Epithelia & Junctions Flashcards
What are the four major tissue types in the body?
Connective, Epithelial, Nervous, Muscular
What are epithelial cells organized into?
Sheets
These cover an external surface or line an internal body cavity
What are the two main types of junctions in epithelial cells?
Cell-cell junctions and cell-extracellular matrix (ECM) junctions
What is the primary function of tight junctions?
Stops diffusion across the epithelial sheet
Which proteins form tight junctions?
Occludin and claudin
What is the significance of functional polarization in epithelia?
Allows apical and basolateral membranes to have different functions and compositions
Epithelia are functionally polarised
What type of transport do tight junctions enable in epithelial cells?
Polarized transport of nutrients
What do hemidesmosomes link to in epithelial cells?
Basal lamina (ECM)
What proteins are used in adherens junctions?
Cadherins
What is the role of cadherins in epithelial cells?
- transmembrane proteins in the plasma membrane
- bind to an identical cadherin in the next cell
- interaction needs calcium (in absence of Ca2+ they will come apart)
What do desmosomes link together in epithelial cells?
Intermediate filaments
What is the function of desmosomes in tissues?
Provide tensile strength
Abundant in tissues under high shear stress such as heart muscle and in tough, exposed epithelia
What are integrins in hemi-desmosomes responsible for?
Linking to keratin intermediate filaments
What is the primary role of cell-cell interactions in tissues?
Help define tissues and allow cells to communicate directly
What are gap junctions and where are they found?
Channels allowing transfer of ions and small molecules, found in most epithelia
What is the plant equivalent of gap junctions?
Plasmodesmata
Large molecules such as proteins and regulatory mRNAs can pass through these
Which cadherin is expressed in epithelial cells?
E-cadherin
Which cadherin is expressed in muscle cells?
N-cadherin
True or False: Cancer cells often continue to express the specific cadherins that keep them in place.
False
What type of molecules can pass through gap junctions?
Inorganic ions and small water-soluble molecules (<1000 daltons)
What do cadherins help cells to do?
Recognize each other and assemble into tissues
Fill in the blank: Epithelia must connect to the underlying tissue via _______.
Hemi-desmosomes
What happens to cancer cells regarding cadherin expression?
They may express N-cadherin instead of E-cadherin, making them highly motile.
What is the composition of the extracellular matrix (ECM)?
A network providing structural and biochemical support to surrounding cells
What is the primary role of the extracellular matrix in connective tissues?
Provides strength
What is transcytosis?
Polarized transport of proteins from one side of the epithelium to the other
What is the importance of cell-cell interactions in the context of cancer?
They help keep cells in the right place and prevent metastasis
Tissue structure in the gut
- epithelial, muscle and nervous tissues
- have little ECM
- intermediate filaments and cell-cell junctions provide strength
- connective tissues with few cells & lots of ECM that provides strength
Role of adherens junctions
Joins an actin bundle in one cell to a similar bundle in a neighbouring cell
Characteristics of tight junctions
- found in vertebrates
- formed of strands of occludin and Claudin proteins
- these complexes allow lipids in the plasma membrane to diffuse freely but not membrane proteins
- protein composition of the apical and basal membranes can be different
Which epithelia have secretion from the apical surface?
- airways
- stomach
- intestine
- mammary gland
Importance of the epithelia being functionally polarised
Allows polarised transport of nutrients
E.g. glucose and amino acids in the intestine
Role of adherens junctions and desmosomes
Link the cytoskeleton of epithelial cells to their neighbours
Using Cadherins
Role of hemidesmosomes
Link the cytoskeleton of epithelial cells to the basal lamina
Using integrins
Cadherin in adherens junctions
- cadherins link to actin filaments
- adhesion belt = continuous band of adherens junctions
- forms an actin network across the epithelium
- can be contractile due to the presence of myosin II
Movement of epithelial cell sheets
Contractility across the epithelium allows epithelial sheets to move
E.g. neurulation —> generates the neural tube in development
Effect of desmin mutations
- desmosomes in adjacent muscle cells are linked to desmin IFs
- desmin is expressed in cardiac, skeletal and smooth muscle
- desmin mutations cause muscular dystrophy and cardiac myopathy
Key characteristic of hemi-desmosomes
Look superficially like half a desmosome but the membrane proteins are integrins
What do integrins in the basal plasma membrane bind to?
- Laminin in the basal lamina
- IFs inside the cell via linker proteins
What are integrins in focal adhesions responsible for?
Link to actin filaments
What is the role of cadherins in cell-cell interactions
- specific cadherins help cells recognise each other
- help keep cells in the correct place
- cancer cells often no longer express specific cadherins that would normally keep a cell in its correct place
- expressing N-cadherin instead of E-cadherin makes cells highly motile
- cancer cells may secrete more matrix proteases (these digest the ECM) than normal, helping them escape through the basal lamina
What percentage of cancer starts in epithelial tissues?
85%
These are carcinomas
What are connexon channels?
Allow direct transfer of inorganic ions and small waste soluble molecules between cells
E.g. IP3 and calcium ions
This results in electrical coupling & gives a regular heartbeat
