Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease. In clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. In contrast, cytopathology examines free cells or tissue micro-fragments. Histopathological examination of tissues starts with biopsy, or autopsy; the tissue is removed from the body or plant, then...often following expert dissection in the fresh state...placed in a fixative which stabilizes the tissues to prevent decay. The most common fixative is formalin; the tissue is prepared for viewing under a microscope using either chemical fixation or frozen section. If a large sample is provided e.g. from a surgical procedure a pathologist looks at the tissue sample and selects the part most to yield a useful and accurate diagnosis - this part is removed for examination in a process known as grossing or cut up.
Larger samples are cut to situate their anatomical structures in the cassette. Certain specimens can undergo agar pre-embedding to assure correct tissue orientation in cassette & in the block & on the diagnostic microscopy slide; this is placed into a plastic cassette for most of the rest of the process. In addition to formalin, other chemical fixatives have been used. But, with the advent of immunohistochemistry staining and diagnostic molecular pathology testing on these specimen samples, formalin has become the standard chemical fixative in human diagnostic histopathology. Fixation times for small specimens are shorter, standards exist in human diagnostic histopathology. Water is removed from the sample in successive stages by the use of increasing concentrations of alcohol. Xylene is used in the last dehydration phase instead of alcohol - this is because the wax used in the next stage is soluble in xylene where it is not in alcohol allowing wax to permeate the specimen; this process is automated and done overnight.
The wax infiltrated specimen is transferred to an individual specimen embedding container. Molten wax is introduced around the specimen in the container and cooled to solidification so as to embed it in the wax block; this process is needed to provide a properly oriented sample sturdy enough for obtaining a thin microtome section for the slide. Once the wax embedded block is finished, sections will be cut from it and placed to float on a waterbath surface which spreads the section out; this is done by hand and is a skilled job with the lab personnel making choices about which parts of the specimen microtome wax ribbon to place on slides. A number of slides will be prepared from different levels throughout the block. After this the thin section mounted slide is stained and a protective cover slip is mounted on it. For common stains, an automatic process is used; the second method of histology processing is called frozen section processing. This is a technical scientific method performed by a trained histoscientist In this method, the tissue is frozen and sliced thinly using a microtome mounted in a below-freezing refrigeration device called the cryostat.
The thin frozen sections are mounted on a glass slide, fixed & in liquid fixative, stained using the similar staining techniques as traditional wax embedded sections. The advantages of this method is rapid processing time, less equipment requirement, less need for ventilation in the laboratory; the disadvantage is the poor quality of the final slide. It is used in intra-operative pathology for determinations that might help in choosing the next step in surgery during that surgical session; this can be done to slides processed by frozen section slides. To see the tissue under a microscope, the sections are stained with one or more pigments; the aim of staining is to reveal cellular components. The most used stain in histopathology is a combination of hematoxylin and eosin. Hematoxylin is used to stain nuclei blue, while eosin stains cytoplasm and the extracellular connective tissue matrix pink. There are hundreds of various other techniques. Other compounds used to color tissue sections include safranin, Oil Red O, congo red, silver salts and artificial dyes.
Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. A performed histochemical technique is the Perls' Prussian blue reaction, used to demonstrate iron deposits in diseases like Hemochromatosis. Antibodies have been used to stain particular proteins and carbohydrates. Called immunohistochemistry, this technique has increased the ability to identify categories of cells under a microscope. Other advanced techniques include in situ hybridization to identify specific DNA or RNA molecules; these antibody staining methods require the use of frozen section histology. These procedures above are carried out in the laboratory under scrutiny and precision by a trained specialist Medical laboratory scientist (Hist
The cell is the basic structural and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the "building blocks of life"; the study of cells is called cellular biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as multicellular; the number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years ago. Cells are of two types: eukaryotic, which contain a nucleus, prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotes include two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling, they are simpler and smaller than eukaryotic cells, lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome, in direct contact with the cytoplasm; the nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: Enclosing the cell is the cell envelope – consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule.
Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma and Thermoplasma which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter; the cell wall consists of peptidoglycan in bacteria, acts as an additional barrier against exterior forces. It prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment; some eukaryotic cells have a cell wall. Inside the cell is the cytoplasmic region that contains the genome and various sorts of inclusions; the genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid.
