Tissues

• Tissues


  Specialised cells embedded in an extracellular matrix
• Key types of tissue

  • Epithelial
  • Connective
  • Muscle
  • Nervous
• Cells

  • Have an internal cytoskeletal network of filaments to give shape and resist tension and compression
  • Secrete extracellular matrix for support, structure, and regulation of cell communication and behaviour
• Extracellular matrix


  • Maintains the differentiated state of cells
  • Maintains normal overall development of tissues and cells
• Epithelial tissue


  • Lines organs, body cavities and covers external skin
  • Provides a protective barrier, and has secretory and absorptive functions
  • Has polarity, apical and basal surfaces
• Basement membrane

  Acts as a scaffold and anchor between epithelial tissue and connective tissue
• Epithelial tissue

  • Is avascular and regenerative
• Types of epithelium

  • Stratified
  • Simple
  • Squamous
  • Columnar
  • Cuboidal
  • Transitional
  • Pseudostratified
• Epithelial cells

  • Can have microvilli and cilia
  • Cilia are much longer and microvilli have an actin core that is an extension of the cell
• Epithelium classification

  • Surface
  • Glandular
• Cell junctions

  • Occluding junctions (tight junctions)
  • Anchoring junctions (zonula Adherens, desmosomes, hemidesmosomes)
  • Communication junctions (gap junctions)
• Tight junctions


  Sit close to the apical membrane, join cells together and control the paracellular pathway
• All cell junctions except for hemidesmosomes are on the lateral side of cell
• Zonula Adherens


  Attach neighbouring cells on lateral borders, connect actin networks of neighbouring cells
• Desmosomes


  Are strong adhesions
• Hemidesmosomes


  Are half desmosomes that anchor cells to underlying basement membrane
• Gap junctions


  Allow communication between cells
• Junctional complex


  Includes a tight junction, zonula Adherens, and desmosome
• Desmosomes have a darker stain in electron microscopy because they are dense and therefore stronger
• Tight junctions


  Control the paracellular pathway which controls what passes between the cells
• Tight junction proteins

  Occludins and claudins
• Claudin-16


  Allows Mg2+ to pass through
• Mutation in Claudin-16


  Produces an abnormal protein which doesn't let Mg2+ into the cell, this begins as magnesium deficiency but can lead to renal failure
• Claudin-1


  Gives us waterproofing and stops water from leaking out of the body through the skin
• Zonula adherens


  Link the cytoskeletons of adjacent cells
• Cadherins


  Ensure cancer doesn't spread around the body, a loss of E-cadherins is linked to metastasis
• Malignant cells have lower levels of E-cadherin expression
• Desmosomes


  Link the cytoskeletons of neighbouring cells, specifically the cytokeratin of intermediate filaments
• Desmosome transmembrane proteins

  Desmoglein, a type of cadherin
• Pemphigus vulgaris


  An autoimmune disease where the body attacks desmoglein-3 proteins which keep the cells bound together, resulting in cells falling apart and sloughing off
• Cadherins maintain structure of epithelium
• Hemidesmosomes


  Are at the basal surface and anchor cells to the basement membrane
• Gap junctions


  Are for intercellular communication
• Connexins


  The transmembrane protein which make up gap junctions
• Gap junctions


  Contain channels called connexons which can open and close to allow passage of small ions and molecules, these make up connexins
• Microvilli


  Finger-like projections of plasma membrane which increase surface area on the apical side for increased absorption, they have a core of actin filaments
• Cilia


  Motile and are embedded in the apical surface, they have a cytoskeleton composed of microtubules
• Basement membrane


  Forms a selective barrier which allows nutrients to pass through and controls growth and differentiation
• Layers of basement membrane

  • Lamina lucida
  • Lamina densa
  • Lamina fibroreticularis
• Some cells have basal foldings to increase surface area, typically found on cells with lots of microvilli (e.g. small intestinal cells)