Epithelial cells Flashcards
What is epithelial tissue?
“frontier’ between the body and external world.
What are the functions of the epithelia
- protection (skin- provide physical protection)
- secretion - pancreas
- absorption kidney
- excretion - lung
In 2, 3, 4 : epithelia: controls permeability, moves fluids, produces specialised secretions - sensory perception - olfatory system - provides sensation
What are the 3 epithelial tissue types based on cell shape (+ describe shape)
Squamous - flat
cuboidal - more cube
columnar - more rectangular
What are the 3 epithelial tissue types based on cell number (+ describe )
Simple- 1 layer
stratified- multiple layers
pseudo stratified- irregular lamina
Where is simple squamous epithelia found
Lungs
capillary endothelium
lining of pleural cavity, pericardium and the peritoneum
bowmen’s capsule
associated with absorption and excretion of ions- need flow
where is Stratified squamous epithelia found
Oral (lip) pharynx esophagus anal canal uterine cervix & vagina Skin (keratinised)
Where is simple cuboidal epithelia found
- follicle of thyroid gland
• collecting ducts of kidney
• salivary glands
• pancreas
Where is Stratified cuboidal epithelia found
- ducts of sweat glands
* ducts of salivary glands
where is Simple columnar epithelia found
• “high cuboidal” gall bladder
• surface epithelium of
stomach
• small intestine
where is Stratified columnar epithelia found
- large excretory duct of salivary glands
- parotid
- oral cavity
- parts of the urethra
Where is Pseudostratified columnar epithelia found
- trachea
* ductus epididymis
How do cells communicate and maintain the structure of the epithelia
Juxtacrine signalling
type of cell to cell (junctions connect the cytoplasm of two adjacent cells) or cell to extracellular matrix (membrane structures connect the cell with the extracellular matrix “messenger’
requires very close contact
What are the six extracellular connections
- tight junctions
- Adhering junctions
- Gap Junctions
- Desmosomes
- Hemidesmosomes
- Focal adhesions
What are the cell to cell extracellular connections
tight junctions
adhering junctions
gap junctions
desmosomes
what are the cell to extracellular matrix connections
Hemidesmosomes
Focal adhesions
At what levels of the cell are the tight junctions found
‘shoulder height’
At what levels of the cell are the tight junctions found
what does the structure involve
‘shoulder height’
Grooves and ridges with extracellular space
connected by strands of transmembrane proteins (e.g. Claudins)
At level of the cell are the adhering junctions
what does the structure involve
below the tight junctions ‘boob height’
actin filaments on either side connected by cadherins in the Extracellular space
(actin filaments attached to the cadherin via catenin)
At level of the cell are the desmosomes
what does the structure involve
waist
intermediate filaments connected to a plaque connected to cadherins in the extracellular space.
At level of the cell are the gap junctions
what does the structure involve
thigh
adjacent plasma membranes connected by connexons
What do tight junctions do/ functions as/ importance
function as a barrier between apical and basal domains
contribute to the maintenance of cell polarity
indicate how permeable epithelia is/ may be most imp way two cells communicate
What do adhering junctions do/ functions as/ importance
adherens maintains normal cell architecture
they occur at the site of intracellular attachment for actin filaments
What do Gap junctions do/ functions as/ importance
signals between cells would pass through an open channel into the next cell.
directly connects cytoplasm of two cells via connexins
A connexon can be open or closed.
What do desmosome do/ functions as/ importance
Connect intermediate filaments together to help maintain epithelium integrity
Attachment plaque inside of cell connects keratin (on inside) and cadherin across the extracellular space.
where are hemidesmosomes located
at the bottom of the cell
it connects the basal lamina (basement membrane) to the basal surface of the cell (bottom)
What do hemidesmosome do/ functions as/ importance
Half- desmosome
similar tissue integrity function as desmosome (distribute forces through epithelial)
Attached to intermediate filaments like keratin
anchored intracellularly to the protein plectin
Hemidesmosomes connects the basal surface to the underlying basal lamina which is one part of the basement membrane
How do focal adhesions and hemidesmosomes differ
what they bind and what filaments they attach to
hemidesmosomes: attached to intermediate filaments like keratin, anchored intracellularly to the protein plectin
focal adhesions: talin and vinculin in cell, integrin across membranes and fibronectin in basement membrane
where are focal adhesions located
base of cell- anchor to basement membrane
Where do the focal adhesions occur
on the cytosolic side of the membrane
???
