Epithelium
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix.Functions of epithelial cells include diffusion, filtration, secretion, selective absorption, germination, and transcellular transport.Epithelial layers contain no blood vessels (avascular), so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.[3] 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.[11] Stratified epithelia (of columnar, cuboidal, or squamous type) can have the following specializations:[11] Epithelial tissue cells can adopt shapes of varying complexity from polyhedral to scutoidal to punakoidal.The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.[14] Tissues that line the inside of the mouth, the esophagus, the vagina, and part of the rectum are composed of nonkeratinized stratified squamous epithelium.Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells.[citation needed] In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle,[15] an outer covering of chitin, the rigidity of which varies as per its chemical composition.Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments.Gap junctions connect the cytoplasm of two cells and are made up of proteins called connexins (six of which come together to make a connexion).Primary cilia on epithelial cells provide chemosensation, thermoception, and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile.[citation needed] The word epithelium uses the Greek roots ἐπί (epi), "on" or "upon", and θηλή (thēlē), "nipple".
PileipellisAnatomical terms of microanatomySquamous epithelial cellSimpleStratifiedColumnar epithelial cellPseudostratifiedCuboidal epithelial cellOlfactoryRespiratoryIntestinalTransitionalVaginalfemaleTable of epithelia of human organsextracellular matrixepidermismesothelialorgansbody cavitiesblood vesselsanimaltissueconnective tissuemuscle tissuenervous tissueglandsdiffusionsecretionabsorptiongerminationtranscellular transportavascularbasement membraneCell junctionsalveolialveolar epitheliumendotheliumpericardiummesotheliumexocrine glandsgerminal epitheliumsweat glandsmammary glandssalivary glandsmicrovillibrush borderciliatedmucociliary clearancefallopian tubescentral canalspinal cordtaste budsHair cellsinner earsstereociliaGoblet cellspharynxsex organsrespiratory tractstratified squamous epitheliumstratified cuboidal epitheliumstratified columnar epitheliumpseudostratified columnar epitheliumTransitional epitheliumKeratinizedpyknoticoral mucosaurotheliumbladderuretersurethrapolyhedralscutoidalhistologybreastluminahuman skinstratified squamousesophagusvaginarectumnonkeratinizedgastrointestinal tractexocrineendocrine glandscornealymphatic vesselspleuraeperitoneumintegumentcuticlechitinnerve supplyblood supplybasal laminareticular laminaparacellular transporttight junctionsadherens junctionsdesmosomeshemidesmosomesgap junctionsconnexinsgerm layersectodermendodermmesodermpathologistssarcomascarcinomascell extrusionradiationtoxinstraumabody cavitycirculatory systemsalivaenzymedigestive juicesrespiratory epitheliumpolarised cellsthermoceptionmechanosensationmotileimmune mediators