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Epithelium

Distinguish between surface epithelium and glandular epithelium.

General characteristics of surface epithelium

 
  • Contains no blood supply
  • Cell are arranged to form cohesive sheet with no or little extracellular matrix in between
  • Displays a free surface apical surface
  • Opposite layer adheres to extracellular basement membrane, basal surface
  • Display polarity apical, luminal and lateral surfaces display structural specialization
  • Epithelial cell are specialised for absorption, secretion or to act as barrier
  • Lateral surfaces display junctional complexes for intercellular cohesion and communication
  • One type of epithelium may change into another type metaplasia eg replacement of the squamous epithelium of the oesophagus by columnar epithelium (Barrett's oesophagus) may result from reflux oesophagitis
  • Coetzee et al p43
  • Stevens and Lowe p33-34
  • Wheater p80

Membrane specializations of epithelia

 

Lateral surface

Specialised structures are present in epithelia which link individual cells together. Two main adhesion types are distinguished:
  1. Cell membrane proteins acting as specialised cell adhesion molecules (CAMs)
  2. Specialised areas of the cell membrane incorporated into cell junctions.

    Three types are recognized: occluding junctions, anchoring or adherence junctions and communicating junctions.

    • Occluding junctions bind cell together to form an impermeable barrier
      • Zonula occludens or tight junction

    • Anchoring junctions link the cytoskeleton of cells to each other and two underlying tissues
      • Zonula adherens provides mechanical strength
      • Macula adherens or desmosomes provides mechanical strength in tissues where there are tensile or shearing stresses, eg skin

    • Communications junctions allow direct cell-cell communication
      • Gap junction or nexus allow rapid communication for coordinated action

Luminal surface

  • Microvilli short finger-like projection of the cell membrane to increased surface area, visible with LM as brush border
  • Cilia hair-like surface projections of cells involved in transport, visible with LM
  • Glycocalyx thin extracellular layer consisting of protein glycoprotein and sugar residues; stains PAS positive; can act as enzyme, CAM or for cell recognition

Basal surface

  • Basal interdigitations or folds greatly enhance surface area
  • Coetzee et al p43-46
  • Stevens and Lowe p35-38
  • Wheater p80

Secretory adaptations

 
  • Protein-secreting epithelial cells have large nuclei and abundant rER.
  • Mucin-secreting epithelial cells have an expanded Golgi system
  • Steroid-secreting epithelial cells have an extensive sER
  • Ion-pumping epithelial cells have many mitochondria and a large surface area

Classification

 

According to number of cell layers

  • Simple single layer
  • Stratified 2 or more layers
  • Pseudostratified single layer but nuclei situated at different levels in the cell. All cells are in contact with the basement membrane, but not all cells reach the apical surface. Both conditions create the illusion of several cell layers.

According to cell shape in sections at right angle to the surface

  • Squamous Multiple layers of flattened thin cells with little cytoplasm and prominent nucleus
  • Endothelium squamous epithelium in cardiovascular system
  • Mesothelium - squamous epithelium of mesodermal origin lining serous membranes and cavities
  • Cuboidal - cell height, width and depth are the same, round centrally placed nucleus
  • Columnar - cell height greater than width, nucleus elliptical or cigar shaped
  • Transitional - stratified, top layer dome or umbrella shaped
  • Coetzee et al p47 & 57
  • Stevens and Lowe p33-34
  • Wheater p80

Identifying tissues microscopically

  There is no mystery to histology.

Cells, tissues and organs can be identified using a few simple and straightforward criteria. Try and construct a mental flowchart: are all the cells the same type (tissues) or are there different cell types specifically arranged (organ): If you decide on an organ, is it hollow or solid?

Let us have a look at epithelium more specifically:

  • Decide if there is one or more than cell layer
  • Decide what shape the cells are
  • Decide what shape the nuclei are
  • Decide where the nuclei are situated
  • Are there specializations visible on the apical surface?
Once you have answer all the questions, you can classify and therefore identify the epithelium.

Remember:

  • Start with lowest possible magnification
  • Never rely on colour, look at the morphology
  • Never memorise images from the atlas, microscope or computer

Diseases affecting epithelium

  Most respiratory tract and renal diseases affect either the epithelial cells or the underlying basement membrane, while epithelial cancers are common.

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© julie 2002 - julie 2008 marius loots