4. cell connection/communication

• A tissue is defined as a coherent grouping of cells bound together by cell junctions or extracellular matrix (ECM)

  
• Cells interact with each other and the extracellular matrix in different ways in different tissues
• Four primary (basic) tissues

  • Epithelium
  • Connective tissue
  • Muscle
  • Nervous tissue
• Extracellular matrix (ECM)


  All the protein and other molecules outside and around cells
• Molecules can be secreted by cells into the extracellular environment
• Functions of the extracellular matrix (ECM)


  • An insoluble barrier or connection between tissues
  • A loose network in which cells are scattered
  • A substratum which cells bind to and form a sheet
  • A substratum over or through which cells migrate
• Major classes of molecules in the extracellular matrix (ECM)


  • Fibrous proteins (mostly collagens)
  • Proteoglycans and glycosaminoglycans (eg hyaluronan)
  • Glycoproteins (eg fibronectin, laminin)

  
• Molecules of the extracellular matrix (ECM) play important mechanical as well as non-mechanical roles
• ECM consists of fibres and non-fibrous molecules that vary from site to site and change during development
• Components of connective tissue

  • Cells of connective tissues
  • Fibroblasts (or other type of specialised cell, eg chondrocyte, osteocyte)
  • Types of immune cells (eg macrophage)
  • Extracellular matrix
  • Fibres
  • Ground substance

  
• Types of fibres in the extracellular matrix

  • Collagen (multiple types)
  • Elastic
  • Reticular (fine, delicate)

  
• Components of ground substance

  • Proteoglycans (eg HSPG, CSPG)
  • Glycosaminoglycans (GAGs)
  • Glycoproteins (eg laminin, fibronectin)
• Classifications of connective tissues

  • Loose (areolar) connective tissue – lamina propria, adipose tissue (fat)
  • Dense connective tissue
  • Dense regular – tendon, ligament
  • Dense irregular - dermis, capsules of some organs (eg. ovary, testis, spleen, prostate, joint spaces)
  • Blood (loose connective tissue)
  • Cartilage – hyaline, elastic, fibro
  • Bone – compact, spongy

  
• Cell junctions


  Occur between two cells or between a cell and the extracellular matrix, with different functions from transmission of stress to channel formation
• Some cell junctions are critical for cell to cell communication as well as signal transduction
• Epithelial cells


  Organized into closely adherent sheets, non-motile, polarised in an apical-basal orientation
• Mesenchymal cells


  Mainly seen in embryogenic morphogenesis, often motile, within extracellular matrix, not polarised, less connected to each other

  
• Six types of cell junctions

  • Tight junctions (zonula occludens)
  • Adherens junctions (zonula adherens)
  • Desmosomes (macula adherens)
  • Gap junctions (nexus)
  • Focal adhesions
  • Hemidesmosomes

  
• Tight junctions (zonula occludens)


  Prevent the passage of molecules and ions through the space between cells, substances must enter the cell to pass through the tissue, block the movement of integral membrane proteins between the apical and basolateral surfaces of the cell
• Adherens junctions (zonula adherens)

  Encircle the cell, provide some adhesion, actin filaments insert into dense plaques on the cytoplasmic surfaces, join an actin bundle in one cell to a similar bundle in a neighbouring cell to form an adhesion belt just under the tight junction

  
• Desmosomes (macula adherens)


  Hold two cells tightly together, plaque-like thickenings on membranes of adjacent cells, intermediate filaments converge on plaque, sites of attachment between cells and for cytoskeleton
• Gap junctions (nexus)


  Intercellular channels ~1.5-2nm in diameter, permit free movement between the cells of ions and small molecules up to MW 1,200, allow for ion flow (electrical coupling) and changes in membrane potential to pass from cell to cell
• Focal adhesions


  Allow adhesion of the cell to extracellular matrix, actin filaments insert into dense plaques on the cytoplasmic surfaces, transmembrane proteins called integrins bind to extracellular matrix molecules (eg collagen, fibronectin)
• Hemidesmosomes


  Connect the basal surface of an epithelial cell to the underlying basal lamina, the extracellular domains of the integrins that mediate the adhesion bind to a laminin protein in the basal lamina, while an intracellular domain binds via an anchor protein (plectin) to intermediate filaments

  
• How tightly cells connect to each other, and in what way they connect to extracellular matrix, affects how the tissue functions
• Epithelial cells


  Tightly connected together to form a layer
• Mesenchymal cells


  Loosely connected to each other, surrounded by extracellular matrix
• Cells are capable of switching between epithelial and mesenchymal states (epithelial to mesenchymal transition, or vice versa)
• Functions of epithelial tissues

  • Protection – skin (epidermis)
  • Absorption – small intestine, kidney
  • Secretion – small and large intestines, trachea
  • Transport – kidney
  • Sensory perception - epidermis
• Dr Sonja McKeown, [e-mail], Department of Anatomy & Developmental Biology
• Epithelial tissues develop from all three germ layers: ectoderm, mesoderm and endoderm
• Acknowledgement to Dr Julia Young
• DEV2011 Cell communication
• I wish to acknowledge the people of the Kulin Nations, on whose land we are gathered today. I pay my respects to their Elders, past, present and emerging.
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• Cell junctions and adhesion

  How tightly cells connect to each other, and in what way they connect to extracellular matrix, affects how the tissue functions

  
• Epithelia (or epithelial cells)


  • Connected tightly together to form a layer
• Mesenchyme (mesenchymal cells)


  • Loosely connected to each other, and surrounded by extracellular matrix

  
• Cells are capable of switching between epithelial and mesenchymal states
• Cell adhesion in different tissues

  • Epithelium
  • Mesenchyme