Cells can reshape their actin cytoskeleton depending on the rigidity of the surface and matrix proteins
What is the function of tight junctions
To act as a selective permeability barrier
What is the function of tight junctions
To act as a selective permeability barrier between the apical and basolateral
v. different environments
What side is the apical membrane
lumen/external environment
What side is the basolateral membrane
(interstitial space)
basal lamina/ basal membrane
What does a tight epithelia do
example where its found
maintains large osmotic gradients (between apical and basal compartments)
urinary bladder
distal tubule of the kidney
What side is the apical membrane
lumen/external environment
FACES LUMEN
What side is the basolateral membrane
(interstitial space)
basal lamina/ basal membrane
FACES INTERSTITIAL SPACE
What are the two types of transport across the epithelia
- transcellular
- paracellular
what is transcellular movement across the epithelia
solutes move across the cell by passing through the apical and basolateral membranes
what is paracellular movement across the epithelia
Solutes bypass the cell by crossing the epithelium through tight junctions
What are the types of transport systems present in cells.
aid in tightly regulating the fluid environment though movement of ions
- uniporters
- symporters
- antiporters
what do uniporters transport
1 molecule types
What do Symporters transport
type of co-transporter
transport different molecules in the same direction
what do antiporters transport
type of co-transporter
transport molecules in the opposite direction
What are the types of ion movement
- passive transport
- active transport
what is passive transport
what is movement determined by
movement of ions and other atomic or molecular substances across cell membranes no requiring energy
Electrochemical gradient (concentration gradient and membrane potential)
what is active transport
movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by proteins and requiring energy.
What are the two types of active transport + how to they differ/ get energy
- primary active transport used energy from ATP hydrolysis to transport molecules against their electrochemical gradients (e.g. Na+/ K+ ATPase)
- secondary active transport uses energy stored in the electrochemical gradient of one solute to transport other solute against its electrochemical gradient (NA+/K+/2CL co-transporter
What type of transporter is Na+/K+ ATPase
what movement of ions does it allow
primary active transporter: uses ATP to transport molecules against electrochemical gradient
3 Na+ out
2 K+ in
describe the enzymatic cycle of the Na-K pump
- Empty: ATP bound, exposed to intracellular space
- 3 Na + ions within the cell bind to sites
- ATP molecule is hydrolysed, phosphorylating the alpha subunit. results in a conformational change closes in cytosolic side and opens in extracellular side.
- release Na+
- two K+ions from outsides the cell binds to the sites releases the phosphate
- conformational change- closed to both sides
- ATP molecule binds - causing conformational change K+ exposed to the cytosolic space and released
What happens when the Na/K ATPase pump is disrupted?
Shows importance in??
with a functioning Na/K+ pump osmotic equilibrium is maintained By an equal number of positive and negative ions moving in and out of the cell
Continued passive leakage without Na/K pump to rectify disrupts the osmotic equilibrium- as a result water flows into the cell, causing it to swell.
What are the driving forces of Na + that make the Na-k pump necessary
what does the flow of positive charge across the apical membrane result in
backleak of Na+ through the tight junction into the apical lumen. Entry of Na+ across the apical membrane into cell.
Exit of Na+ across the basolateral membrane is against the electrochemical gradient- thus needs ATP.
The flow of positive charge across the apical membrane sightly depolarises the apical membrane (-67mV) relative to the basolateral membrane (-70mV)
describe the enzymatic cycle of the Na-K pump
- Empty: ATP bound, exposed to intracellular space
- 3 Na + ions within the cell bind to sites
- ATP molecule is hydrolysed, phosphorylating the alpha subunit. results in a conformational change closes in cytosolic side and opens in extracellular side.
- release Na+
- two K+ions from outsides the cell binds to the sites releases the phosphate
- conformational change- closed to both sides
- ATP molecule binds - causing conformational change K+ exposed to the cytosolic space and released
What does the Nernst Equation describe?
The conditions when an ion is in equilibrium across a membrane.
What can the Nernst equation be used to calculate?
equation
Equilibrium potentials (cell potentials)
E= (RT/zF) ln([X]o/[X]i)
Explain the differences in tight junctions
between epithelia and its consequences
leaky and tight tight junctions result in leaky and tight epithelia
tight: maintains large osmotic gradients (between apical and basal compartments)
leaky: allows molecules transport/exchange.
