1. Cell adhesion Flashcards
How is physical stress transmitted between cells in a tissue?
- ECM
- cytoskeleton and cell-cell adhesion
What is physical stress in cells?
Changes/fluctuations of environment (solutes/ pressure/toxins/light/nutrients)
How is physical stress managed in plant vs animal cells?
- PLANT: predominantly ECM
- ANIMAL: use both ECM and cytoskeleton + cell-cell adhesion in diff tissues
What are the supportive and space filling matrices in plants?
Supportive matrix - cell wall
Space filling matrix - pectin (cell adhesion molecule)
Explain the synthesis of plant cell wall
- plant cells synthesise the cell wall themselves on the outside of the cell by enzyme complexes embedded in the membrane
- enzyme complexes are directed by microtubules aligned exactly like microfibrils -> cytoskeleton controls the modelling of plant tissues
What is the driving force of plant cell growth?
Turgor pressure
What are microfibrils composed of?
Cellulose microfibrils - long, unbranched chains of glucose -> 16 cellulose molecules assemble a microfibril
What macromolecules compose the plant cell wall?
- cellulose microfibrils
- pectin fibers
- lignin ligning (not all cell walls)
What is pectin composed of? What is the function?
- pectin - long, complex polysaccharide - forms plant cell matrix
- to resist compression
Compare the structures of primary and secondary plant cell walls
PRIMARY:
- weaker - grows into needed size
- pectin
- cellulose
- middle lamella
SECONDARY cell wall - more rigid - starts forming when primary wall reaches the size - secondary wall forms by:
(1) thickening of primary wall
(2) deposition of new layers under old layers (ex: lignin in wood)
- when plant cells become specialised - produce cell wall specific to the cell type: wax for epidermis / hard, thick, woody for xylem
What does the orientation of cellulose microfibrils influence?
Because cellulose microfibrils resist stretching - orientation influences in which direction the cell elongates - length / width
Explain the synthesis of plant cell wall
- plant cells synthesise the cell wall themselves on the outside of the cell by enzyme complexes embedded in the membrane
- composition of the cell wall depends on the plant (hard thick wall - wood, thin flexible wall - leaf)
Which animal tissue links all other animal tissues?
Connective tissue (+ basal lamina / basement membrane)
What are the examples of animal connective tissues and their characteristics?
All animal connective tissues are abudant in ECM
- tendons - tough and flexible
- bone - hard
- dermis - soft and flexible
- cartilage - shock absorbing
- vitreous humour - soft and transparent
Explain the structure formed in bones
Bone - connective tissue - osteoblasts secrete collagen - Ca / Mg / P ions incorporated into matrix - hard, flexible, not brittle - form osteons
What is an osteon?
Osteons - mineralised matrix deposited around central canal with a blood vessel and nerves
Explain the structure of cartilage
Large amounts of **ECM **(abudant in collagen and proteoglycan) - no mineralisation
Explain the structure of vitreous humour
Clear, viscous gel - water, collagen, hyaluronic acid
What is the major protein of ECM?
Collagen - fibrous proteins
What cells synthesise collagen?
Fibroblasts
What is the structure of collagen?
Triple stranded helical structure - rope like superhelix -> collagen fibril -> collagen fiber
MONOMER -> TRIMER -> FIBRIL -> FIBER
What is collagen arrangement in skin?
In skin collagen is arranged both longitudinally + transversally -> allows skin to resist stress in different directions
What is the collagen arrangement in tendons?
In tendons collagen fibers are aligned parallely, along the axis of extension
How do fibroblasts influence the alignement of collagen?
In collagen secretion fibroblasts pull apart and shape it - different collagen alignments based on tissue (ex: skin vs tendon) - also vice versa collagen alignment influences fibroblast distribution
Explain collagen secretion by fibroblasts
- collagen secreted as precursor form outside the cell - procollagen
- enzymes add peptide extensions - assembles into collagen into fibrils
What could be caused by faulty collagen assembly?
Disorders - Ehlers-Danlos syndrome (EDS)
How do cells attach to ECM?
Via integrin proteins - they attach to cytoskeleton inside the cell and collagen in ECM
What is the mechanism of integrin attachment inside the cell?
Integrin proteins (transmembrane receptor) attaches to the cytoskeleton via actin filaments (part of cytoskeleton)
What is the mechanism of integrin attachment outside the cell?
Integrins attach to collagen in ECM via fibronectins
How integrins can be activated / deactivated?
Activated: both subunits are extended - triggered by binding to ECM (fibronectin) or adaptor proteins to cytoskeleton inside the cell
Deactivated: both subunits are folded
REVERSIBLE
How do integrins coordinate cell movement?
Integrins create new attachment points at the front - attachments at the back are released -> cell crawls
Compare the functions of collagen and GAGs?
Collagen: provide tensile strength to resist stretching
GAGs: resist compression
What are GAGs?
Glycosaminoglycans (GAGs)
- neg charged (draws in H2O, ions) polysaccharide chains
- usually form proteoglycans
- common brush structure GAG aggregate
- more commonly found in soft ECM (ex vitreus humour not bone - mostly collagen)
What is epithelium and what are its functions?
- epithelium - sheet of cells
- created barriers - external and internal lining of cavities - controls movement of molecules / microbes - maintains different environments on both sides
What are the forms of epithelium cells? Relate to their function, give organ examples
- Columnar : secretion, absorption (ex: intestine)
- Cuboidal : secretion, absorption (ex: kidney)
- Squamous : filtration (ex: lung)
- Stratified : protection (ex: skin)
What is basal lamina and what is its function?
