Epithelium tissue

• Epithelial tissue


  • Covers body surfaces
  • Lines hollow organs, cavities and ducts (like our intestines)
  • Forms the glands of the body
• Functions of epithelial tissue

  • Barrier that limits or aids transfer
  • Allows fluids or other compounds onto a free surface
  • Protective especially from abrasion
• Cells of an epithelium

  • Held together by various junctions:

  Tight Junctions

  Adherens Junctions

  Gap Junctions

  Desmosomes

  Hemidesmosomes

  Arranged in continous sheets as single or multiple layers
• Cell junctions


  The way that the cells hold together or are linked to adjacent cells, and/or the membranes they sit on
• Cytoskeleton


  Cells have various molecules coursing through it that provides a skeleton or structure to the cell

  Cytoskeleton contains microfilaments that help with:

  Bundles beneath cell membrane and cytoplasm

  Cell strength to alter shape

  Links cytoplasm to membrane

  Cells tied together

  Muscle contractions

  Contains intermediate filaments like keratin that help give structure to cells as they are thick

  Provide strength
• Tight junctions


  Located in the apical end of the cell

  Comprised of two membrane and sown together by proteins

  Main proteins are claudins and occludins; they bind the cell together

  Electrically tight meaning highly conducting molecules don't move between cells

  Like a gate at the top of the cells

  Joins cytoskeleton of adjacent cells via ZO-1 protein for example

  Polarity maintained by prevention of migration of proteins between apical and basal surfaces
• Adherens Junction


  Belt junction because looks like a belt runs around the cell

  Don't bring the two layers of membrane as close to each other

  Leaves a gap for proteins to span

  Cadherin protein traverses the two adjacent plasma membranes held together

  Contains plaque which is a layer of protein that joins actin to cadherin

  Cadherin is joined with catenin as it spans the plasma membrane

  This locks onto actin

  Skeleton of one cell is connected to another through adherens junction at the level of actin

  Stabilises cell from mechanical trauma

  Prevents cell separation from tension forces like contractions
• Desmosome Junction


  Button like junction

  Gap in between membrane

  Plaque is more circular

  Cadherin is the main protein and spansn the gap

  Cadherin spans a slightly different molecule called desmoplakin

  Binds to keratin instead of actin

  Keratin spans from one desmosome to another on other side of the cell

  Important for structural integrity

  Desmosome bind and join keratin from different cells
• Gap junctions


  Help cells coordinate their activities instead of sealing and holding

  Made up of 6 connexin protein, they lock together to form a connexon hemichannel

  Each cell makes a hemichannel, and its cytoplasm migrates to the membranes

  They move around in the membrane till they find one beside them and join together

  Thats when they open their tunnel

  Nature of proteins forming the channel determine the control of what goes in between different cells
• Hemidesmosomes


  Other junctions focus on things that go in between cells. This would be good if the cells were all locked together, otherwise, the cells would just slide on us.

  Ties the cells down to the rest of us

  It is the way cells adjacent to the basement membrane can lock themselves onto that

  Links keratin to the basement membrane

  Integrin spans the gap between two membrane

  Spans the gap between plaque and basement membrane

  Integrin binds to another protein in the basement membrane called laminin

  Laminin eventually binds to keratin which is an intermediate filament in cytoplasm
• Basement membrane


  Cells sit on them to keep their structure

  Consists of two parts:

  Basil lamina

  Reticular lamina

  Basal and reticular lamina form a junction between epithelium and connective tissue so its got important functional components.
• Basal lamina


  Secreted by epithelial cells

  Contain collagen, laminin, other proteoglycans, glycoproteins, etc.
• Reticular Lamina


  Produced by cells on underlying connective tissue called fibroblasts

  Contains fibrous proteins such as fibronectic, collagen etc.
• Epithelia contains nerves but they don't contain blood vessels (avascular). Nerves can pass through the basement membrane, but blood vessels don't. So how do the cells in the epithelia get nutrients? It all diffuses from the basement membrane, so oxygen, nutrients and other things can pass through the porous nature of the basement membrane and sustain the cells in the epithelium.
• The exchange of nutrients and waste takes place by diffusion from the vessels in the connective tissue.
• Key function of basement membrane


  Provide template surface for epithelia cells during injury

  Reconstitutes itself quickly after healing

  Cells will migrate across basement membrane as epithelial cells start to divide and reform the epithelium

  Acts a physical barrier as it provides protection if its still intact

  Barrier to invasion by malignant melanoma

  Penetration of barrier brings chances of metastasis occurring increase