nanometers for visible light microscopes. Developments in ultraviolet microscopes, led by Köhler and Rohr, increased resolving power by a factor of two; however this required expensive quartz optics, due to the absorption of UV by glass. It was believed that obtaining an image with sub-micrometer information was not possible due to this wavelength constraint. In 1858 Plücker observed the deflection of "cathode rays" with the use of magnetic fields.
This effect was used by Ferdinand Braun in 1897 to build simple cathode ray oscilloscopes measuring devices. In 1891 Riecke noticed that the cathode rays could be focused by magnetic fields, allowing for simple electromagnetic lens designs. In 1926 Hans Busch published work extending this theory and showed that the lens maker's equation could, with appropriate assumptions, be applied to electrons. In 1928, at the Technical University of Berlin, Adolf Matthias, Professor of High voltage Technology and Electrical Installations, appointed Max Knoll to lead a team of researchers to advance the CRO design; the team consisted of several PhD students including Bodo von Borries. The research team worked on lens design and CRO column placement, to optimize parameters to construct better CROs, make electron optical components to generate low magnification images. In 1931 the group generated magnified images of mesh grids placed over the anode aperture; the device used two magnetic lenses to achieve higher magnifications, arguably creating the first electron microscope.
In that same year, Reinhold Rudenberg, the scientific director of the Siemens company, patented an electrostatic lens electron microscope. At the time, electrons were understood to be charged particles of matter; the research group was unaware of this publication until 1932, when they realized that the De Broglie wavelength of electrons was many orders of magnitude smaller than that for light, theoretically allowing for imaging at atomic scales. In April 1932, Ruska suggested the construction of a new electron microscope for direct imaging of specimens inserted into the microscope, rather than simple mesh grids or images of apertures. With this device successful diffraction and normal imaging of an aluminium sheet was achieved; however the magnification achievable was lower than with light microscopy. Magnifications higher than those available with a light microscope were achieved in September 1933 with images of cotton fibers acquired before being damaged by the electron beam. At this time, interest in the electron microscope had increased, with other groups, such as Paul Anderson and Kenneth Fitzsimmons of Washington State Univ
Low-density lipoprotein is one of the five major groups of lipoprotein which transport all fat molecules around the body in the extracellular water. These groups, from least dense, compared to surrounding water to most dense, are chylomicrons low-density lipoprotein, intermediate-density lipoprotein, low-density lipoprotein and high-density lipoprotein. LDL delivers fat molecules to the cells and can drive the progression of atherosclerosis if they become oxidized within the walls of arteries, it is important to note that LDL is not "bad cholesterol". LDL is not cholesterol at all, not bad, it is an essential transport system for lipids the human body needs to survive, including cholesterol. There is both "large" and "small" particle LDL, while only small is associated with cholesterol-related issues, neither is "bad". "small" LDL is necessary to conduct nutrients to vessels that "large" LDL can't reach. Lipoproteins transfer lipids around the body in the extracellular fluid, making fats available to body cells for receptor-mediated endocytosis.
Lipoproteins are complex particles composed of multiple proteins 80–100 proteins/particle. A single LDL particle is about 220–275 angstroms in diameter transporting 3,000 to 6,000 fat molecules/particle, varying in size according to the number and mix of fat molecules contained within; the lipids carried include all fat molecules with cholesterol and triglycerides dominant. For years, it was believed that LDL particles posed a risk for cardiovascular disease when they invaded the endothelium and became oxidized, since the oxidized forms would be more retained by the proteoglycans, but there is growing evidence that this belief was supported by bad methodology, that there is no actual correlation between LDL and heart disease. A complex set of biochemical reactions regulates the oxidation of LDL particles, chiefly stimulated by presence of necrotic cell debris and free radicals in the endothelium. Increased concentrations of LDL particles is associated with the development of atherosclerosis over time.
Each native LDL particle enables emulsification, i.e. surrounding/packaging all fatty acids being carried, enabling these fats to move around the body within the water outside cells. Each particle contains a single apolipoprotein B-100 molecule, along with 80 to 100 additional ancillary proteins; each LDL has a hydrophobic core consisting of polyunsaturated fatty acid known as linoleate and hundreds to thousands esterified and unesterified cholesterol molecules. This core carries varying numbers of triglycerides and other fats and is surrounded by a shell of phospholipids and unesterified cholesterol, as well as the single copy of Apo B-100. LDL particles are 22 nm to 27.5 nm in diameter and have a mass of about 3 million daltons. Since LDL particles contain a variable and changing number of fatty acid molecules, there is a distribution of LDL particle mass and size. Determining the structure of LDL has been a tough task because of its heterogeneous structure; the structure of LDL at human body temperature in native condition, with a resolution of about 16 Angstroms using cryogenic electron microscopy, has been described.
