EPithelial

• Four basic tissue types are: epithelial, connective, muscle, and nervous
• Epithelial tissue:

  • Composed of aggregated polyhedral cells
  • Small amount of extracellular matrix
  • Functions include lining the surface of body cavities and glandular secretion
• Connective tissue:

  • Composed of several types of fixed and wandering cells
  • Abundant amount of extracellular matrix
  • Functions include support and protection of tissues or organs
• Muscle tissue:

  • Composed of elongated contractile cells
  • Moderate amount of extracellular matrix
  • Functions include strong contraction and body movements
• Nervous tissue:

  • Composed of elongated cells with extremely fine processes
  • Very small amount of extracellular matrix
  • Functions include transmission of nerve impulses
• Epithelial tissue is tissue in which cells are bound tightly together structurally and functionally to form a sheetlike or tubular structure
• Epithelial tissue lines all external and internal surfaces of the body
• Epithelial tissue is avascular, lacking blood vessels, and relies on connective tissue for nutrient supply
• Functions of epithelial tissue include:

  • Protecting the body from abrasion and injury (e.g., skin and esophagus)
  • Lines the internal and covers the external surfaces of the body
  • Absorption of material from a lumen (e.g., tubules in the kidney, small and large intestines.)
  • Transportation of material along a surface (e.g., cilia- mediated transport in the trachea.)
  • Secretion of mucus, hormones, and proteins (e.g., glands)
  • Gas exchange (e.g., alveoli in lungs)
  • Lubrication between two surfaces (e.g., mesothelium or pleural cavity)
• Basement membrane:

  • Structure near epithelial cells
  • Thin extracellular layer of specialized proteins
  • Acts as a filter and provides support for epithelial cells
  • Attaches epithelial tissue to underlying connective tissue
• Basement membrane has two parts which can only be viewed by the Transmission Electron Microscope:

  • Basal lamina: thin meshwork of type IV collagen and laminin
  • Reticular lamina: contains type III collagen and anchoring fibrils of VII collagen
• Types of junctional complexes:

  • Occluding or tight junctions (zonula occludens)
  • Adherent junction (zonula adherens)
  • Desmosome or macula adherens
  • Hemidesmosome
  • Gap or communicating junctions
• Apical structures of epithelial cells include:

  • Microvilli: small membrane projections with cores of actin filaments
  • Stereocilia: long microvilli with specialized mechanosensory function
  • Cilia: larger projecting structures with a core of microtubules for ciliary movement
• Ciliary movement in epithelial surfaces is caused by a well-organized core of microtubules with dynein-based sliding, propelling material along the surface
• Motile cilia exhibit a rapid beating pattern that moves substances into our tissue
• Axoneme is a 9 + 2 assembly of microtubules responsible for the beating pattern in ciliary movement
• Epithelial cells can be classified based on shape:

  • Squamous: flat, sheet-like appearance
  • Cuboidal: square/cuboid shape with equal width, height, and depth
  • Columnar: tall/rectangular shape taller than wide
• Epithelial cells can also be classified based on the number of layers:

  • Pseudostratified epithelium: closely packed cells that appear to be in many layers due to different sizes and orientations of their nucleus
  • Transitional epithelium or Urothelium: multiple cell layers that can change shape depending on organ distension
• Based on surface modification, epithelial cells can be:

  • Ciliated for movement of substances
  • Keratinized/non-keratinized for protection and gradual movement of cells
• Functional epithelium types include:

  • Mesothelium: serous lining of cavities for lubrication such as pericardium, pleura, and peritoneum
  • Endothelium: flat cells lining blood vessels
  • Myoepithelium: basket cells for scar remodeling
  • Endometrium: lining of the uterus
  • Germinal epithelium: layer of ovaries and seminiferous tubules
  • Glandular epithelium for secretion, also known as secretory epithelium
• Summary of types of epithelial tissue:

