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Epithelial Tissues: covers exposed surfaces, lines internal passageways, chambers and forms glands

1. It includes
  1. Epithelia: layer of cell that covers internal and external surfaces. - covers exposed surfaces of body - form surface of skin and digestive, respiratory, reproductive and urinary tracts - more delicat ethipelia line the internal cavaties and passageways, chest cavity, spaces in the brain, inner surfaces of blood vessels, chambers of the heart
  2. Glands: structure that produce fluid secretion and a are either attached/derived from epithelial
2. Functions
  1. Cellularity — epithelial made of mostly cells bound together by interconnections called cell junctions
  2. Polarity — polarity refers to the presence of structural and functional differences between exposed and attached surfaces epithelial with one layer of cells has an exposed apical surface and an attached basal surface — both have different membranes
  3. Control permeability — substances entering/ leaving the body must pass through an epithelial tissue — some are impermeable/permeable to certain things like large proteins
  4. Provide Sensation — epithelia are sensitive and the sensory nerves prove info about external/ internal enviro. — ex. a mosquito on the skin will make you swap at it
  5. Avascularity — epithelial are avascular, meaning that they lack blood vessels epithelial tissues get their nutrients via diffusion/ absorption
  6. Provide Physical Protection — epithelia protect exposed and internal surfaces from abrasion, dehydration and destruction by chemical/ biological agents
  7. Regeneration — epit. cells which are damaged/ lost are replaced via cell division
  8. Attachment — base of epithelium is attached to a thin noncellular, complex structure basement membrane (aka basal lamina) which is made by basal surface & connective tissue
  9. Produce Specialized Secretions — epithelial cells produce secretions called gland cells Glandular epithelium — inside, all of epithelial cells produce secretions that either provide physical protection/ temp. regulation OR act as messengers
3. Specializations
  1. Production of secretion that provide physical protection/ act as chemical messengers
  2. Movement of fluids over epithelial surface, providing protection and lube
  3. Movement of fluids through epithelium, to control permeability
4. functional regions
  1. Apical Surface — cell is exposed to an internal/ external enviro.
  2. Basolateral Surface — include both the base, where call attached to underlying epithelial cells/ deeper tissues AND the sides, where cell contacts its neighbours
  3. Epithelia cells which line internal passageways have microvilli, mostly present on surfaces that involve absorption & secretion — like digestive system and kidneys Ciliated Epithelium — surfaces covered by cilia (abt 250) — moves substances over epithelial surface — ex. cilia in lungs help mucus go into the nasal passage where it captures particles
5. Intercellular Connections
  1. Cell junctions specialized areas of cell membranes that attach a ell to another call or to extracellular material
    1. Tight junctions — aka occluding junction; lipid portions of two plasma membranes are tightly bound together by interlocking membrane proteins — prevents water & solutes from passing b/w cells — isolates contents of lumen (space inside cell)
    2. Gap junctions — two cells are held together by two interlocking transmembrane proteins called connexons v
    3. Desmosome — are strong and can resist stretching and twisting - within each cell is a complex known as dense area - dense area is connected to cytoskeleton, which gives desmosome its strength
      1. Hemidesmosomes — attaches to extracellular filaments in the basement membrane which elks stabilize position of epithelial cell and anchors it to underlying tissue it to underlying tissue
      2. Spot desmosomes — small discs connected to bands of intermediate filaments — filaments stabilize shape of cell
  3. Extensive areas of opposing plasma membrane Extensive areas of opposing plasma membrane are connected by transmembrane proteins called cell adhesion molecules (CAMs), which bind to each other and extracellular materials Membranes of adjacent cells may also be bonded by a thin layer of proteoglycans that contain polysaccharide derivatives known as hyaluronan
6. Classification of Epithelia
  1. Simple epithelium— when one layer of cells cover basement membrane they are thin and fragile — cannot mechanically protect line only protected areas inside body like internal passageways and compartments — like pericardial, pleural, peritoneal cavities (heart chambers, blood vessels) present where secretion and absorption occur ex. intestines
  2. Stratified epithelium — several layers of cells cover the basement membrane located in areas that are exposed to mechanical/ chemical stresses — ex. skin and lining of mouth
  3. Squamous epithelium — thin, flat, irregular shape (jigsaw puzzle)
    1. Stratified squamous epithelium — series of layers located where mechanical stresses are severe protects against physical & chemical attacks — ex. lining of mouth, surface of skin, esophagus, anus
