Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of plasma membrane, which buds off inside the cell to form a vesicle containing the ingested material. Endocytosis includes phagocytosis, it is a form of active transport. The term was proposed by De Duve in 1963. Phagocytosis was discovered by Élie Metchnikoff in 1882. Endocytosis pathways can be subdivided into four categories: namely, receptor-mediated endocytosis, caveolae and phagocytosis. Clathrin-mediated endocytosis is mediated by the production of small vesicles that have a morphologically characteristic coat made up of the cytosolic protein clathrin. Clathrin-coated vesicles are found in all cells and form domains of the plasma membrane termed clathrin-coated pits. Coated pits can concentrate large extracellular molecules that have different receptors responsible for the receptor-mediated endocytosis of ligands, e.g. low density lipoprotein, growth factors and many others.
Study in mammalian cells confirm a reduction in clathrin coat size in an increased tension environment. In addition, it suggests that the two distinct clathrin assembly modes, namely coated pits and coated plaques, observed in experimental investigations might be a consequence of varied tensions in the plasma membrane. Caveolae are the most common reported non-clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types, they consist of the cholesterol-binding protein caveolin with a bilayer enriched in cholesterol and glycolipids. Caveolae are small flask-shape pits in the membrane, they can constitute up to a third of the plasma membrane area of the cells of some tissues, being abundant in smooth muscle, type I pneumocytes, fibroblasts and endothelial cells. Uptake of extracellular molecules is believed to be mediated via receptors in caveolae. Potocytosis is a form of receptor-mediated endocytosis that uses caveolae vesicles to bring molecules of various sizes into the cell.
Unlike most endocytosis that uses caveolae to deliver contents of vesicles to lysosomes or other organelles, material endocytosed via potocytosis is released into the cytosol. Pinocytosis, which occurs from ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which pinches off into the cell to form a vesicle filled with a large volume of extracellular fluid and molecules within it; the filling of the pocket occurs in a non-specific manner. The vesicle travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes. Phagocytosis is the process by which cells bind and internalize particulate matter larger than around 0.75 µm in diameter, such as small-sized dust particles, cell debris, micro-organisms and apoptotic cells. These processes involve the uptake of larger membrane areas than clathrin-mediated endocytosis and caveolae pathway. More recent experiments have suggested that these morphological descriptions of endocytic events may be inadequate, a more appropriate method of classification may be based upon the clathrin-dependence of particular pathways, with multiple subtypes of clathrin-dependent and clathrin-independent endocytosis.
Mechanistic insight into non-phagocytic, clathrin-independent endocytosis has been lacking, but a recent study has shown how Graf1 regulates a prevalent clathrin-independent endocytic pathway known as the CLIC/GEEC pathway. The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface, or sort them to degradation; the principal components of the endocytic pathway are: Early endosomes are the first compartment of the endocytic pathway. Early endosomes are located in the periphery of the cell, receive most types of vesicles coming from the cell surface, they have a characteristic tubulo-vesicular structure and a mildly acidic pH. They are principally sorting organelles where many endocytosed ligands dissociate from their receptors in the acid pH of the compartment, from which many of the receptors recycle to the cell surface, it is the site of sorting into transcytotic pathway to compartments via transvesicular compartments.
