Module 6 - Interactions between cells in multicellular systems Flashcards
Four major tissue types
Muscle
Nervous
Connective
Epithelia
The gut: the tissues comprising it and its significant features
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
The skin: the tissues comprising it and its significant features
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
Types of epithelia
Simple squamous (flat)
Simple cuboidal (cuboidal)
Simple columnar (columnal)
Non-keratinised stratified squamous
Keratinised stratified squamous
Pseudostratified columnar with goblet cells
Tight junction: what are they and what other key features do they have?
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
Gap junction
Connexon channels (2-4nm) between adjacent cells allowing ions and small molecules (<1000 daltons) to pass through
Adheres junction
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
Desmosomes
Intermediate filament bonding between neighbouring cells using cadherins
- Strong tensile strength
- Abundant in heart muscle and tough, exposed epithelia
Hemidesmosomes
Intermediate filaments anchored to the basal lamina
Integrins used
Types of cell connections between animal cells
Gap junctions
Tight junctions
Adheres junctions
Desmosomes
Hemidesmosomes
Plasmodesmata
Gap junction equivalent in plants
The only type of cell connections within plants
Transcytosis
Polarised transport of proteins from one end of the epithelia to the other
Cadherins
Transmembrane proteins in the plasma membrane which bind to an identical cadherin in the next cell
Interaction needs calcium (Ca²⁺)
Integrins
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
Protrusions and focal points
Protrusions adhere to the surface via:
- Focal contacts containing trans-membrane plasma membrane proteins (integrins)
- Contractile stress fibres (actin bundles) attached to focal contacts
Different types of cadherins
Epithelial cadherins - E-cadherins
Muscle cadherins - N-cadherins
Expressing N-cadherin instead of E-cadherin makes cells highly motile.
Cancer and cell-cell interactions
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
Carcinomas
Epithelial cancer cells - these are where 85% of cancers start
Extracellular matrix
Tough and flexible (skin, tendon)
Hard and dense (bone)
Shock-absorbing (cartilage)
Soft and transparent (in the eye)
Collagen synthesis
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))
Procollagen
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
SUN and KASH proteins
Link filaments in the cytoplasm to nuclear lamins inside the nucleus
Affects genes through alteration in response to the ECM environment
Osteoblasts
Cells that deposit the ECM in bone
Calcium phosphate fills gaps
Basal lamina
Laminin and collagen IV are the main components of the basal lamina
Collagen organisation
- Properly aligned in an oriented way
- Cells rearrange fibres after secretion by pulling on them
ECM diseases
- Abnormally stretchy skin
- Brittle bone disease
- Skeletal disease
Collagen linker proteins
Fibronectin in focal adhesions (when the cells are protruding)
Laminin in the basal lamina (keeps the cells)
Changes in mitosis
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
Integrin on/off switching
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)
GAGs
Glucosaminoglycans: hydrophobic, large, negatively charged polysaccharides that resist compression and occupy a large volume (in comparison to their mass) - the empty spaces between cells
Proteoglycans: what are they and how are they synthesised?
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
Cartilage
Tough, resist compression
Used to counteract swelling pressure caused by GAGs (because they bind water molecules)
Hyaluronan: what is it and how is it synthesised?
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
Plant ECM
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)
Plant cell walls: primary cell walls
Relatively thin, allowing the cell to grow as cell expansion is driven by turgor pressure
Plant cell walls: secondary cell walls
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
Cellulose fibre structure
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)
Cellulose fibre organisation
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)
Cellulose synthesis
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
Cellulose synthase complex
Used to organise cellulose fibres after production using microtubules in the cells as a reference