- basal lamina / basement membrane - thin, tough sheet of ECM
- separates epithelial cells and connective tissue
What is basal lamina composed of? What is its structure?
- type IV collagen <- form conn tissue
- laminin <- from epithelial cells
Explain the structure and function of laminin
- laminin - cross-like trimer structure
- supplies adhesive sites in basal lamina for integrins from epithelial cells (linking role)
Explain epithelial polarity
- environments on basal - apical epithelial surfaces are different (epithelium creates barriers)
-> ‘POLES’ of different environments are formed
=> epithelial apical-basal polarity
Why is epithelial polarity necessary?
Epithelial polarity maintains barriers - necessary for:
- absorption : brush-border cells (microvilli) - take up nutrients
- secretion : goblet cells - secrete mucus
What are the main cell junctions between epithelial cells?
- tight junctions
- adherens junctions
- desmosomes
- hemidesmosomes
- gap junctions
Honestly,** T**as Adomas Geria visa Diena
What is the role of tight junctions in epithelial cells?
- barrier function - prevent leakage
- maintain epithelial polarity
What proteins act in tight junctions?
Occludin
Claudin
How do tight junctions maintain epithelial polarity?
- prevent diffusion of membrane proteins within cell membrane
- allow glucose transport both in and out the cell
Explain the structure and function of adherens junctions
- cadherins (need Ca2+) - key components - link cytoskeletons (actin) of adjacent cells
- homophilic binding (two same cadherins bind)
What are adhesion belts?
Adhesion belts - actin filament bundles which bind neighbouring cells via adherens junctions
How are adherens junctions involved in epithelial sheet bending?
Connections between adherens junctions and cytoskeleton (adhesion belts) allow sheets to change shape
Explain the structure and function of desmosomes
- join intermediate filaments (keratin) of neighbouring cells - join cells
- contain cadherins (different to adherens junctions)
Explain the structure and function of hemidesmosomes
- join intermediate filaments (keratin) to basal lamina
- use integrins to connect keratin
- looks like a half desmosome - hemidesmosome
Explain the structure and function of gap junctions
- direct channels for cytosolic communication
- tunnels of aqueous connectivity between cells - small, water-soluble molecules pass
- connexon proteins line up to form the water-filled channel - gap junction
How permeable are gap junctions?
- gap junctions are gated - selective
- permeability regulated by external signals (ex dopamine)
What is the equivalent of gap junctions in plants?
- plasmodesmata in cell walls
- no other cell-cell junctions
How do cell adhesions influence cell behaviour?
Assist in cell movement in tissue growth and development
What is compaction in embryo development?
Compaction - the 8-cell stage in embryo development when adherens junctions (cadherin) form between adjacent cells
What proteins are essential in embryo compaction?
- E-cadherins essential for forming adherens junctions
- E-cadherin localisation at basolateral cell-cell conatct sites
What are the experiments which defined cadherin mediated adhesions?
1) when cadherin binding antibodies were introduced in embryo - cells didn’t adhere - fell apart
2) when cadherin genes were deleted in embryos - cells didn’t adhere, embryo fell apart
What are the main types of cadherins? What cells express them?
- E-cadherin (epithelial cells)
- N-cadherin (neurons)
- R-cadherin (nerve, muscle, lens (retina))
- Cadherin-6 (placenta, epidermis)
Different cells express different cadherins
CERN
Explain homophilic vs heterophilic binding
HOMOPHILIC : like-to-like (ex: same cadherins)
HETEROPHILIC : different molecule bind (ex: ligand and receptor)
How cell adhesions help cells sort out?
- cells sort out according to type - recognise similar cells by cell adhesion molecule
- cells adhere expressing same cadherin types
- levels of specific cadherin expression also influence cell-cell interactions
- cells with higher cadherin levels adhere more tightly
How do cadherins act in cell sorting during development?
In neural tube formation:
1) ectoderm cells express E-cadherin
2) when neural tube pinches off - turn off E-cadherin production, turn on N-cadherin expression
3) migratin neural crest cells turn off N-cadherin expression and turn on cadherin-7 expression
Explain epithelial to mesenchymal cell transition
Epithelial-to-mesenchymal transition (EMT):
- cell assembly into epithelium is reversible
- EMT found in developing tissues, tissue regeneration, wound healing
- EMT involved in cancer
Explain what are selectins and their function
Selectins :
- cell surface proteins that bind to carbohydrates
- expressed in WBCs and endothelial cells
- act in inflammation
- form relatively weak cell adhesion in blood
Explain selectin and integrin function in inflammation
Selectins WBC movement into tissues at inflammation sites:
- selectins on endothelial cell surface recognise WBCs sugars - form weak adhesion and rolling of WBCs along endothelial cells
- rolling by selectins done - strong adhesions formed with integrins at the site of inflammation the WBCs squeeze into tissue
What problems can arise if selectins or integrins are defective?
Affects WBCs recognition at inflammation sites - leukocyte adhesion deficiency (LAD)
Why would cells degrade ECM?
- cells degrade ECM to allow cells to pass between endothelial cells to the inflammation site
- ECM remodelling (for development, wound healing)
What are the main groups of ECM degrading enzymes?
- matrix metalloproteinases (MMPs)
- serine proteases
- localised degradation - highly specific enzymes (ex: collagenase)
Why does the activity of ECM degrading enzymes must be localised?
ECM degradation must be highly controlled by localising ECM degrading enzymes - not to destroy ECM where not needed
How is the activity of ECM degrading enzymes is controlled?
1) Enzymes expressed in inactive form (localised activators)
2) Enzymes confined by cell-surface receptors
3) Enzymes are inhibited by locally secreted inhibitors