LDL particles are formed as VLDL lose triglyceride through the action of lipoprotein lipase and they become smaller and denser, containing a higher proportion of cholesterol esters. When a cell requires additional cholesterol, it synthesizes the necessary LDL receptors as well as PCSK9, a proprotein convertase that marks the LDL receptor for degradation. LDL receptors are inserted into the plasma membrane and diffuse until they associate with clathrin-coated pits; when LDL receptors bind LDL particles in the bloodstream, the clathrin-coated pits are endocytosed into the cell. Vesicles containing LDL receptors bound to LDL are delivered to the endosome. In the presence of low pH, such as that found in the endosome, LDL receptors undergo a conformation change, releasing LDL. LDL is shipped to the lysosome, where cholesterol esters in the LDL are hydrolysed. LDL receptors are returned to the plasma membrane, where they repeat this cycle. If LDL receptors bind to PCSK9, transport of LDL receptors is redirected to the lysosome, where they are degraded.
LDL interfere with the quorum sensing system that upregulates genes required for invasive Staphylococcus aureus infection. The mechanism of antagonism entails binding Apolipoprotein B to a S. aureus autoinducer pheromone, preventing signaling through its receptor. Mice deficient in apolipoprotein B are more susceptible to invasive bacterial infection. LDL can be grouped based on its size: large low density LDL particles are described as pattern A, small high density LDL particles are pattern B. Pattern B has been associated by some with a higher risk for coronary heart disease; this is thought to be because the smaller particles are more able to penetrate the endothelium of arterial walls. Pattern I, for intermediate, indicates that most LDL particles are close in size to the normal gaps in the endothelium. According to one study, sizes 19.0–20.5 nm were designated as pattern B and LDL sizes 20.6–22 nm were designated as pattern A. Other studies have shown no such correlation at all; some evidence suggests the correlation between Pat
Anatomical terms of microanatomy
Anatomical terminology is used to describe microanatomical structures. This helps describe the structure and position of an object, minimises ambiguity. An internationally accepted lexicon is Terminologia Histologica. Epithelial cells line body surfaces, are described according to their shape, with three principal shapes: squamous and cuboidal. Squamous epithelium has cells. Cuboidal epithelium has cells whose height and width are the same. Columnar epithelium has cells taller. Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall, it is a thin layer of single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells. Epithelium can be arranged in a single layer of cells described as "simple", or more than one layer, described as "stratified".
By layer, epithelium is classed as either simple epithelium, only one cell thick or stratified epithelium as stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium that are two or more cells thick, both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia; this kind of epithelium is therefore described as pseudostratified columnar epithelium. Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs, it consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is of endodermal origin and is continuous with the skin at various body openings such as the eyes, inside the nose, inside the mouth, the urethral opening and the anus.
Some mucous membranes a thick protective fluid. The function of the membrane is to stop pathogens and dirt from entering the body and to prevent bodily tissues from becoming dehydrated; the submucosa consists of a dense and irregular layer of connective tissue with blood vessels and nerves branching into the mucosa and muscular layer. It contains the submucous plexus, enteric nervous plexus, situated on the inner surface of the muscular layer; the muscular layer consists of two layers of the inner and outer layer. The muscle of the inner layer is arranged in circular rings around the tract, whereas the muscle of the outer layer is arranged longitudinally; the stomach has an inner oblique muscular layer. Between the two muscle layers are the myenteric or Auerbach's plexus; this controls peristalsis. Activity is initiated by the pacemaker cells; the gut has intrinsic peristaltic activity due to its self-contained enteric nervous system. The rate can of course be modulated by the rest of the autonomic nervous system.
The layers are not longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Serosa / Adventitia -- these last two tissue types differ in form and function according to the part of the gastrointestinal tract they belong to; the hollow inner part of a body organ or tube is called the lumen. The side of a cell facing the lumen is called the apical surface.