Module 6 - Interactions between cells in multicellular systems

Question 1

Four major tissue types

Answer 1

Muscle
Nervous
Connective
Epithelia

Question 2

The gut: the tissues comprising it and its significant features

Answer 2

Epithelia at the top (absorbing food) and bottom, connective tissue connecting epithelia and smooth muscle (for peristalsis) with nerve cells connecting as well

• Low ECM
• Intermediate filaments and cell-cell junctions provide strength

Question 3

The skin: the tissues comprising it and its significant features

Answer 3

Epithelia at the top, connective tissue connecting epithelia and muscle with nerve cells connecting as well

• Low ECM
• Intermediate filaments and cell-cell junctions provide strength

Question 4

Types of epithelia

Answer 4

Simple squamous (flat)
Simple cuboidal (cuboidal)
Simple columnar (columnal)
Non-keratinised stratified squamous
Keratinised stratified squamous
Pseudostratified columnar with goblet cells

Question 5

Tight junction: what are they and what other key features do they have?

Answer 5

Seals neighbouring cells together in an epithelial sheet to prevent leakage of extracellular molecules between them - allowing functional polarisation

• Found in vertebrates
• Formed from occludin and claudin proteins
• Allow lipids in the plasma membrane to diffuse freely, but NOT membrane proteins
• Protein composition of the apical and basal membranes can be different

Question 6

Gap junction

Answer 6

Connexon channels (2-4nm) between adjacent cells allowing ions and small molecules (<1000 daltons) to pass through

Question 7

Adheres junction

Answer 7

Two actin bundles joined together between neighbouring cells using cadherins

• Forms adhesion belts (continuous bands of adheres junctions)
• Can be contractile when myosin II interacts

Question 8

Desmosomes

Answer 8

Intermediate filament bonding between neighbouring cells using cadherins

• Strong tensile strength
• Abundant in heart muscle and tough, exposed epithelia

Question 9

Hemidesmosomes

Answer 9

Intermediate filaments anchored to the basal lamina

Integrins used

Question 10

Types of cell connections between animal cells

Answer 10

Gap junctions
Tight junctions
Adheres junctions
Desmosomes
Hemidesmosomes

Question 11

Plasmodesmata

Answer 11

Gap junction equivalent in plants

The only type of cell connections within plants

Question 12

Transcytosis

Answer 12

Polarised transport of proteins from one end of the epithelia to the other

Question 13

Cadherins

Answer 13

Transmembrane proteins in the plasma membrane which bind to an identical cadherin in the next cell

Interaction needs calcium (Ca²⁺)

Question 14

Integrins

Answer 14

Trans-membrane proteins in the membrane that link the ECM to the cell’s cytoskeleton

Type of cytoskeleton linkage depends on context - migrating/collagen-secreting cells (focal adhesion) or epithelial monolayers (hemidesmosomes)

Protrusions adhere to the surface via:

focal contacts containing trans-membrane plasma membrane proteins called integrins

contractile actin bundles (stress fibres) attach to focal contacts

Question 15

Protrusions and focal points

Answer 15

Protrusions adhere to the surface via:

• Focal contacts containing trans-membrane plasma membrane proteins (integrins)
• Contractile stress fibres (actin bundles) attached to focal contacts

Question 16

Different types of cadherins

Answer 16

Epithelial cadherins - E-cadherins
Muscle cadherins - N-cadherins

Expressing N-cadherin instead of E-cadherin makes cells highly motile.