Understand that an electrochemical gradient
exists to aid in ion transport
-3mV : lumen
-70mV: in cell
0mV: interstitial space
The charge difference allows movement of charged particles. Will try and balance out charge differences as well and [] difference
. Explain how the Sodium/Potassium ATPase
works and its action in ion transport
mechanisms
How it works:
- Empty: ATP bound, exposed to intracellular space
- 3 Na + ions within the cell bind to sites
- ATP molecule is hydrolysed, phosphorylating the alpha subunit. results in a conformational change closes in cytosolic side and opens in extracellular side.
- release Na+
- two K+ions from outsides the cell binds to the sites releases the phosphate
- conformational change- closed to both sides
- ATP molecule binds - causing conformational change K+ exposed to the cytosolic space and released
Gets 3Na+ out and two K+ in
in basolateral membrane
It prevents the cell from bursting: water follows salt, if too much Na+ in cell water would follow- increase Volume.
What is the nephron
describe parts
the functional unit of the kidney
Bowman’s capsule (with glomerulus) –> proximal tubule –> loop of henle–> distal tubule –> collecting duct
What type of epithelia is found in bowmans capsule
simple squamous epithelial cells
How does filtration within the glomerulus occur
fenestrations let in blood in except for cells. Across basement membrane into the lumen of the capsule
What is reabsorbed in proximal tubule
100% glucose 100% AA 70% sodium 70% potassium 70% calcium 70% water
What is created in the loop of henle
what does this aid in
A salt gradient
Water retention in the collecting duct - water follows salt.
Function of the proximal tubule
reabsorbs
recovers the largest fraction of fluid and solutes filtered from glomerular filtrate
Na+, Cl-, NaHCO3, glucose, AA, and water to go back into blood
In the kidney basolateral membrane faces
blood
in kidney apical membrane faces
Lumen- glomerulus filtrate
How is Na+ reabsorbed in the proximal tubule
draw diagram
Flows down concentration gradient into cell (high Na+ out of cell 150mM–> low Na+ inside cell ~5mM), Na/K+ ATPase pump in the basolateral membrane pumps Na+ out into blood- against gradient (maintains the gradient)
How is Na+ reabsorbed in the proximal tubule
draw diagram
Flows down concentration gradient into cell via Na-H exchanger (high Na+ out of cell 150mM–> low Na+ inside cell ~5mM), Na/K+ ATPase pump in the basolateral membrane pumps Na+ out into blood- against gradient (maintains the gradient) + Na/HCO3 cotransporter pumps out too
What are the pumps in the proximal tubule that facilitate Na+ reasorbtion
- Na-H exchanger= apical membrane (into cell from lumen)
- Na-K ATPase in basolateral membrane - 3Na+ out of cell (2K+ in)
- Na/CHO3 co-transporter basolateral membrane- out
where does the H and HCO3- come from for the Na-H exchanger and Na/CHO3 co-transporter in the proximal tubule (to get Na back in blood)
CO2+ H2O = CA
breaks into H + HCO3-
How is glucose recovered in the proximal tubule
convoluted
and
straight tubule
Convoluted: SGLT2 98% reabsorbed here
Straight: SGLT1 2% reabsorbed here
These are Na/glucose co-transporters
How is glucose recovered in diabetes
Diabetes: defined by high glucose in the blood–> high [glucose] in glomerular filtrate
SGLT becomes saturated:
- excess glucose in the urine
- increase in urine secretion
SGLT inhibitors= new drug, prevent reabsorption
How is glucose recovered in the proximal tubule
convoluted
and
straight tubule
Convoluted: SGLT2 (in apical membrane + GLUT2 in basolateral) 98% reabsorbed here
Straight: SGLT1 (in apical membrane and GLUT1 in basolateral) 2% reabsorbed here
These are Na/glucose co-transporters
How is water reabsorbed in the proximal convoluted tubule
paracellular - via tight junctions
transcellular via aquaporins in both apical and basolateral membrane
Explain water transport in the collecting ducts
- this part regulates how much water is recovered
- last segment where urine constituents can be modified
- tight junctions are not permeable to water
- transcellular transport through water channels
> aquaporin 2 (apical)
> aquaporin 3, 4, etc (basolateral) - required on both apical and basolateral sides
> constantly present on basal side
What is the mechanism that cycles aquaporins to the surface of the apical membrane
(which aquaporin)
ADH is released in response to low H2O in the blood.
binds ADH receptor in the basolateral membrane of the collecting duct. G protein coupled receptor__> cyclic AMP–> protein kinase A triggers aquaporin 2 to move to the surface of the apical membrane.
