epithelium

General Characteristics:  

  • Largely or entirely of closely packed cells with little extracellular material between cells 

  • Arranged in continuous sheets of either simple or stratified cells in thickness 

  • Anything that enters or leaves the body must pass through an epithelium 

  • Have a free surface which is exposed to external or internal space and basal surface which is attached to a basal lamina 

  • Are avascular 

  • Have a nerve supply 

  • Have a high capacity for renewal 

 

Identify the functions of epithelia 

  • Mechanical protection 

  • Absorption (gut) 

  • Secretion of materials (gut, kidney) 

  • Lubrication of surfaces (knee joint) 

  • Formation of a surface for diffusion 

  • Other functions: transportation, excretion, sensory reception and reproduction 

 

Define the two types of epithelia 

  • Covering 

  • Glandular 

  • Almost all epithelial tissue cells rest on connective tissue 

  • Epithelia lining the cavity of internal organs rest on the lamina propria (proprial layer) 

  • Evaginations in the connective tissue form papillae 

    • This is to increase surface area in order to: 

      • Better anchor to epithelial tissue 

      • To supply blood & nutrients to epithelial tissue 

 

Know the cell to cell junctions we studied on the basal and lateral surfaces of 

epithelial cells. 

•  Specialised intercellular junctions  

  • Functions varies by location and structure varies due to their functions 

  • Seals: closest to the free surface  

    • Zonulae occludens (tight junctions) 

      • Closes off intercellular space  

      • Prevents diffusion between cells  

      • Holds apical edges of cells together tightly using transmembrane proteins occludin, claudin 

        • Ulcers: break the seal between cells, causing unwanted substances like acids to enter where they are not supposed to be 

  • Adhesions: 

    • Zonula adherens (adhering junctions): 

      • Anchors cells together 

      • Connects cytoskeleton (actin filaments) of one cell to another 

    • Desmosome (macula adherens) 

      • Strong spot links between cells 

      • Attach to strong intermediate filaments 

    • Hemidesmosomes: 

      • Join epithelial cells to basal lamina 

      • Involve integrins (transmembrane proteins) 

  • Cell adhesion molecules (CAM) 

    • Cadherins: attach to actin filaments 

      • Anchors cells together in zonula adherens 

      • Role in suppressing epithelial tumor cells 

    • Integrins: cells attach to basal lamina 

      • Interact with basal lamina and extracellular matrix of connective tissue, affect cell movement and shape 

    • Selectins: when WBC leaves blood cells and enter into tissue 

  • Gap junctions: form small diffusion channel (connexon) between adjacent cells 

    • Allows signalling molecules to pass through, allowing intercellular communication 

    • Each connexon consists of 6 integral proteins called connexins 

 

Define the basal lamina and its collagen 

  • Basement membrane 

    • Visible with light microscope using a specific stain 

    • Usually formed by 2 laminae:  

      • Basal lamina: epithelial base 

        • Only seen with an electron microscope 

        • Binds epithelial cells to underlying connective tissue 

        • Composed of: 

          • Type IV collagen:  while most collagen is made by connective tissue, type IV is made by epithelial tissue 

          • Glycoproteins:  

            • Laminin: binds type IV collagen and integrins (hemidesmosomes) 

          • Proteoglycans secreted by epithelial cells 

      • Reticular lamina: connective tissue  

        • Synthesised by the connective tissue of the lamina propria 

        • Made of type III collagen and connecting collagen 

          • Type III: facilitates junction between the 2 collagens 

 

Describe the types of free surface specializations of an epithelial cell 

  • Cilia: move mucus & other substances 

    • Primarily found in trachea, bronchi, oviduct 

    • Made up of microtubules  

    • At base, there are engines that move cilia 

      • Lose cilia: can't move mucus out of the airway 

  • Microvilli: increase surface area 

    • Brush border of kidney tubule cell 

    • Straited border of intestinal absorptive cell 

    • Held up straight by actin filaments & anchored at terminal web 

  • Stereocilia (long microvilli): 

    • Only in epididymis and sensory cells of inner ear 

    • Cant move unlike other 2 

 

What is the terminal web? 

  • a contractile structure that influences the spatial distribution of microvilli as well as the development and morphogenesis of tissues containing polarized epithelial cells. 

  • Merges with actin filaments to anchor microvilli 

 

Describe the different types of covering epithelia and their functions 

  • Classification by number of layers 

    • Simple: single cell layer 

    • Stratified: multiple cell layers 

  • Classification by shape 

    • Squamous: flat  

    • Cuboidal: cube or hexagonal shape 

    • Columnar: tall and cylindrical shape 

  • Simple squamous: 

    • Allow active and passive movements of substances through tissue 

      • Endothelium: internal lining of heart and blood vessels 

      • Mesothelium (internal lining of ventral body cavities  

    • e.g: lining of alveoli to allow gas exchange 

  • Simple cuboidal: 

    • Secretions (sweat or tears) and absorption (water) 

  • Simple columnar: 

    • Protective, secretion and absorption 

    • Types: 

      • Ciliated: moves mucous and other substances 

      • Non-ciliated: 

        • Goblet cells (secretes mucus) 

        • Absorptive cells (have microvilli) 

  • Stratified squamous: 

    • Superficial layers are flat, deeper layers more cudboidal, deepest have continuous cell division 

    • Types: 

      • Keratinised, cornified, dry (tough, water-resistant)  

        • Outer layer of skin 

      • Non-keratinsied, non-cornified, wet  

        • Oral cavity 

  • Pseudostratified columnar: 

    • Appears to be multiple layers but is not, all cells attached to basal lamina 

    • Some cells do not reach free surface 

    • Secrete or may be ciliated 

    • Seen mostly in respiratory system 

      • Chronic bronchitis/emphysema: ciliated cells get destroyed but goblet cells still secreting mucus, so mucus is not moved and remains in the bronchus 

  • Transitional epithelium: only in urinary tract 

    • Specialised to protect from harshness of urine 

    • Cells change shape according to the degree of distension (enlargement) of the organ 

 

Differences between exocrine and endocrine glands 

  • Exocrine:  

    • Secrete into ducts so hollosed out shape 

      • Serous: watery (long duct) 

      • Mucous: thick, gooey (short duct) 

      • mixed 

    • Usually divided into lobes and lobules 

  • Endocrine: 

    • Ductless, connection with epithelium disintegrates 

    • Secrete into extracellular diffuse into blood 

       

Types of exocrine glands 

  • Merocrine: salivary glands, palms for sweat 

  • Holocrine: under the skin for hair follicles 

  • Apocrine: in the breast, secretes hormones 

  • Unicellular:  

    • Found in GI and respiratory system 

    • Main: goblet cells (secrete mucins that form mucus) 

  • Multicellular: 

    • Secretory sheet (lining of stomach) 

    • Tubular invagination 

      • Duct:  

        • Simple: colon, sweat gland 

        • Compound (branching): sebaceous gland, labial gland 

      • Secretory part of gland 

        • Tubular  

        • Acinar or alveolar (shaped like a blind sac) 

 

• Some cells we have studied can change phenotype. What happens to them? 

  • Osteoblasts change phenotype as they calcify 

    1. Secretory vesicles in osteoblasts 

    2. Release matrix vesicles and collagen fibers 

    3. Early mineralisation around vesicles 

    4. Matrix becoming confluent (flowing) between vesicles