Late endosomes receive endocytosed material en route to lysosomes from early endosomes in the endocytic pathway, from trans-Golgi network in the biosynthetic pathway, from phagosomes in the phagocytic pathway. Late endosomes contain proteins characteristic of nucleosomes, mitochondria and mRNAs including lysosomal membrane glycoproteins and acid hydrolases, they are acidic, are part of the trafficking pathway of mannose-6-phosphate receptors. Late endosomes are thought to mediate a final set of sorting events prior the delivery of material to lysosomes. Lysosomes are the last compartment of the endocytic pathway, their chief function is to break down cellular waste products, carbohydrates and other macromolecules into simple compounds. These are returned to the cytoplasm as new cell-building materials. To accomplish this, lysosomes use some 40 differe
In biology, an organism is any individual entity that exhibits the properties of life. It is a synonym for "life form". Organisms are classified by taxonomy into specified groups such as the multicellular animals and fungi. All types of organisms are capable of reproduction and development, some degree of response to stimuli. Humans are multicellular animals composed of many trillions of cells which differentiate during development into specialized tissues and organs. An organism may be either a eukaryote. Prokaryotes are represented by two separate domains -- archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles. Fungi and plants are examples of kingdoms of organisms within the eukaryotes. Estimates on the number of Earth's current species range from 10 million to 14 million, of which only about 1.2 million have been documented. More than 99% of all species, amounting to over five billion species, that lived are estimated to be extinct.
In 2016, a set of 355 genes from the last universal common ancestor of all organisms was identified. The term "organism" first appeared in the English language in 1703 and took on its current definition by 1834, it is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kant's 1790 Critique of Judgment. An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, fungus or bacterium, capable of growth and reproduction". Many definitions exclude viruses and possible man-made non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction. A superorganism is an organism consisting of many individuals working together as a single functional or social unit. There has been controversy about the best way to define the organism and indeed about whether or not such a definition is necessary.
Several contributions are responses to the suggestion that the category of "organism" may well not be adequate in biology. Viruses are not considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism; this controversy is problematic because some cellular organisms are incapable of independent survival and live as obligatory intracellular parasites. Although viruses have a few enzymes and molecules characteristic of living organisms, they have no metabolism of their own; this rules out autonomous reproduction: they can only be passively replicated by the machinery of the host cell. In this sense, they are similar to inanimate matter. While viruses sustain no independent metabolism and thus are not classified as organisms, they do have their own genes, they do evolve by mechanisms similar to the evolutionary mechanisms of organisms; the most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly.
Some scientists argue. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible; this is not true for cells. If viruses did not exist, the direction of cellular evolution could be different, but cells would be able to evolve; as for the reproduction, viruses rely on hosts' machinery to replicate. The discovery of viral metagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong in the tree of life; the presence of these genes suggested. However, it was found that the genes coding for energy and protein metabolism have a cellular origin. Most these genes were acquired through horizontal gene transfer from viral hosts. Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival; the same laws that govern non-living chemistry govern the chemical processes of life.
It is the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes. Organisms owe their origin and many other internal functions to chemical phenomena the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles. Organisms are semi-closed chemical systems. Although they are individual units of life, they are not closed to the environment around them. To operate they take in and release energy. Autotrophs produce usable energy using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment; the primary chemical element in these compounds is carbon. The chemical properties of this element such as its grea
The blastula is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele is formed does the early embryo become a blastula; the blastula precedes the formation of the gastrula. A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoele. In mammals the blastula is referred to as a blastocyst; the blastocyst contains an embryoblast that will give rise to the definitive structures of the fetus, the trophoblast, which goes on to form the extra-embryonic tissues. During the blastula stage of development, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, regulate gene expression.
In many animals such as Drosophila and Xenopus, the mid blastula transition is a crucial step in development during which the maternal mRNA is degraded and control over development is passed to the embryo. Many of the interactions between blastomeres are dependent on cadherin expression E-cadherin in mammals and EP-cadherin in amphibians; the study of the blastula and of cell specification has many implications on the field of stem cell research as well as the continued improvement of fertility treatments. Embryonic stem cells are a field which, though controversial, have tremendous potential for treating disease. In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling. By manipulating the cell signals during the blastula stage of development, various tissues can be formed; this potential can be instrumental in regenerative medicine for injury cases. In vitro fertilisation involves implantation of a blastula into a mother's uterus.
