Epithelia and cell junctions

Question 1

What are epithelia?

Answer 1

avascular tisses with cells organised into sheets/tubules, attached to underlying ECM basement membrane

Question 2

Types of epithelia

Answer 2

Simple, stratified, columnar, cuboidal, squamous

Question 3

cuboidal

Answer 3

kidney tubules

Question 4

columnar

Answer 4

small intestine

tall and thin

Question 5

squamous

Answer 5

lung alveolus

flat, like paving slabs

Question 6

stratified

Answer 6

oesophagus
thicker, multiple layers of cells
only bottom layer in contact with basement / basal lamina
Proliferative cell constantly renewing cells above- stratified differentiated across

Question 7

Functions of epithelia

Answer 7

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)

Question 8

Epithelial cells are polarised

Answer 8

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

Question 9

Gut specific example of cell polarisation

Answer 9

Microvilli on apical membrane. Goblet cells secreting mucus on apical side

Question 10

How is physical integrity of epithelium maintained

Answer 10

cells held together by cell junctions

Question 11

Cell junctions

Answer 11

specialised site on a cell at which it is attached to another cell or the ECM

Question 12

Anchoring junctions

Answer 12

Linking cells together or to to ECM

Question 13

Occluding junctions

Answer 13

Seal gaps between cells

Prevent things moving across epithelium (between cells)

Question 14

Channel forming junctions

Answer 14

create passageways linking cytoplasm of adj cells

Question 15

signal relaying junctions

Answer 15

Allow signals to be communicated from cell to cell

Question 16

Adherens junction

Answer 16

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

Question 17

Desmosome junction

Answer 17

cell-cell
associated with intermediate filaments
also have cadherins
Plakoglobin/desmoplakin are electron dense plaques (type of linker?)

Question 18

Types of anchoring junction

Answer 18

Adherens, desmosome, focal adhesion, hemidesmosome

Question 19

Focal adhesion

Answer 19

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?)

Question 20

Hemidesmosomes

Answer 20

Cell-ECM
intermediate filaments
integrins
Dystonin plaques- allow linking of integrins to cytoskeleton

Question 21

Homophilic mechanism

Answer 21

2 molecules which are the same pair up eg cadherins pair up, both calcium dependent

Question 22

adhesion belts

Answer 22

many adheren junctions

Question 23

invagination

Answer 23

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

Question 24

Pemphis Vulgaris

Answer 24

autoimmune destruction of desmosomal protein
blistering, dehydration, infection and death
Gaps open up between cells and barrier function of skin fails

Question 25

Occluding junctions | more detail

Answer 25

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

Question 26

Loss of barrier function of occluding junctions

Answer 26

Crohn's disease: inflammation of bowels, permeability disorder

Question 27

Loss of fence function

Answer 27

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

Question 28

Channel forming junctions | more detail

Answer 28

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

Question 29

cardiac muscle channel forming junctions example

Answer 29

gap junctions allow passage of ions which means changes in mem potential can pass cell to cell- rhythmic contraction of heartbeat

Question 30

what are channel forming junctions in plants called

Answer 30

plasmodesmata cytoplasm physically connected Smooth ER desmotubule projects across

Question 31

Signal relaying junctions more detail

Answer 31

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

Question 32

What happens when signal relaying junctions are lost?

Answer 32

Myasthenia Gravis Autoimmune destruction of neuromuscular junction Droopy eye Severe muscle weakness