Epithelial tissues

Epithelial tissues

1. Simple cuboidal

   • Represents an intermediate between simple squamous and simple columnar
   • In the section perpendicular to basement mebranes cells appear sequare
     • On the surface = polygonal in shape
       • Nucleus usually round and in the centre
   • Line small ducts and tubules
     • Tubules that excrete/ absorb
       • e.g. in collecting tubules of kidney

2. Simple squamous

   • Made of flattened irregularly shaped cells
   • Pavememented epithelium
   • Found lining surfaces involved in passive transport
     • e.g. lungs and capillary walls

3. Simple columnar ciliated epithelium

   • Have surface cilia / villi on majority of cells
     • Cells that are not ciliated - have secretory function
       • Cilia = finger like projection of plasma membrane
         • Cilia contain microtubules
           • Each cell may have up to 300 cilia that move fluid or small particles over epithelial surface
   • Cilia = larger than microvilli
   • Nuclei located towards basal aspect of cell
   • Found in female reproductive tract
     • Facilitates ovum movement to uterus

4. Stratified squamous epithelium

   • Variable number of cell layers that exhibit changes
     • Cuboidal at the basal surface
       • Squamous at the apical surface
         • Cells are shed here
       • Continually dividing stem cells
   • Lines oral cavitiy / pharynx

5. Transitional epithelium - sepcialised functions

   • e.g. cells that line the bladder
     • Enables them to stretch and to withstand toxicity of urine

6. Psuedostratified columnar with cilia and microvilli

   • Majority of cells ciliated / microvilli
     • Give the impression of being stratified but isn't
       • All cells rest on the basement membrane
         • Nuclei at different levels within the cells
   • Scattered stem cells in the epithelium
     • Generally don't have cilia
     • Exhibit polarity
   • Respiratory epithelium

7. Intracellular junctions

   • Occluding junctions / tight junctions
     • Apical part of the cell
       • Actin --> actin filaments
     • Seal intracellular spaces to limit paracellular transport
       • Maintain cell polarity
   • Adhering junctions
     • Desmosomes
       • Keratin intermediate filaments
         • Below adherens
     • Adherins junctions
       • Just below tight junctions
         • Actin microfilaments
     • Bind adjacent cells together
       • Act as anchorage sites
         • So cells can withstand movement
   • Gap junctions
     • Formed of connexins
       • Form a channel between cells
         • Communicating junctions

8. Connections to the basement membrane

   • Hemidesmosomes
     • Keratin intermediate filaments
   • Focal adhesions
     • Actin microfilaments

9. Form a barrier between host and 'free surface'

   • e.g. External environment - skin
     • e.g. digesta - GIT
       • e.g. air - respiratory tract
   • Simple
     • Absorption
     • Secretion
     • Diffusion
   • Stratified
     • Physical protection

10. Avascular tissue

    • No blood vessels
    • Relies on diffusion through BM --> CT

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