Blastula cell implantation could serve to eliminate infertility. The blastula stage of early embryo development begins with the appearance of the blastocoel; the origin of the blastocoele in Xenopus has been shown to be from the first cleavage furrow, widened and sealed with tight junctions to create a cavity. In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore from the mother. In many organisms including Xenopus and Drosophila, the mid-blastula transition occurs after a particular number of cell divisions for a given species, is defined by the ending of the synchronous cell division cycles of the early blastula development, the lengthening of the cell cycles by the addition of the G1 and G2 phases. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle; the addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created.
This transition begins the growth in size of the organism. The mid-blastula transition is characterized by a marked increase in transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as SMAUG in Drosophila or by microRNA; these two processes shift the control of the embryo from the maternal mRNA to the nuclei. A blastula is a sphere of cells surrounding a blastocoele; the blastocoele is a fluid filled cavity which contains amino acids, growth factors, sugars and other components which are necessary for cellular differentiation. The blastocoele allows blastomeres to move during the process of gastrulation. In Xenopus embryos, the blastula is composed of three different regions; the animal cap forms the roof of the blastocoele and goes on to form ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate into mesodermal tissue.
The vegetal mass is composed of the blastocoel floor and develops into endodermal tissue. In the mammalian blastocyst there are three lineages that give rise to tissue development; the epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac. In mouse embryo, blastocoele formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient, the result of Na+/K+ ATPases that produce a high Na+ gradient on the basolateral side of the trophectoderm; this movement of water is facilitated by aquaporins. A seal is created by tight junctions of the epithelial cells. Tight junctions are important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to paracellular transport, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation; these tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification.
Within the blastula, inner blastomeres are non-polar while epithelial cells demonstrate polarity. Mammalian embryos undergo compaction around the 8-cell stage where E-cadherins as well
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are pronounced; some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as chordates produce a third layer, between these two layers. Making them triploblastic. Germ layers give rise to all of an animal’s tissues and organs through the process of organogenesis. Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. In 1817, Heinz Christian Pander discovered three primordial germ layers while studying chick embryos. Between 1850 and 1855, Robert Remak had further refined the germ cell layer concept, stating that the external and middle layers form the epidermis, the gut, the intervening musculature and vasculature; the term "mesoderm" was introduced into English by Huxley in 1871, "ectoderm" and "endoderm" by Lankester in 1873. Among animals, sponges show the simplest organization. Although they have differentiated cells, they lack true tissue coordination.
Diploblastic animals and Ctenophora, show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues. All higher animals are triploblastic, possessing a mesoderm in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs. Fertilization leads to the formation of a zygote. During the next stage, mitotic cell divisions transform the zygote into a hollow ball of cells, a blastula; this early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers. In all vertebrates, these progenitor cells differentiate into all adult organs. In the human embryo, after about three days, the zygote forms a solid mass of cells by mitotic division, called a morula; this changes to a blastocyst, consisting of an outer layer called a trophoblast, an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida and undergoes implantation.
The inner cell mass has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears; the epiblast in this region moves towards the primitive streak, dives down into it, forms a new layer, called the endoderm, pushing the hypoblast out of the way The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm; the endoderm is one of the germ layers formed during animal embryonic development. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm; the endoderm consists at first of flattened cells. It forms the epithelial lining of the whole of the digestive tract except part of the mouth and pharynx and the terminal part of the rectum, it forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas. The endoderm forms: the pharynx, the esophagus, the stomach, the small intestine, the colon, the liver, the pancreas, the bladder, the epithelial parts of the trachea and bronchi, the lungs, the thyroid, the parathyroid.
The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm; the formation of a mesoderm leads to the development of a coelom. Organs formed inside a coelom can move and develop independently of the body wall while fluid cushions and protects them from shocks; the mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, chorda-mesoderm. The chorda-mesoderm develops into the notochord; the intermediate mesoderm develops into gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, dermis; the lateral plate mesoderm develops into the circulatory system, the wall of the gut, wall of the human body. Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of differentiation; the mesoderm forms: muscle, cartilage, connective tissue, adipose tissue, circulatory system, lymphatic system, genitourinary system, serous membranes, notochord.
