Epithelia and Cell Junctions

Question 1

Epithelia

Definition

Answer 1

• Avascular tissues composed of cells, usually organised into sheets or tubules, attached to an underlying ECM basement membrane.
• Epithelia cover both the internal and external surfaces of the body
• Some epithelia are modified to form glandular structures.

Question 2

Types of epithelia

(5)

Answer 2

• Simple
• Stratified
• Columnar
• Cuboidal
• Squamous

Question 3

Functions of Epithelia

(7)

Answer 3

1. Mechanical protection
2. Permeability barrier
3. Absorption
4. Filtration
5. Secretion
6. Diffusion of gases or fluids
7. Sensory

Question 4

How are epithelia polarised?

Answer 4

Polarised in terms of morphology.
Might have specialised apical surface for example.
Distribution of cellular components in the apical and basal-lateral regions are distinct.

Question 5

How is the physical integrity of the epithelium maintained?

Answer 5

Cell Junctions

Question 6

What are cell junctions?

Answer 6

Specialised sites on a cell at which it is attached to another cell, or to the ECM.
They hold the epithelia together, and control its function.

Question 7

Types of cell junctions

(4)

Answer 7

• Anchoring junctions
• Occluding junctions
• Channel-forming juctions
• Signal-relaying junctions

Question 8

Anchoring junctions

Answer 8

Link cells together or to the extracellular matrix.
* Adherens
* Desosomes
* Focal adhesions
* Hemidesmosomes

Question 9

Adherens junctions

Answer 9

• Interact with actin filaments, connecting the filaments in neighboring cells.
• Wrap around cell in the apical region
• Each cadherin is tethered to actin filaments via several linker proteins

Question 10

Actin is contractile so…

Adherens

Answer 10

… pulls cells closer together.
This allows for the epithelial sheets to form tubes or vesicles.

Question 11

Desosomes

Answer 11

• Form spot-like structures
• Cadherins link to keratin.
• Interact with intermediate filaments in the cytoskeleton
• Strengthen cells against streching forces

Question 12

Core components of desosomes

(2)

Answer 12

plakoglobin and desmoplakin

Question 13

Cadherins mediate…

Answer 13

… cell-cell attachments.
These are homophilic mechanisms

Question 14

Integrin molecules play a central role in …

Answer 14

… mediating cell-matrix contacts

Question 15

Focal Adhesions

Answer 15

Allow the cell to hold on to the ECM and exert a pull on it.

Question 16

Hemisdesmosomes

Answer 16

• Interact with intermediate filaments
• Integrin molecules do the “holding on”
• Adaptor complex of proteins - plaque
• Anchor epithelial cells to the basal lamina

Question 17

Pemphigus Vulgaris

Answer 17

Autoimmune disorder that causes the destruction of desmosomal proteins.
Causes severe blistering, dehydration, infection, and death.

Question 18

Occluding (tight) junctions

Answer 18

• Define boundary between basolateral and apical membranes
• Made of claudin and occludin
• Have a barrier and a fence function
• Form networks of strands across the cells, which press the cells together.
• No space for proteins to pass through

Question 19

Claudin and occludin

Answer 19

Proteins that make up occluding junctions.
They undergo homodimerisation and physically interact.

Question 20

Barrier function

Occluding junctions

Answer 20

Occluding junctions regiulate the passage of ions, water and macromolecules.

Question 21

Fence function

Occluding junctions

Answer 21

Maintains cell polarity.

Question 22

Problems with barrier functions cause…

Answer 22

… Crohns disease, which causes inflammation of the bowels. This is a permeability disorder.

Question 23

Problems with the fence function can be caused by…

Answer 23

… cancer. It causes a loss of cell polaroty and cell contacts and increases motility, which causes metastasis

Question 24

Channel-forming (gap) junction

Answer 24

• Made of connexins
• Two connexons on neighbouring cells bind to each other.
• Allow ions through, but not larger molecules.
• Homophillic interactions

Question 25

Connexins | Channel-forming junctions

Answer 25

6 connexins make a connexon, these are like tubes and can open and close.

Question 26

Abnormal gap junctions

Answer 26

Cause some inherited disorders, such as cataracts and Vohwinkel syndrome

Question 27

Channel-forming junctions in plants

Answer 27

Cell walls prevent these kind of connections between cells. A channel-forming junctions exists called a plasmodesmata which are regions where the cytoplasm of two cells are directly connected via a pore.

Question 28

Signal-relaying junctions

Answer 28

* Allow communication of signals between cells. * Contains specialised protein components which form pairs which effect each other - control things like spacing. * Establish an area where communication between cells can take place using vesicles. * Allows for transport of species larger than ions.

Question 29

Myasthenia gravis

Answer 29

An autoimmune destruction of the neuromuscular junctions. Causes severe muscle muscle weakness