Junctions and tissues

• types of junction:

  impermeable

  adhesive

  communicating
• impermeable junction - prevent the passage of molecules between cells
• Adhesive junctions: these mechanically hold cells together (adheren and desmosomes)
• Communicating junction: passage of small molecules between cells (gap junctions or chemical synapses)
• Tight junctions:

  • barrier function: prevents molecule from leaking between adjacent cells
  • Fence function: separates the apical plasma membrane the basal plasma membrane, thus allowing them to have different compositions, this also gives rise to cellular polarity
• Partietal cells in the stomach

  Apical surface contains a H+ and Cl- pump which generates HCl

  Tight junctions keep the apical membrane components separate from the basal membrane components

  in the gut this stops acid being pumped into the blood stream
• Transmembrane proteins in tight junctions

  physical barrier

  adhesion

  permeability
• cytosolic proteins in tight junction

  • scaffolding
  • signalling
  • polarity
• Adherens junctions

  Cadherins

  connect cells to actin filaments

  intracellular signalling
• Desmosomes

  cadherins

  connect cells to intermediate filament

  structural integrity, withstand mechanical stress
• Focal adhesions

  integrins

  connect ECM (extra cellular matrix) to actin filament
• Hemi-desmosomes

  integrins

  connect ECM to intermediate filament
• spot desmosomes

  spot-weld cells together attached on insides of the cell to keratin filaments, which serve to spread the stresses from the spot desmosomes throughout the cell
• Gap junctions are the most common junction type
• Gap junctions are composed of proteins called connexins (12)
• The permeability of gap junctions is changed by decreased intercellular pH or increased intracellular calcium
• Connexin forms cylindrical channels in gap junctions
• Medical significance of gap junctions:

  Propagation of the pattern of current flow that governs regular rhythm of a healthy heart

  Gap junction channels are reduced in number or changed in distribution in a variety of cardiac disease
• Cellular function of connexin:

  • mediate electrical communication to effect contraction and motility
  • allow passage of molecules and ions between cells
  • modulate cell polarity and directional migration
• Putative role in disease

  • inflammatory bowel
  • gastrointestinal infection
  • autism spectrum disorder
• Tissue: any of the distinct types of material of which animals or plants are made, consisting of specialized cells and their products
• epithelial tissue consists of sheets tightly packed cells, it covers the outside of the body lines organs and cavities inside the body
• Epithelial cells are fastened together by desmosomes and sealed via tight junction
• Epithelium protects against mechanical injury and also provides a barrier against microbes and fluid loss
• Epithelia can be simple or stratified
• function of epithelial, lines digestive tracts which secrets mucus (glandular epithelium), it can also be ciliated as in the respiratory tract
• Type of epithelial

  • cuboidal
  • simple columnar
  • pseudostratified columnar
  • simple squamous
  • stratified squamous
• Connective tissue

  mechanical role, provides strength binds and supports other tissue

  consists of extracellular matrix which cells are sparsely scattered
• Dense connective tissue is made up of:

  1. fibrous protein (collagen and elastin)
  2. ground substance usually proteoglycans
• Loose connective tissue holds small glands and epithelia together and includes the basal lamina of cells
• Blood and adipose are also connective tissues but dont fit very well into these categories
• Muscle tissue role

  support and movement
• Types of muscle fibre (myofibril)

  1. actin (Thin)
  2. myosin (thick)
• types of muscle tissue

  skeletal

  smooth

  cardiac
• nervous tissue

  senses stimuli and transmits signals throughout the animal

  Neurons or nerve cells that transmit nerve impluse

  Gilal cell or glia that help nourish, insulate and replenish neurons
• Structure of neuron

  cell body (soma)

  axon

  dendrites

  schwann cells
• Dendrites are processes which conduct impulses towards the nerve body
• Axons transmit impulses away from the nerve body
• Schwann cells wrap around an axon to form a multilayered membrane sheath to provide electrical insulation
• Dense connective tissue = bone, cartilage