The Epithelial Tissue

The Epithelial Tissue

1. Characteristics

   • Avascular tissue
   • Covers the body surfaces, cavities and forms glands
   • It is a barrier between the exterior and the underlying connective tissue
   • Epithelial cells
     • Closely apposed to each other
     • Bound by cell junctions
   • Tissue polarity
     • Apical domain
     • Lateral domain
     • Basal domain
       • Basement membrane

2. Functions

   • Secretion
   • Absorption
   • Transportation
   • Mechanical protection
   • Receptor function

3. Classification of the epithelium

   • Depending on the amount of layers
     • Simple
       • Monolayer
     • Stratified
       • Two or more layers
     • Pseudostratified
       • Monolayer with different heights
   • Depending on the form
     • Squamous
     • Cuboidal
     • Columnar
     • Transitional
   • The last layer is the one that defines the epithelium

4. Apical domain

   • Mircofilaments
     • Microvilli (1-3 µm)
       • Core of actin filaments
       • Anchored to the terminal web
       • Passive movement
         • Contraction of the terminal web
       • Localization
         • Epithelial cells
         • Increase absorptive surface
     • Stereocilia (120 µm)
       • Core of actin filaments
       • Anchored to the terminal web
       • Passive movement
         • Fluid flow
         • Vibration of endolymph
       • Localization
         • In sensory hais cells in the inner ear
   • Microtubules
     • Cilia
       • Motile (5-10 µm)
         • Structure
           • Axoneme
           • Basal body
           • 9+2 pattern of microtubules
         • Active movement
         • Localization
           • Flagela
       • Primary (2-3 µm)
         • Structure
           • Axoneme
           • Basal body
           • 9+0 pattern of microtubules
         • Passive movement
           • Fluid flow
       • Nodal (5-6 µm)
         • Structure
           • Axoneme
           • Basal bodie
           • 9+0 pattern of microtubules
         • Active movement
           • Rotational movement
         • During enbryonal development

5. Lateral domain (cell to cell adhesions)

   • Occluding junctions
     • No communication between cells
       • Barrier
     • Near the apical domain
     • Zip conformation
   • Anchoring junctions
     • Provide mechanical strength
     • Types
       • Zonula adherens
         • Binding to actin filaments
         • Proteins
           • Cadherines
             • Calcium dependent
           • Nectins
             • Calcium independent
       • Macula adherens
         • Binding to intermediate filaments
           • Proteins
             • Desmocollin
             • Desmoglein
   • Communicating junctions
     • There is communication between cells
     • Connexins
       • Stimuli regulated

6. Basal domain

   • Basement domain
     • Dense layer of a proteinic extracelular matrix (no cells)
     • Functions
       • Attachment site to the undelying connective tissue
       • Filters substances
       • Scaffold during tissue regeneration
       • Cell signalling
     • Layers
       • Basal lamina
         • Laminin polymer (most abundant)
         • Type IV collagen
       • Reticular lamina (not always present)
         • Type VII collagen
         • Reticular fibrils (most abundant)
         • Fibrillin microfibril
         • Elasitc fiber
     • Infoldings
       • Increase the cell surface
       • Facilitate interactions
   • Cell to extracellular matrix junctions
     • Focal adhesions
       • Actin filaments
       • They cross the whole basement membrane
     • Hemidesmosomes
       • Intermediate filaments
       • To the basement membrane

7. Glands

   • Types
     • Exocrine glands
       • Secretion to the lumen
       • Types
         • Merocrine
           • Exocytosis
           • No cell damage
           • Pancreas
         • Apocrine
           • The product takes part of the cell
           • Loss of cytoplasm
           • Mamary glands
         • Holocrine
           • Programmed necrosis
           • Cell death
           • Cebaceous glands
     • Endocrine glands
       • Secretion into the connective tissue
       • Example
         • Hormones
   • Types of signaling
     • Paracrine signaling
       • The signal reaches the epithelial cell that is next to it
       • Mediated by receptors
     • Autocrine signaling
       • Own regulation
       • Negative feedback
       • Mediated by receptors

Images and files in the map