Epithelia and cell junctions Flashcards
What are epithelia?
avascular tisses with cells organised into sheets/tubules, attached to underlying ECM basement membrane
Types of epithelia
Simple, stratified, columnar, cuboidal, squamous
cuboidal
kidney tubules
columnar
small intestine
tall and thin
squamous
lung alveolus
flat, like paving slabs
stratified
oesophagus
thicker, multiple layers of cells
only bottom layer in contact with basement / basal lamina
Proliferative cell constantly renewing cells above- stratified differentiated across
Functions of epithelia
mechanical protection (skin), permeability barrier (small intestine), absorption (small intestine), filtration (epi of renal corpuscle), secretion (sweat glands), diffusion of gases/fluids (lung alveoli), sensory (retina)
Epithelial cells are polarised
Top of the cell is a zone where cells are in direct contact- held close together
Basolateral membrane in contact with basal lamina- secreted by these cells
Membrane at apical vs basolateral membrane different- composition of lipids, membrane proteins
Gut specific example of cell polarisation
Microvilli on apical membrane. Goblet cells secreting mucus on apical side
How is physical integrity of epithelium maintained
cells held together by cell junctions
Cell junctions
specialised site on a cell at which it is attached to another cell or the ECM
Anchoring junctions
Linking cells together or to to ECM
Occluding junctions
Seal gaps between cells
Prevent things moving across epithelium (between cells)
Channel forming junctions
create passageways linking cytoplasm of adj cells
signal relaying junctions
Allow signals to be communicated from cell to cell
Adherens junction
associated with actin filaments
cell-cell
actin(cytoskeleton)-linker-cadherin-cadherin-linker-actin
Linkers intracellular, cadherins across plasma membrane
Linkers are alpha-catenin and beta-catenin
Desmosome junction
cell-cell
associated with intermediate filaments
also have cadherins
Plakoglobin/desmoplakin are electron dense plaques (type of linker?)
Types of anchoring junction
Adherens, desmosome, focal adhesion, hemidesmosome
Focal adhesion
cell-ECM
integrins instead of cadherins, used to hold onto ECM
Integrins have a specialised receptor for components outside the cell
Associated with actin filaments
Kinase allows attachment to be controlled (plaques?)
Hemidesmosomes
Cell-ECM
intermediate filaments
integrins
Dystonin plaques- allow linking of integrins to cytoskeleton
Homophilic mechanism
2 molecules which are the same pair up eg cadherins pair up, both calcium dependent
adhesion belts
many adheren junctions
invagination
sheet bends due to organised tightening along adhesion belts
Epithelial tube pinches of
If 1 cell contracts, exert force on adjacent cells because cytoskeletons connected by adherens
Pemphis Vulgaris
autoimmune destruction of desmosomal protein
blistering, dehydration, infection and death
Gaps open up between cells and barrier function of skin fails
Occluding junctions
more detail
seal gaps between epithelial cells
2 transmembrane proteins at core of junction: claudin and occludin- pair ie claudin-claudin
Barrier (no diffusion, leakage of molecules), Fence (maintaining apical polarity) function
Loss of barrier function of occluding junctions
Crohn’s disease: inflammation of bowels, permeability disorder
Loss of fence function
Cancer: loss of cell polarity and cell contacts (epithelial- mesenchymal transition-EMT) incr in motility and eventually metastasis (cancer spreading to new part of the body)
EMT: change from being tightly held to dispersed
Channel forming junctions
more detail
Gap junctions made of connexin proteins (T proteins)
Connexon composed of 6 subunits, have a hollow tube between then, join up with another 6 on neighbouring cell- forms channel
cardiac muscle channel forming junctions example
gap junctions allow passage of ions which means changes in mem potential can pass cell to cell- rhythmic contraction of heartbeat
what are channel forming junctions in plants called
plasmodesmata
cytoplasm physically
connected
Smooth ER desmotubule projects across
Signal relaying junctions more detail
Communication between cells
eg connection between axon and its target cell/ another axon
NT release to signal to adj cell and stimulate it
Proteins holding membranes and appropriate distance so electrical transmission can occur
What happens when signal relaying junctions are lost?
Myasthenia Gravis
Autoimmune destruction of neuromuscular junction
Droopy eye
Severe muscle weakness