    2. Simple squamous epithelium — most delicate type of epithelium located in protected areas where absorption & diffusion occurs OR where a slick, slipped surface reduces friction — ex. alveoli in the lungs, lining of heart/ blood vessels
      1. Mesothelium — simple squamous epithelium that lines the body cavities enclosing the lungs, heart , abdominal organs pleura, pericardium, peritoneum contain a layer of mesothelium
      2. Endothelium — SSE that lines inner surface of heart and blood vessels
  4. Cuboidal Epithelia cells resemble hexagonal boxes from their apical surfaces w/ nuclei in the centre
    1. Simple cuboidal epithelium — provides limited protection & occurs where secretion/ absorption occurs — like, kidney tubules\
    2. Stratified cuboidal epithelia — rare, located along ducts of sweat glands and mammary glands
  5. Columnar Epithelia — rectangular the densely packed cells are hexagonal, taller & more slender than cuboidal epithelia cells elongated nuclei crowded into narrow band close to basement membrane height is a huge distance b/w adjacent nuclei
    1. Stratified columnar epithelia — rare, protects portions of pharynx, epiglottis, anus, urethra, large excretory ducts
    2. Simple columnar epithelium — found where absorption/ secretion occurs & protect against chemical stresses— ex. small intestine, stomach
    3. Pseudostratified columnar epithelium — found in portions of respiratory tract & male reproductive tract have cilia — in, nasal cavity, trachea. bronchi includes many cells w/ many shapes and functions can appear to be layered/ stratified — it is not stratified bc every epithelial cell contacts the basement membrane Stratified columnar epithelia — rar
  6. Transitional Epithelia — plump & cuboidal unusual — tolerates repeated stretching and recoiling w/o damage found in urinary system — like, bladder, where are volumes increase and the lining is stretched causing the epithelia to look flat
7. Glandular Epithelia
  1. Endocrine glands — ductless glands produces endocrine secretions called hormones, which enter bloodstream Hormones — regulate/coordinate the activities of various tissues, organs, organ systems glands include: thyroid gland & pituitary gland can also be part of epithelial surfaces — like, lining of digestive tract, pancreas, thyroid gland, thymus
  2. Exocrine glands — produce exocrine secretions which are emptied onto skin surface or epithelium lining an internal passageway via tubular ducts
    1. Merocrine secretions — most common mode of exocrine secretion product released from secretory vesicles via exocytosis Mucin — merocrine secretion that mixes with water to form mucus — lube that captures microorganisms, foreign particles AND can also cant food and reduce friction merocrine sweat glands produce watery perspiration that helps cool the body
    2. Apocrine secretions — involves loss of cytoplasm and secretory product apical portion of cytoplasm has secretory vesicles and is then shed milk production in mammary glands is a combo of merocrine and apocrine secretions
    3. Holocrine secretions — destroys gland cell During holocrine secretion: Cell packed w/ secretory vesicles —> burst —> releasing secretion —> killing cell further secretion depends on replacing destroyed gland cells by division of stem cells Sebaceous glands — associated with hair follicles and produce oily hair coating
  3. Types of secretion:
    1. Serous glands — secrete a watery solution that contains enzymes — ex. parotid salivary glands
    2. Mixed endocrine gland — more than one type of gland cell, which may produce TWO diff. exocrine secretions, one serous and other mucous — ex. submandibular salivary gland
    3. Mucous gland — secrete mucins that hydrate to form mucus — ex. sublingual salivary glands & submucosal glands of small intestine
  4. Gland Structure
    1. Unicellular exocrine glands — individual secretory glands only UEG in the body are Mucous (goblet) cells, which secrete mucins mucus cells scattered along epithelial cells pseudostratified ciliated columnar epithelium that lines trachea and columnar epithelium of the small & large intestine have a lot of mucous cells
    2. Multicellular exocrine glands — secretory sheet, where gland cells form an epithelium that releases secretions into inner compartment secretion travels through one or more ducts to the surface — ex. salivary glands protect organ from its own acids and enzymes by producing mucin
      1. Structure of the Duct — gland is simple if it does not divide on its way to gland cells — it is compound if duct divides
      2. Shape of secretory portion of gland — glands whose glandular cells form tubes are tubular (tube shaped), can be straight/ coiled — those that form blind pockets are alveolar/ acinar — glands that form tubes and pockets are tubuloacinar
      3. Relationship b/w ducts and glandular areas — gland is branched if many secretory areas share a duct

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Epithelial Tissues: covers exposed surfaces, lines internal passageways, chambers and forms glands

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	Created by Jeannene Rumball about 9 years ago