Plasmids encode additional genes, such as antibiotic resistance genes. On the outside and pili project from the cell's surface; these are structures made of proteins that facilitate communication between cells. Plants, fungi, slime moulds and algae are all eukaryotic; these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles in which specific activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA; this nucleus gives the eukaryote its name, which means "true kernel". Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may not be present; the eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins.
All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation and thermosensation. Cilia may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." Motile eukaryotes can move using motile flagella. Motile cells are absent in flowering plants. Eukaryotic flagella are more complex than those of prokaryotes. All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out, maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells possess DNA, the hereditary material of genes, RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery.
There are other kinds of biomolecules in cells. This article lists these primary cellular components briefly
A biomaterial is any substance, engineered to interact with biological systems for a medical purpose - either a therapeutic or a diagnostic one. As a science, biomaterials is about fifty years old; the study of biomaterials is called biomaterials science or biomaterials engineering. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into the development of new products. Biomaterials science encompasses elements of medicine, chemistry, tissue engineering and materials science. Note that a biomaterial is different from a biological material, such as bone, produced by a biological system. Additionally, care should be exercised in defining a biomaterial as biocompatible, since it is application-specific. A biomaterial, biocompatible or suitable for one application may not be biocompatible in another. Biomaterials can be derived either from nature or synthesized in the laboratory using a variety of chemical approaches utilizing metallic components, ceramics or composite materials.
They are used and/or adapted for a medical application, thus comprises whole or part of a living structure or biomedical device which performs, augments, or replaces a natural function. Such functions may be passive, like being used for a heart valve, or may be bioactive with a more interactive functionality such as hydroxy-apatite coated hip implants. Biomaterials are used every day in dental applications and drug delivery. For example, a construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time. A biomaterial may be an autograft, allograft or xenograft used as a transplant material; the ability of an engineered biomaterial to induce a physiological response, supportive of the biomaterial's function and performance is known as bioactivity. Most in bioactive glasses and bioactive ceramics this term refers to the ability of implanted materials to bond well with surrounding tissue in either osseoconductive or osseoproductive roles.
Bone implant materials are designed to promote bone growth while dissolving into surrounding body fluid. Thus for many biomaterials good biocompatibility along with good strength and dissolution rates are desirable. Bioactivity of biomateirals is gauged by the surface biomineralisation in which a native layer of hydroxyapatite is formed at the surface. Self-assembly is the most common term in use in the modern scientific community to describe the spontaneous aggregation of particles without the influence of any external forces. Large groups of such particles are known to assemble themselves into thermodynamically stable, structurally well-defined arrays, quite reminiscent of one of the 7 crystal systems found in metallurgy and mineralogy; the fundamental difference in equilibrium structure is in the spatial scale of the unit cell in each particular case. Molecular self-assembly is found in biological systems and provides the basis of a wide variety of complex biological structures; this includes an emerging class of mechanically superior biomaterials based on microstructural features and designs found in nature.
Thus, self-assembly is emerging as a new strategy in chemical synthesis and nanotechnology. Molecular crystals, liquid crystals, micelles, phase-separated polymers, thin films and self-assembled monolayers all represent examples of the types of ordered structures which are obtained using these techniques; the distinguishing feature of these methods is self-organization. Nearly all materials could be seen as hierarchically structured since the changes in spatial scale bring about different mechanisms of deformation and damage. However, in biological materials this hierarchical organization is inherent to the microstructure. One of the first examples of this, in the history of structural biology, is the early X-ray scattering work on the hierarchical structure of hair and wool by Astbury and Woods. In bone, for example, collagen is the building block of the organic matrix — a triple helix with diameter of 1.5 nm. These tropocollagen molecules are intercalated with the mineral phase forming fibrils that curl into helicoids of alternating directions.