The ectoderm generates the outer layer of the embryo, it forms from the embryo's epiblast. The ectoderm develops into the surface ectoderm, neural crest, the neural tube; the surface ectoderm develops into: epidermis, nails, lens of the eye, sebaceous glands, tooth enamel, the epithelium of the mouth and nose. The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, facial cartilage, dentin of teeth; the neural tube of the ectoderm develops into: brain, spinal cord, posterior pituitary, motor neurons, retina. Note: The anterior pituitary develops from the ectodermal tissue of Rathke's pouch; because of its great importance, the neural crest is sometimes considered a fourth germ layer. It is, derived from the ectoderm. Histogenesis
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata. It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords; the subphylum was at one time called Urochordata, the term urochordates is still sometimes used for these animals. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits; some tunicates live as solitary individuals, but others replicate by budding and become colonies, each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding, they take in water through the incurrent siphon and expel the filtered water through the excurrent siphon. Most adult tunicates are sessile and permanently attached to rocks or other hard surfaces on the ocean floor. Various species of the subphylum tunicata are known as ascidians, sea squirts, sea pork, sea livers, or sea tulips.
The earliest probable species of tunicate appears in the fossil record in the early Cambrian period. Despite their simple appearance and different adult form, their close relationship to the vertebrates is evidenced by the fact that during their mobile larval stage, they possess a notochord or stiffening rod and resemble a tadpole, their name derives from their unique outer covering or "tunic", formed from proteins and carbohydrates, acts as an exoskeleton. In some species, it is thin and gelatinous, while in others it is thick and stiff. About 2,150 species of tunicate exist in the world's oceans, living in shallow water; the most numerous group is the ascidians. Some are solitary animals leading a sessile existence attached to the seabed, but others are colonial and a few are pelagic; some are supported by a stalk, but most are attached directly to a substrate, which may be a rock, coral, mangrove root, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, peaches, barrels, or bottles.
One of the largest is a stalked sea tulip, Pyura pachydermatina, which can grow to be over 1 metre tall. The Tunicata were established by Jean-Baptiste Lamarck in 1816. In 1881, Francis Maitland Balfour introduced a second name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates. No doubt because of his influence, various authors supported the term, either as such, or as "Urochordata", but this usage is invalid because "Tunicata" has precedence, grounds for superseding the name never existed. Accordingly, the current trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, the name Tunicata is invariably used in modern scientific works, it is accepted as valid by the World Register of Marine Species but not by the Integrated Taxonomic Information System. Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks. Sea squirts are so named because of their habit of contracting their bodies and squirting out water when disturbed.
Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat. Tunicates are more related to craniates, than to lancelets, hemichordates, Xenoturbella or other invertebrates; the clade consisting of tunicates and vertebrates is called Olfactores. The Tunicata contain 3,051 described species, traditionally divided into these classes: Ascidiacea Thaliacea Appendicularia Members of the Sorberacea were included in Ascidiacea in 2011 as a result of rDNA sequencing studies. Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group of paraphyletic status. Undisputed fossils of tunicates are rare; the best known and earliest unequivocally identified species is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, near Kunming. There is a common bioimmuration, of a possible tunicate found in Upper Ordovician bryozoan skeletons of the upper midwestern United States.
Three enigmatic species were found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia acidiformis, one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians. Ausia and Burykhia lived in shallow coastal waters more than 555 to 548 million years ago, are believed to be the oldest evidence of the chordate lineage of metazoans; the Russian Precambrian fossil Yarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia, so this identification has been questioned. Fossils of tunicates are rare because their bodies decay soon after death, but in some tunicate families, microscopic spicules are present, which may be preserved as microfossils; these spicules have been found in Jurassic and rocks, but, as few palaeontologists are familiar with them, they may have been mistaken for sponge spicules.