  • Simple squamous: lining of Loop of Henle, alveoli, blood vessels
  • Simple cuboidal: ducts of glands, lining of kidney tubules
  • Simple columnar: uterine tube, stomach lining, gall bladder, fallopian tube, small intestine
  • Stratified squamous: non-keratinized in vagina, cervix, mouth, esophagus; keratinized in skin
  • Stratified cuboidal: ducts of sweat glands
  • Stratified columnar: male urethra
  • Transitional: urinary bladder, urinary tract
  • Pseudostratified columnar: trachea
• Secretory epithelia and glands:

  • Exocrine glands have ducts carrying secretions, can be simple or compound
  • Modes of secretion by exocrine glands include merocrine, holocrine, and apocrine
  • Endocrine glands lack ducts and secrete hormones which are carried throughout the body by interstitial fluid and blood, with specificity produced by the hormone receptors of target cells. It secrete products directly into the bloodstream.
• Mucous and serous glands:

  • Mucous glands produce mucus, often through goblet cells. The oligosaccharide components of mucus stain poorly with routine dyes but stain well with periodic acid-Schiff (PAS) stain
  • Serous glands produce enzymes and stain darkly with H&E due to rough endoplasmic reticulum content and secretory granules.
• Epithelial cells show polarity:

  • Basal Pole or Surface - a cell region facing the connective tissue.
  • Apical Pole or Surface - a cell region facing the lumen or surface.
• • The number and shape of stained nuclei are important indicators of cell shape and density. It is a primary morphologic criterion for classifying Epithelia.
  • The cell's size and morphology are generally dictated by their function.
  • epithelial cells are variable in shape and dimensions
  • All epithelial cells lie in the basement membrane
• OCCLUDING JUNCTION OR TIGHT JUNCTION OR ZONULA OCCLUDENS

  • Seals adjacent cells to one another, controlling the passage of molecules between them.
  • separates apical and basolateral membrane domains
  • Contains major transmembrane links proteins: occludins, claudins, ZO proteins.
• ADHERENT JUNCTION OR ZONULA ADHERENS

  • Provides points linking the cytoskeleton of adjacent cells
  • Strengthens and stabilizes nearby tight juntion
  • the sight of strong cellular adhesion
  • Contains transmembrane link proteins: E-cadherin, catenin complexes.
• DESMOSOME OR MACULA ADHERENS

  • provides points of strong intermediate filament coupling between adjacent cells, strengthening the tissue
  • resembles or forms a belt around the cells
  • contains major transmembrane link proteins: cadherin family proteins (desmogleins and desmocollin)
• HEMIDESMOSOME

  • anchors cytoskeleton to the basal lamina
  • contains transmembrane link proteins: integrins
• GAP OR COMMUNICATING JUNCTIONS

  • Allows direct transfer of small molecules and ions from one cell to another
  • focuses more on intercellular communication instead of cell-to-cell adhesion
  • contains transmembrane link protein: Connexin wherein each connexon is made up of 6 subunits of connexin and one channel is made up of 1 connexon(1.5 nanometer per diameter)
• The motile cilia exhibit a rapid beating pattern that moves certain substances into our tissue. The one responsible for that pattern or movement is the AXONEME. Axoneme contains an assembly of 11 microtubules (9 and 2).
• Merocrine secretion

  • releases products usually containing proteins by means of exocytosis at the apical end of the secretory cells
  • most exocrine glands are merocrine
  • no part of the cell is lost (e.g., salivary glands)
• Holocrine Secretion

  • Secretion is produced by the disintegration of the secretory cells themselves as they complete their terminal differentiation, which involves becoming filled with product.
  • The whole cell detaches during secretion; complete destruction of the glandular epithelium.
  • Sebaceous glands of hair follicles are the best example of holocrine glands
• Apocrine Secretion

  • Involves loss of membrane-enclosed apical cytoplasm usually containing one or more lipid droplets.
  • Apocrine secretion, along with merocrine secretion, is seen in mammary glands
  • partial destruction of cells