These "osteons" are the basic building blocks of bones, with the volume fraction distribution between organic and mineral phase being about 60/40. In another level of complexity, the hydroxyapatite crystals are mineral platelets that have a diameter of 70–100 nm and thickness of 1 nm, they nucleate at the gaps between collagen fibrils. The hierarchy of abalone shell begins at the nanolevel, with an organic layer having a thickness of 20–30 nm; this layer proceeds with single crystals of aragonite consisting of "bricks" with dimensions of 0.5 and finishing with layers 0.3 mm. Crabs are arthropods whose carapace is made of a mineralized hard component and a softer organic component composed of chitin; the brittle component is arranged in a helical pattern. Each of these mineral ‘rods’ contains chitin–protein fibrils with 60 nm diameter; these fibrils are made of 3 nm diameter canals which link the exterior of the shell. Biomaterials are used in: Joint replacements Bone plates Intraocular lenses for eye surgery Bone cement Artificial ligaments and tendons Dental implants for tooth fixation Blood vessel prosth
A medical device is any device intended to be used for medical purposes. Thus what differentiates. Medical devices benefit patients by helping health care providers diagnose and treat patients and helping patients overcome sickness or disease, improving their quality of life. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assurance before regulating governments allow marketing of the device in their country; as a general rule, as the associated risk of the device increases the amount of testing required to establish safety and efficacy increases. Further, as associated risk increases the potential benefit to the patient must increase. Discovery of what would be considered a medical device by modern standards dates as far back as c. 7000 BC in Baluchistan where Neolithic dentists used flint-tipped drills and bowstrings. Study of archeology and Roman medical literature indicate that many types of medical devices were in widespread use during the time of ancient Rome.
In the United States it wasn't until the Federal Food and Cosmetic Act in 1938 that medical devices were regulated. In 1976, the Medical Device Amendments to the FD&C Act established medical device regulation and oversight as we know it today in the United States. Medical device regulation in Europe as we know it today came into effect in the 1993 by what is collectively known as the Medical Device Directive. On May 26th, 2017 the Medical Device Regulation replaced the MDD. Medical devices vary in both indications for use. Examples range from simple, low-risk devices such as tongue depressors, medical thermometers, disposable gloves, bedpans to complex, high-risk devices that are implanted and sustain life. One example of high-risk devices are those with Embedded software such as pacemakers, which assist in the conduct of medical testing and prostheses. Items as intricate as housings for cochlear implants are manufactured through the deep drawn and shallow drawn manufacturing processes; the design of medical devices constitutes a major segment of the field of biomedical engineering.
The global medical device market reached $209 billion USD in 2006 and was estimated to be between $220 and $250 billion USD in 2013. The United States controls ~40% of the global market followed by Europe and the rest of the world. Although collectively Europe has a larger share, Japan has the second largest country market share; the largest market shares in Europe belong to Germany, Italy and the United Kingdom. The rest of the world comprises regions like Australia, China and Iran; this article discusses what constitutes a medical device in these different regions and throughout the article these regions will be discussed in order of their global market share. A global definition for medical device is difficult to establish because there are numerous regulatory bodies worldwide overseeing the marketing of medical devices. Although these bodies collaborate and discuss the definition in general, there are subtle differences in wording that prevent a global harmonization of the definition of a medical device, thus the appropriate definition of a medical device depends on the region.
A portion of the definition of a medical device is intended to differentiate between medical devices and drugs, as the regulatory requirements of the two are different. Definitions often recognize In vitro diagnostics as a subclass of medical devices and establish accessories as medical devices. Section 201 of the Federal Food Drug & Cosmetic Act defines a device as an "instrument, implement, contrivance, implant, in vitro reagent, or other similar or related article, including a component part, or accessory which is: recognized in the official National Formulary, or the United States Pharmacopoeia, or any supplement to them Intended for use in the diagnosis of disease or other conditions, or in the cure, treatment, or prevention of disease, in man or other animals, or Intended to affect the structure or any function of the body of man or other animals, andwhich does not achieve its primary intended purposes through chemical action within or on the body of man or other animals and, not dependent upon being metabolized for the achievement of its primary intended purposes.
The term'device' does not include software functions excluded pursuant to section 520." According to Article 1 of Council Directive 93/42/EEC, ‘medical device’ means any "instrument, appliance, material or other article, whether used alone or in combination, including the software intended by its manufacturer to be used for diagnostic and/or therapeutic purposes and necessary for its proper application, intended by the manufacturer to be used for human beings for the purpose of: diagnosis, monitoring, treatment or alleviation of disease, monitoring, alleviation of or compensation for an injury or handicap, replacement or modification of the anatomy or of a physiological process, control of conception,and which does not achieve its principal intended action in or on the human body by pharmacological, immunological or metabolic means, but which may be assisted in its function by such means. The New Approach, defined in a European Council Resolution of May 1985, represents an innovative way of technical harmonisation.
