histology of epithelial tissue

• 2 types of epithelial tissues

  covering and glandular
• covering epithelium


  found as a sheet of cells (a continuous layer of cells) covering the body's external and internal surfaces, lining every vessel and tube/ tract of the body.
• glandular epithelium


  formed of secretory epithelial cells which form exocrine or endocrine glands of the body. Arise from covering epithelium - form when covering epithelium invaginates.
• exocrine glands


  secrete product into a gland.
• endocrine glands


  secrete product into blood stream.
• how do epithelial cells form a functional layer of cells?

  by adhering to neighbouring cells through cell junctions, which are anchored to the basement membrane.
• structure of epithelial tissues

  - cell is smallest functional unit

  - Cells can form a continuous sheet via cell to cell junctions

  - This sheet is sat on a scaffold/ structure called the basement membrane

  - The side of the cell that is anchored to the basement membrane is the basal side of the cell

  - The other side of the cell (forms the lumen or is in contact with environment) is called the apical surface / luminal surface
• basement membrane


  - Extracellular matrix scaffold on which the epithelial cells can adhere to

  - Provides structural support to the epithelial cells - provides an anchor which the epithelial cells can adhere to

  - Forms a barrier to connect epithelium above to the tissues below

  - Barrier only lets water and small molecules through
• Basal lamina


  part of the basement membrane
• what are covering epithelial tissues characterised on?

  - the number of cell layers: simple or stratified.

  - the cell shape: cuboidal, columnar or squamous

  - any cell specialisations present: ciliated or keratinised
• simple vs stratified


  simple = one cell thick, line ducts and tubes - secretory or absorbing functions

  
  

  stratified/compound = multiple layers, gives a functional advantage of protection - areas where abrasion occurs
• different shapes of epithelial cells

  - cuboidal - round, cube shaped - secretory/ absorbing functions

  
  

  - columnar - taller than wider - found in highly absorbative areas e.g. digestive system and in ducts of glands

  
  

  - squamous - flat - little metabolic activity - involved in diffusion e.g. in capillaries, alveoli, lining surfaces not in touch with environment
• specialisations of epithelial cells

  cilia, keratinisation
• ciliated epithelial cells


  - cilia found on apical surface of some cells - are motile - help to waft/move cell debris and mucus over top of epithelial tissue

  - found in respiratory tract
• keratinisation


  a specialisation found on skin (squamous epithelial cells) in which there is an excess production of keratin - provides an extra layer of protection - helps to waterproof it and provide strong and flexible outer coating

  
  

  keratin = cytoskeletal protein
• how do you name epithelium?

  specialisation, number, shape, epithelium

  
  

  - characterise by uppermost cell seen
• exceptions to epithelial naming system

  pseudostratified epithelial and transitional epithelial tissue
• pseudostratified epithelial tissue


  Every cell is sat on basement membrane but nuclei are at different levels - looks stratified

  e.g. in respiratory tract
• transitional epithelial tissue


  Only found in urinary system - in urethra and bladder

  
  

  Cells at top flatten when bladder is full - allows stretch in bladder - transition
• specialisations to apical side of epithelium

  keratinisation, cilia, microvilli etc.
• basal side of epithelium

  anchors to basement membrane

  - allows cells to receive nutrients

  - allows cells to receive nervous innervation
• lateral/intercellular sides of epithelium

  connecting epithelial cells together
• how is cell polarity achieved?

  - different cell junctions achieve cell polarity

  - especially tight junctions - prevent paracellular transport - set up differences between apical, basal and lateral surfaces of the cell
• cell polarity


  the asymmetrical organisation of cellular content to allow functional differences at different poles of the cell
• cell junctions


  - tight junctions (TJ)

  - adhering junctions (ZA)

  - communicating junctions (CJ)

  - desmosomes (D)

  - hemidesmosomes (HD)
• tight junctions


  Prevent paracellular diffusion - diffusion of nutrients between the cells - leakage of luminal contents into surrounding tissue

  Important in forming a selective epithelial barrier - selective in which nutrients it takes up into epithelial cell
• adhering junctions


  tightly bind adjacent cells together and act as anchoring sights for the cytoskeleton in epithelium cells

  - Ensure epithelium is cohesive and can withstand movement
• communicating junctions


  formed by a ring of proteins - forms a channel in which small ions can be directly transferred between the two cells - allow swap of cytoplasmic contents between cells - forms functional bridge between cells
• desmosomes


  connect cytoplasm together, dotted around the cell
• hemidesmosomes


  interact only with extracellular matrix - responsible for adhering the cells to the basement membrane below
• cilia


  - Made of microtubules (9 doublets around outside and a central pair)

  - Are long in comparison to microvilli

  - Motile - powered by ATP

  - Only present in a few regions around body (respiratory/reproductive tracts)
• microvilli


  - Made of actin filaments

  - Shorter than cilia

  - Less motile

  - Found in many absorbative tissues (brush border)
• glandular epithelia


  - examples - adrenal glands, thyroid, pituitary, salivary, pancreas

  - secrete products

  - exocrine or endocrine (hormones)
• neoplasia


  uncontrolled, abnormal proliferation/ growth of cells
• metaplasia


  the reversible transformation from one differentiated cell to another
• carcinomas


  epithelial cancers