Epithelial Polarty

Epithelial Polarty

1. Apical surface specializations

   • Microville
     • They are finger -like projections
     • Site
       • Proximal convoluted tubules of the kidney
         • Small intestine
     • LM
       • Brush border
       • Acidophilic striated
     • EM
       • Cylindrical process
       • Actin filaments
       • The cell membrane
       • Terminal web
       • Actin
       • Intermediate filaments
       • The Fimbrin
       • Glycocalyx
     • Function
       • They increase surface area for absorption by about 15-30 folds .
   • cilia
   • Flagella
   • Stereocili
     • They are long microvilli but not non -motile
     • Em
       • LIke mircovilli
     • LM
       • Striated border
     • Function
       • 1- Absortive e.g. epidyimis
       • 2- Receptors e.g. those of hair cells in ear

2. Leteral surfaces specializations

   • According to the distance btw the cell membranes
     • a-TIght junction (Occluding junction )
       • 1- provide strong attachments to the adjacent cells
       • 2- Prevent passages of materials through the intercellular
       • 3-Prevent movement of integral protein from apex to basolateral membrane
     • b- Adhering junction (anchoring junction)
       • 1- Provide strong attachment to the adjacent cells
       • 2-Prevent passages of materials through the inercellular space .
       • 3- Links the cytoskeleton of the two cells .
     • c-Gap Junction (communicating Junction)
       • 1-Provides strong attachment to the adjacent sells .
       • 2- prevent passages of materials through the intercellular space
       • 3- Maintain direct communication btw cells for transmission of ions ,amino acids and hormones
       • 4- Allow spreading of excitation waves throughout the heart and smooth muscles
       • 5- Transmit impulse at certain synapse
   • According to the shape of the junction
     • a- Zonula junction (Gridle or ring junction)
     • b- Fascia junction (Band junction)
     • c- Macula junction (spot junction)

3. Basel surface specializations

   • Basement membrane
     • supporting layer present btw epithelium and the underlying connective tissue
     • LM
       • It cannot be seen by H&M
       • It needs special stain as PAS , silver or trichorme stains
     • EM
       • The basement membrane (the basal lamina)
         • It is secreted by the cells themselves
           • Lamina lucida
             • Adjacent to the base of epithelial cell membrane
             • Formed of glycoproteins
             • Appears electron lucent
           • Lamina densa
             • Present adjacent to the underlying connective tissue
             • Formed of IV collagen fibers
             • Appears electron dense
         • Function
           • 1- support of the overlying epithelium
           • 2- Forms , attachment btw epithelium and underlying connective tissue
           • 3- prevents the passages of large molecules and allows Passages of small molecules (i.e filtration)as in the kidney
           • 4-Regulates epithelial cell proliferation and guides their migration during repair after injury
       • Reticular lamina ( lamina reticularis )
   • Basel infoldings
   • Hemidemosomes