It aims to remo
Macrophages are a type of white blood cell, of the immune system, that engulfs and digests cellular debris, foreign substances, cancer cells, anything else that does not have the type of proteins specific to healthy body cells on its surface in a process called phagocytosis. These large phagocytes are found in all tissues, where they patrol for potential pathogens by amoeboid movement, they take various forms throughout the body. Besides phagocytosis, they play a critical role in nonspecific defense and help initiate specific defense mechanisms by recruiting other immune cells such as lymphocytes. For example, they are important as antigen presenters to T cells. In humans, dysfunctional macrophages cause severe diseases such as chronic granulomatous disease that result in frequent infections. Beyond increasing inflammation and stimulating the immune system, macrophages play an important anti-inflammatory role and can decrease immune reactions through the release of cytokines. Macrophages that encourage inflammation are called M1 macrophages, whereas those that decrease inflammation and encourage tissue repair are called M2 macrophages.
This difference is reflected in their metabolism. However, this dichotomy has been questioned as further complexity has been discovered. Human macrophages are about 21 micrometres in diameter and are produced by the differentiation of monocytes in tissues, they can be identified using flow cytometry or immunohistochemical staining by their specific expression of proteins such as CD14, CD40, CD11b, CD64, F4/80 /EMR1, lysozyme M, MAC-1/MAC-3 and CD68. Macrophages were first discovered by Élie Metchnikoff, a Russian zoologist, in 1884. A majority of macrophages are stationed at strategic points where microbial invasion or accumulation of foreign particles is to occur; these cells together as a group are known as the mononuclear phagocyte system and were known as the reticuloendothelial system. Each type of macrophage, determined by its location, has a specific name: Investigations concerning Kupffer cells are hampered because in humans, Kupffer cells are only accessible for immunohistochemical analysis from biopsies or autopsies.
From rats and mice, they are difficult to isolate, after purification, only 5 million cells can be obtained from one mouse. Macrophages can express paracrine functions within organs that are specific to the function of that organ. In the testis for example, macrophages have been shown to be able to interact with Leydig cells by secreting 25-hydroxycholesterol, an oxysterol that can be converted to testosterone by neighbouring Leydig cells. Testicular macrophages may participate in creating an immune privileged environment in the testis, in mediating infertility during inflammation of the testis. Cardiac resident macrophages participate in electrical conduction via gap junction communication with cardiac myocytes. Macrophages can be classified on basis of the fundamental activation. According to this grouping there are classically activated macrophages, wound-healing macrophages and regulatory macrophages. Macrophages that reside in adult healthy tissues either derive from circulating monocytes or are established before birth and maintained during adult life independently of monocytes.
By contrast, most of the macrophages that accumulate at diseased sites derive from circulating monocytes. When a monocyte enters damaged tissue through the endothelium of a blood vessel, a process known as leukocyte extravasation, it undergoes a series of changes to become a macrophage. Monocytes are attracted to a damaged site by chemical substances through chemotaxis, triggered by a range of stimuli including damaged cells and cytokines released by macrophages at the site. At some sites such as the testis, macrophages have been shown to populate the organ through proliferation. Unlike short-lived neutrophils, macrophages survive longer in the body, up to several months. Macrophages are professional phagocytes and are specialized in removal of dying or dead cells and cellular debris; this role is important in chronic inflammation, as the early stages of inflammation are dominated by neutrophils, which are ingested by macrophages if they come of age. The neutrophils are at first attracted to a site, where they proliferate, before they are phagocytized by the macrophages.
