Quiz 2 and 3

• epithelium is a tissue in which cells are bound tightly together structurally and functionally to form a sheetlike or tubular structure with little extracellular material between the cells.
• Cells in epithelia each have an apical side facing the sheet’s free surface and a basal side facing a basement membrane and under- lying connective tissue.
• Epithelia are often specialized for absorption or transcytosis, pinocytosis of material at the apical side and exocytosis at the baso- lateral side (or vice versa).
• Cells of most epithelia exhibit continuous renewal, with the locations of stem cells and rates of cell turnover variable in various spe- cialized epithelia.
• basement membrane of all epithelia is a thin extracellular layer of specialized proteins, usually having two parts: a basal lam- ina and a more fibrous reticular lamina.
• basal lamina is a thin meshwork of type IV collagen and laminin produced by the epithelial cells.
• reticular lamina contains type III collagen and anchoring fibrils of VII collagen, all secreted by cells of the immediately adja- cent connective tissue.
• these components attach epithelia to connective tissue, regulate (filter) substances passing from connective tissue into epithelia, provide a guide or scaffold during tissue regeneration after injury, and compartmentalize epithelial cells from other tissues.
• The cells’ size and morphology are generally dictated by their function
• Epithelial tissues


  • Have a small amount of extracellular matrix
• Columnar cells generally have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei
• Epithelial tissue function


  Lining of external and internal surfaces or body cavities; glandular secretion
• Epithelial tissues are composed mainly of aggregated polyhedral cells, adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface
• Characteristic features of epithelial cells

  Shapes and dimensions can be tall columnar cells, cuboidal cells, low squamous cells
• Specific cells of certain epithelia may be contractile (myoepithelial cells) or specialized sensory cells, such as those of taste buds or the olfactory epithelium
• Most epithelia are adjacent to connective tissue containing blood vessels from which the epithelial cells receive nutrients and O2
• The lipid-rich membranes of epithelial cells are frequently indistinguishable by light microscopy; the number and shape of their stained nuclei are important indicators of cell shape and density
• Functions of Epithelial tissues

  • Covering, lining, and protecting surfaces
  • Absorption
  • Secretion
• Papillae projects from the connective tissue into the epithelium and occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue
• Epithelial cell nuclei vary in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape
• Epithelial cells generally show polarity, with organelles and membrane proteins distributed unevenly within the cell
• Lamina propria is a connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems
• Basal pole is the region of the cell contacting the ECM and connective tissue, while the apical pole is the opposite end, usually facing a space
• Regions of cuboidal or columnar cells that adjoin neighboring cells comprise the cells’ lateral surfaces; cell membranes here often have numerous folds that increase the area and functional capacity of that surface
• Basement membrane serves as a scaffold that allows rapid epithelial repair and regeneration and is a semipermeable filter for substances reaching epithelial cells from below
• Basement membrane is the basal surface of all epithelia that rests on a thin extracellular, felt-like sheet of macromolecules
• With the transmission electron microscope (TEM), two parts of the basement membrane may be resolved: Basal lamina and reticular lamina
• Glycoproteins and other components in the basement membrane structure can often be stained and visualized with the light microscope
• Basal lamina


  • Includes Type IV collagen, Laminin, Nidogen, and perlecan
  • Type IV collagen self assembles into a two-dimensional network
  • Laminin attaches to transmembrane integrin proteins and projects through the mesh formed by Type IV collagen
  • Nidogen and perlecan cross-link laminins to the Type IV collagen network, providing the basal lamina's three-dimensional structure and determining its porosity
• Laminin, a major glycoprotein within basal laminae, is shown by immunohistochemistry and identifies the basement membranes of the stratified epithelium (E) and the simple epithelium lining a small blood vessel (V)
• Hemidesmosomes (H) bind the basal surface of the epithelial cell (C) to the basal lamina
• The dense basal lamina (BL), 20-100 nm thick, may appear with thin clear zones on each side and is anchored to a thicker, more diffuse reticular lamina (RL) containing collagen III fibers
• Basement membrane structure

  1. Basal lamina - nearest on the epithelial cells

  2. Reticular lamina - beneath the basal lamina

  3. Basement membrane and basal lamina are sometimes used interchangeably

  4. Basement membrane usually denotes the fine extracellular layer seen ultrastructurally

  5. Macromolecules of the basement membrane are secreted from the basal sides of the epithelial cells and form a sheetlike array

  6. Basal lamina helps organize integrins and other proteins in the plasma membrane of epithelial cells, maintaining cell polarity and helping to localize endocytosis, signal transduction, and other activities
• Basal laminae (external laminae) exist as thin sleeves surrounding muscle cells, nerves, and fat-storing cells
• Basal laminae serve as semipermeable barriers regulating macromolecular exchange between enclosed cells and connective tissue, provide structural support for epithelial cells, and attach epithelia to underlying connective tissue
• Epithelial cells adhere strongly to neighboring cells and basal laminae, particularly in epithelia subject to friction or other mechanical forces
• The more diffuse meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both produced by cells of the connective tissue
• Lateral surfaces of epithelial cells have complexes of several specialized intercellular junctions with different functions: Tight or occluding junctions, Adherent or anchoring junctions, Gap junctions
• Tight junctions prevent passive flow of material between cells but are not very strong, close to adherent junctions, maintain two distinct membrane domains (apical and basal)
• Apical cell membranes


  • Part of the luminal compartment of a tissue or an organ