Epithelia & cell junctions

Question 1

What are epithelia?

Answer 1

Avascular tissues composed of cells, usually organised into sheets/tubule, attached to an underlying ECM basement membrane

Question 2

What are epithelia modified to form?

Answer 2

Glandular structures (exocrine glands)

Question 3

How many types of epithelia are there?

What are they?

Answer 3

5

Layers:
> simple
>stratified

Cell shape:
>columnar
>cuboidal
>squamous

Question 4

Where are simple cuboidal epithelia found?

Answer 4

Around kidney tubules

Question 5

Where are simple columnar epithelia found?

Answer 5

Small intestine

Question 6

Where are squamous epithelia found?

Answer 6

Lung alveolus

Question 7

Where are stratified epithelia found?

Answer 7

Oesophagus

Question 8

What are the 7 functions of epithelia?

Answer 8

> mechanical protection
> permeability barrier
> absorption 
> filtration
> secretion 
> diffusion
> sensory

Question 9

How are epithelia polarised?

Answer 9

Apical-basolateral

Question 10

How are the apical & basal membranes different?

Answer 10

Composition of lipids & membrane proteins

Question 11

How is the cytoskeleton organised in epithelia?

Answer 11

So that secreting vesicles are produced & driven towards apical surface ready to be secreted

Question 12

How is physical integrity of the epithelium maintained?

Answer 12

Epithelia held together by cell junctions

= specialised site attached to another cell or extracellular matrix

Question 13

What are the 4 different types of cell junctions?

Answer 13

> anchoring junctions
occurring junctions
channel-forming junctions
signal-relaying junctions

Question 14

What is the role of anchoring junctions?

Answer 14

Linking cells together or to the ECM

Question 15

What is the role of occluding junctions?

Answer 15

Sealing gaps between cells

Question 16

What is the role of channel-forming junctions?

Answer 16

Creating passageways (for ions & small molecules) linking the cytoplasm of adjacent cells

Question 17

What is the role of signal-relaying junctions?

Answer 17

Allowing signals to be communicated from cell to cell

Question 18

What are the 2 types of ‘strengthening’ junctions?
(between cells)

What are the 2 types of anchoring junctions?
(between cell & basal lamina)

Answer 18

> Adherens junction
Desmosome

> Focal adhesion
Hemidesmosome

Question 19

What are the 3 components of an adherens junction?

Answer 19

> Actin filaments

> Cadherin

> alpha & beta catenin

Question 20

What are the 3 components of a desmosome?

Answer 20

> intermediate filaments

> cadherin (desmoglein)

> plakoglobin & desmoplakin

Question 21

What are the 3 components of a focal adhesion?

Answer 21

> actin filaments

> integrin

> focal adhesion kinase

Question 22

What are the 3 components of a hemidesmosome?

Answer 22

> intermediate filaments

> integrin/collagen

> dystonin

Question 23

What do cadherins do?

Answer 23

Mediate cell-cell attachment

Question 24

Which type of junction links cadherin to the actin filaments of the cytoskeleton?

Answer 24

Adherens junction

Question 25

How can epithelial sheets form a tube/vesicle?

Answer 25

Invagination of sheet due to an organised tightening along adhesion belts in selected regions of sheet

–> epithelial tub pinches off from sheet of cells

Question 26

Which type of junction links cadherins to intermediate filaments of cytoskeleton?

Answer 26

Desmosomes

Question 27

What is the role of integrin?

Which junction is it found in?

Answer 27

Mediates cell-matrix contacts

Hemidesmosomes
- anchor epithelial cells to basal lamina

Question 28

Give an example of a disease caused by defective desmosomes

What are the symptoms?

Answer 28

Pemphigus vulgaris
- autoimmune destruction of desmosomal protein

> severe blistering 
> dehydration & infection 
- gaps open up between cells & fill w/ fluid
--> blisters burst = infection 
> death

Question 29

What are the 2 functions of occluding junctions?

Answer 29

Barrier: regulate passage of ions, water & macromolecules

Fence: maintains cell polarity

Question 30

What are the 2 components of occluding junctions?

Answer 30

> Claudin & occludin

> Zona occludin scaffold protein

Question 31

Which disease is loss of barrier function in occluding junctions associated with?

Answer 31

Crohn’s disease

• inflammation of bowels
• permeability disorder

Question 32

Which disease is loss of fence function in occluding junctions associated with?

Answer 32

Cancer

• loss of cell polarity
• loss of cell contacts
• increase in motility
• eventual metastasis

Question 33

What are the components of channel-forming junctions?

Answer 33

Connexin formed of 6 subunits

Question 34

Explain the role of channel-forming junctions in cardiac muscle

Answer 34

Allow passage of ions
–> permits changes in membrane potential to pass from cell to cell
= rhythmic contraction of heartbeat

Question 35

Which inherited disorders can abnormal gap junctions cause?

Answer 35

Cataracts

Vohwinkel Syndrome

Question 36

What causes cataracts?

Answer 36

Dominant mutation in CX50
= no gap junctions, but hemichannels form
–> disrupted connections

Question 37

What causes Vohwinkel Syndrome?

Answer 37

Upregulated CX26 in epidermis
–> hyper proliferation
(over production of epidermal cells)

Question 38

What are channel-forming junctions in plants called?

Which organelle projects across them?

Answer 38

Plasmodesmata
- cytoplasm physically connected across junction

Smooth endoplasmic reticulum

Question 39

What are the components of a signal-relaying junction in a neuromuscular junction between 2 neurones?

Answer 39

> cadherin

> neuroligin & neurexin

Question 40

What happens when signal-relaying junctions are lost?

What are the symptoms?

Answer 40

Myasthenia Gravis
= autoimmune destruction of neuromuscular junction

> droopy eye
severe muscle weakness