When at the site, the first wave of neutrophils, after the process of aging and after the first 48 hours, stimulate the appearance of the macrophages whereby these macrophages will ingest the aged neutrophils. The removal of dying cells is, to a greater extent, handled by fixed macrophages, which will stay at strategic locations such as the lungs, neural tissue, bone and connective tissue, ingesting foreign materials such as pathogens and recruiting additional macrophages if needed; when a macrophage ingests a pathogen, the pathogen becomes trapped in a phagosome, which fuses with a lysosome. Within the phagolysosome and toxic peroxides digest the pathogen. However, some bacteria, such as Mycobacterium tuberculosis, have become resistant to these methods of digestion. Typhoidal Salmonellae induce their own phagocytos
In biochemistry, immunostaining is any use of an antibody-based method to detect a specific protein in a sample. The term "immunostaining" was used to refer to the immunohistochemical staining of tissue sections, as first described by Albert Coons in 1941. However, immunostaining now encompasses a broad range of techniques used in histology, cell biology, molecular biology that use antibody-based staining methods. Immunohistochemistry or IHC staining of tissue sections, is the most applied immunostaining technique. While the first cases of IHC staining used fluorescent dyes, other non-fluorescent methods using enzymes such as peroxidase and alkaline phosphatase are now used; these enzymes are capable of catalysing reactions that give a coloured product, detectable by light microscopy. Alternatively, radioactive elements can be used as labels, the immunoreaction can be visualized by autoradiography. Tissue preparation or fixation is essential for the preservation of cell morphology and tissue architecture.
Inappropriate or prolonged fixation may diminish the antibody binding capability. Many antigens can be demonstrated in formalin-fixed paraffin-embedded tissue sections. However, some antigens will not survive moderate amounts of aldehyde fixation. Under these conditions, tissues should be fresh frozen in liquid nitrogen and cut with a cryostat; the disadvantages of frozen sections include poor morphology, poor resolution at higher magnifications, difficulty in cutting over paraffin sections, the need for frozen storage. Alternatively, vibratome sections do not require the tissue to be processed through organic solvents or high heat, which can destroy the antigenicity, or disrupted by freeze thawing; the disadvantage of vibratome sections is that the sectioning process is slow and difficult with soft and poorly fixed tissues, that chatter marks or vibratome lines are apparent in the sections. The detection of many antigens can be improved by antigen retrieval methods that act by breaking some of the protein cross-links formed by fixation to uncover hidden antigenic sites.
This can be accomplished by using enzyme digestion. One of the main difficulties with IHC staining is overcoming non-specific background. Optimisation of fixation methods and times, pre-treatment with blocking agents, incubating antibodies with high salt, optimising post-antibody wash buffers and wash times are all important for obtaining high quality immunostaining. In addition, the presence of positive and negative controls for staining are essential for determining specificity. A flow cytometer can be used for the direct analysis of cells expressing one or more specific proteins. Cells are immunostained in solution using methods similar to those used for immunofluorescence, analysed by flow cytometry. Flow cytometry has several advantages over IHC including: the ability to define distinct cell populations by their size and granularity. However, flow cytometry can be less effective at detecting rare cell populations, there is a loss of architectural relationships in the absence of a tissue section.
Flow cytometry has a high capital cost associated with the purchase of a flow cytometer. Western blotting allows the detection of specific proteins from extracts made from cells or tissues, before or after any purification steps. Proteins are separated by size using gel electrophoresis before being transferred to a synthetic membrane via dry, semi-dry, or wet blotting methods; the membrane can be probed using antibodies using methods similar to immunohistochemistry, but without a need for fixation. Detection is performed using peroxidase linked antibodies to catalyse a chemiluminescent reaction. Western blotting is a routine molecular biology method that can be used to semi-quantitatively compare protein levels between extracts; the size separation prior to blotting allows the protein molecular weight to be gauged as compared with known molecular weight markers. The enzyme-linked immunosorbent assay or ELISA is a diagnostic method for quantitatively or semi-quantitatively determining protein concentrations from blood plasma, serum or cell/tissue extracts in a multi-well plate format.