A multi-taxon molecular study in 2010
Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology encompasses the biology of regeneration, asexual reproduction and the growth and differentiation of stem cells in the adult organism. In the late 20th century, the discipline transformed into evolutionary developmental biology; the main processes involved in the embryonic development of animals are: regional specification, cell differentiation and the overall control of timing explored in evolutionary developmental biology: Regional specification refers to the processes that create spatial pattern in a ball or sheet of similar cells. This involves the action of cytoplasmic determinants, located within parts of the fertilized egg, of inductive signals emitted from signaling centers in the embryo; the early stages of regional specification do not generate functional differentiated cells, but cell populations committed to develop to a specific region or part of the organism. These are defined by the expression of specific combinations of transcription factors.
Morphogenesis relates to the formation of three-dimensional shape. It involves the orchestrated movements of cell sheets and of individual cells. Morphogenesis is important for creating the three germ layers of the early embryo and for building up complex structures during organ development. Cell differentiation relates to the formation of functional cell types such as nerve, secretory epithelia etc. Differentiated cells contain large amounts of specific proteins associated with the cell function. Growth involves both an overall increase in size, the differential growth of parts which contributes to morphogenesis. Growth occurs through cell division but through changes of cell size and the deposition of extracellular materials; the control of timing of events and the integration of the various processes with one another is the least well understood area of the subject. It remains unclear; the development of plants involves similar processes to that of animals. However plant cells are immotile so morphogenesis is achieved by differential growth, without cell movements.
The inductive signals and the genes involved are different from those that control animal development. Cell differentiation is the process. For example, muscle fibers and hepatocytes are well known types of differentiated cell. Differentiated cells produce large amounts of a few proteins that are required for their specific function and this gives them the characteristic appearance that enables them to be recognized under the light microscope; the genes encoding these proteins are active. Their chromatin structure is open, allowing access for the transcription enzymes, specific transcription factors bind to regulatory sequences in the DNA in order to activate gene expression. For example, NeuroD is a key transcription factor for neuronal differentiation, myogenin for muscle differentiation, HNF4 for hepatocyte differentiation. Cell differentiation is the final stage of development, preceded by several states of commitment which are not visibly differentiated. A single tissue, formed from a single type of progenitor cell or stem cell consists of several differentiated cell types.
Control of their formation involves a process of lateral inhibition, based on the properties of the Notch signaling pathway. For example, in the neural plate of the embryo this system operates to generate a population of neuronal precursor cells in which NeuroD is expressed. Regeneration indicates the ability to regrow a missing part; this is prevalent amongst plants, which show continuous growth, among colonial animals such as hydroids and ascidians. But most interest by developmental biologists has been shown in the regeneration of parts in free living animals. In particular four models have been the subject of much investigation. Two of these have the ability to regenerate whole bodies: Hydra, which can regenerate any part of the polyp from a small fragment, planarian worms, which can regenerate both heads and tails. Both of these examples have continuous cell turnover fed by stem cells and, at least in planaria, at least some of the stem cells have been shown to be pluripotent; the other two models show only distal regeneration of appendages.
These are the insect appendages the legs of hemimetabolous insects such as the cricket, the limbs of urodele amphibians. Considerable information is now available about amphibian limb regeneration and it is known that each cell type regenerates itself, except for connective tissues where there is considerable interconversion between cartilage and tendons. In terms of the pattern of structures, this is controlled by a re-activation of signals active in the embryo. There is still debate about the old question of whether regeneration is a "pristine" or an "adaptive" property. If the former is the case, with improved knowledge, we might expect to be able to improve regenerative ability in humans. If the latter each instance of regeneration is presumed to have arisen by natural selection in circumstances particular to the species, so no general rules would be expected; the sperm and egg fuse in the process of fertilization to form a fertilized egg, or zygote. This undergoes a period of divisions to form a ball or sheet of similar cells called a blastula or blastoderm.
These cell divisions are rapid with no growth so the daughter cells are half the size of the mother cell and the whole embryo stays ab
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