Question 17

Cancer and cell-cell interactions

Answer 17

Cell-cell interactions keep cells where they should be by using specific cadherins

Specific cadherins help cells recognise each other

Cancer cells lose the specific cadherins and create tumours everywhere

Cancer cells may secrete more matrix proteases than normal, helping them to escape through the basal lamina

Question 18

Carcinomas

Answer 18

Epithelial cancer cells - these are where 85% of cancers start

Question 19

Extracellular matrix

Answer 19

Tough and flexible (skin, tendon)
Hard and dense (bone)
Shock-absorbing (cartilage)
Soft and transparent (in the eye)

Question 20

Collagen synthesis

Answer 20

Fibrous protein - a key component of connective tissue synthesised by either osteoblasts (bone) or fibroblasts (skin, tendon)

~40 different collagen genes in humans (90% of collagen mass is collagen I (key component in bones))

Question 21

Procollagen

Answer 21

The precursor to collagen, three of these single-stranded procollagen fibres are trimerised in the ER using ascorbic acid (VC) to form three-helix procollagen fibres

Procollagen only becomes collagen after cleavage outside of the cell by proteases

Question 22

SUN and KASH proteins

Answer 22

Link filaments in the cytoplasm to nuclear lamins inside the nucleus

Affects genes through alteration in response to the ECM environment

Question 23

Osteoblasts

Answer 23

Cells that deposit the ECM in bone

Calcium phosphate fills gaps

Question 24

Basal lamina

Answer 24

Laminin and collagen IV are the main components of the basal lamina

Question 25

Collagen organisation

Answer 25

* Properly aligned in an oriented way * Cells rearrange fibres after secretion by pulling on them

Question 26

ECM diseases

Answer 26

* Abnormally stretchy skin * Brittle bone disease * Skeletal disease

Question 27

Collagen linker proteins

Answer 27

Fibronectin in focal adhesions (when the cells are protruding) Laminin in the basal lamina (keeps the cells)

Question 28

Changes in mitosis

Answer 28

Cells become rounded and less well attached to the surface while dividing Actin and myosin filaments are drastically rearranged in mitosis, allowing cell movement Integrins are phosphorylated and weaken their grip on the extracellular matrix

Question 29

Integrin on/off switching

Answer 29

Can be switched on and off both ways - by both intracellular signals and extracellular matrix signalling * When activated they take on an extended conformation * Microtubules do not interact with integrins (actins do)

Question 30

GAGs

Answer 30

Glucosaminoglycans: hydrophobic, large, negatively charged polysaccharides that resist compression and occupy a large volume (in comparison to their mass) - the empty spaces between cells

Question 31

Proteoglycans: what are they and how are they synthesised?

Answer 31

Extracellular proteins that have GAGs covalently linked Protein component made in the ER, followed by glycosylation which is then completed in the Golgi apparatus and delivered to the plasma membrane by constitutive secretion

Question 32

Cartilage

Answer 32

Tough, resist compression Used to counteract swelling pressure caused by GAGs (because they bind water molecules)

Question 33

Hyaluronan: what is it and how is it synthesised?

Answer 33

Composed only of carbohydrates, no proteins are involved in the compound Synthesised by hyaluronan synthase at the plasma membrane and extruded directly into the extracellular space

Question 34

Plant ECM

Answer 34

Plants have no intermediate filaments so they rely on cell walls only to support their cells and give them strength and resist turgor pressure (the force exerted by the cytoplasm onto the membrane)

Question 35

Plant cell walls: primary cell walls

Answer 35

Relatively thin, allowing the cell to grow as cell expansion is driven by turgor pressure

Question 36

Plant cell walls: secondary cell walls

Answer 36

This wall has its effect specialised dependent on the location of the cell Wood - hard and thick (crossed mesh of lignin) Leaves - thin, flexible, and waxy

Question 37

Cellulose fibre structure

Answer 37

Polysaccharide of D-glucose containing 16 long fibres held together by hydrogen bonds Interwoven with other polysaccharides occasionally - pectin (forms a gel that resists compression) and lignin (cross-linked mesh used for strength)

Question 38

Cellulose fibre organisation

Answer 38

Cellulose fibres resist stretching so their orientation determines the axis of growth (high cellulose concentration at the top and bottom of cells promotes lateral growth)

Question 39

Cellulose synthesis

Answer 39

Similar to hyaluronan: synthesis starts at the membrane and is made in the ER, transported into the Golgi, and then secreted out of the membrane

Question 40

Cellulose synthase complex

Answer 40

Used to organise cellulose fibres after production using microtubules in the cells as a reference