Broadly, proteins in solution are adsorbed to ELISA plates. Antibodies specific for the protein of interest are used to probe the plate. Background is minimised by optimising blocking and washing methods, specificity is ensured via the presence of positive and negative controls. Detection methods are colorimetric or chemiluminescence based. Electron microscopy or EM can be used to study the detailed microarchitecture of cells. Immuno-EM allows the detection of specific proteins in ultrathin tissue sections. Antibodies labelled with heavy metal particles can be directly visualised using transmission electron microscopy. While powerful in detecting the sub-cellular localisation of a protein, immuno-EM can be technically challenging and require rigorous optimisation of tissue fixation and processing methods. Protein biotinylation in vivo was proposed to alleviate the problems caused by frequent incompatibility of antibody staining with fixation protocols that better preserve cell morphology. In immunostaining methods, an antibody is used to detect a specific protein epitope.
These antibodies can be monoclonal or p
Monocytes are a type of leukocyte, or white blood cell. They are the largest type of leukocyte and can differentiate into macrophages and myeloid lineage dendritic cells; as a part of the vertebrate innate immune system monocytes influence the process of adaptive immunity. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors. Monocytes are amoeboid in appearance, have a granulated cytoplasm. Containing unilobar nuclei, these cells are one of the types of mononuclear leukocytes which shelter azurophil granules; the archetypal geometry of the monocyte nucleus is ellipsoidal. Contrast to this classification occurs in polymorphonuclear leukocytes. Monocytes compose 2% to 10% of all leukocytes in the human body and serve multiple roles in immune function; such roles include: replenishing resident macrophages under normal conditions. In an adult human, half of the monocytes are stored in the spleen; these change into macrophages after entering into appropriate tissue spaces, can transform into foam cells in endothelium.
There are at least three types of monocytes in human blood: The classical monocyte is characterized by high level expression of the CD14 cell surface receptor The non-classical monocyte shows low level expression of CD14 and additional co-expression of the CD16 receptor. The intermediate monocyte with high level expression of CD14 and low level expression of CD16. While in humans the level of CD14 expression can be used to differentiate non-classical and intermediate monocytes, the slan cell surface marker was shown to give an unequivocal separation of the two cell types. Ghattas et al. state that the "intermediate" monocyte population is to be a unique subpopulation of monocytes, as opposed to a developmental step, due to their comparatively high expression of surface receptors involved in reparative processes as well as evidence that the "intermediate" subset is enriched in the bone marrow. After stimulation with microbial products the CD14+CD16++ monocytes produce high amounts of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12.
Said et al. showed that activated monocytes express high levels of PD-1 which might explain the higher expression of PD-1 in CD14+CD16++ monocytes as compared to CD14++CD16- monocytes. Triggering monocytes-expressed PD-1 by its ligand PD-L1 induces IL-10 production which activates CD4 Th2 cells and inhibits CD4 Th1 cell function. In mice, monocytes can be divided in two subpopulations. Inflammatory monocytes, which are equivalent to human classical CD14++ CD16− monocytes and resident monocytes, which are equivalent to human non-classical CD14low CD16+ monocytes. Resident monocytes have the ability to patrol along the endothelium wall in the steady state and under inflammatory conditions. In man a monocyte crawling behavior, similar to the patrolling in mice, has been demonstrated both for the classical and the non-classical monocytes. Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and typically move into tissues throughout the body where they differentiate into macrophages and dendritic cells.
They constitute between eight percent of the leukocytes in the blood. About half of the body's monocytes are stored as a reserve in the spleen in clusters in the red pulp's Cords of Billroth. Moreover, monocytes are the largest corpuscle in blood. Monocytes which migrate from the bloodstream to other tissues will differentiate into tissue resident macrophages or dendritic cells. Macrophages are responsible for protecting tissues from foreign substances, but are suspected to be important in the formation of important organs like the heart and brain, they are cells that possess a large smooth nucleus, a large area of cytoplasm, many internal vesicles for processing foreign material. In vitro, monocytes can differentiate into dendritic cells by adding the cytokines granulocyte macrophage colony-stimulating factor and interleukin 4. Monocytes and their macrophage and dendritic-cell progeny serve three main functions in the immune system; these are phagocytosis, antigen presentation, cytokine production.
Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes can perform phagocytosis using intermediary proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are capable of killing infected host cells via antibody-dependent cell-mediated cytotoxicity. Vacuolization may be present in a cell that has phagocytized foreign matter. Many factors produced by other cells can regulate other functions of monocytes; these factors include most chemokines such as monocyte chemotactic protein-1 and monocyte chemotactic protein-3.