Function of the epithelial cells in the lung
- protection and secretion of ions
- regulate thickness of the epithelial lining fluid and pH of the fluid
How do epithelial cells function to maintain their function
- chloride movement
- sodium potassium 2 chloride co-transporter
How do epithelial cells function to maintain their function
Draw
- chloride movement
(moves out apical membrane via Cl- transporter and CFTR (Cl- HCO3- co-transporter) - sodium potassium 2 chloride co-transporter (in basolateral membrane- facing blood)
CFTR inhibits ENaC- which prevents Na+ absorption into cell (prevents dehydration- water follows salt)
What causes cystic fibrosis
mutations in the CFTR gene that encodes for the CFTR channel leads to defective expression and function of CFTR
what do Cystic fibrosis patients present with
what is this caused by
Present with dehydrated mucus in the lungs which cannot be cleared (leads to airway obstruction, chronic infection and inflammation and respiratory failure)
mutated CFTR removes inhibition on ENaC (Na+ transporter into cell) –? enhance Na+ absorption
Enhance Na+ absorption at apical membrane causes dehydration of airway surface liquid by osmosis
Accumulation of mucus due to impaired ciliarity clearance
Understand the molecular mechanisms of
cystic fibrosis
Defective CFTR gene–> removes inhibition on Na transporter–> increased Na absorption into cell–> dehydration (water follows salt, osmosis) –> ciliarity can’t move mucus –> accumulation
Describe epithelial cell ion transport in
the lung, and how this contributes to
pathogen defence
Cl- into airway surface liquid
keep airway surface liquid hydrated, allowing it to flow: CFTR transporter (HCO3-/ Cl-) inhibits Na absorption no water flows out.
What is digestion and absorption in the GIT dependent on
Apical microvilli- these have enzymes that complete digestion
How are peptides and amino acids absorbed in the SI?
Pancreatic proteases and peptidases (fond on microvilli) break proteins down into peptides in the lumen
peptides are absorbed into enterocyte with the help of Na.
Cytosolic peptidases break peptides into Amino acids
Amino acids are absorbed into blood
Follows Na+
Na/K ATPase pump mantains Na gradient.
Understand and describe ion transport and
protein secretion mechanisms in the
exocrine pancreas
hdj
How are Carbohydrates absorbed in the SI
Glycogen broken down in lumen
Fructose absorbed via GLUT5 into enterocyte- released out into blood via GLUT2
Lactose broken down by lactase in the apical membrane
Glucose and galactose absorbed into enterocyte via SGLT1 : sodium cotransporter.
absorbed into blood via GLUT 2
Na/K ATPase maintains Na gradient.
layers of the stomach
Mucosa
muscularis mucosae
Submucosa
What signals to the parietal cells to release parietal juice
vagus nerve and gastrin
Describe the process of acid secretion by parietal cells
- Stimulation: Gastrin –> CCK2 receptor –> Ca+ influx
Acetylcholine –> M3 receptor –> Ca+ influx
Histamine –> H2 receptor –> cAMP
(must induce Cl- uptake) - Cl- uptake from basolateral side:
Cl- pump
Na+ K+ and Cl- co-transporter
Cl-/ HCO3- anti-porter - Cl release into apical lumen
- Via CFTR receptor - water and CO2 uptake
from basolateral side
- where protons come from
form CA: HCO3- is pumped back out of cell via Cl-/ HCO3- anti-porter - Proton release into apical lumen
via H+/ K+ antiporter
proton + Cl-= HCl
Stimulated and resting parietal cell look very different. Describe cause
Resting: low acid secretion
Stimulated: High acid secretion cAMP –> increase rate and increase number of H-K pumps
Pancreas structure
Acinus
intercalated duct
Acinar cell
duct–> centroacinar cell–> pancreatic acinar cell
What are the acinar cells in the pancreas
Specialised protein synthesising cells
Pancreas structure
what does pancreas produce?
Acinus
intercalated duct
Acinar cell
duct–> centroacinar cell–> pancreatic acinar cell
produces insulin
What are the acinar cells in the pancreas
what is released
Specialised protein synthesising cells
zymogen granules
triggers of pancreatic secretion
Stimulation of NaCl secretion by pancreatic acinar cells.
- protein and lipid breakdown products stimulate a vagovagal reflex that stimulated primarily the acinar cells
- H+ stimulates S cells to secrete secretin, which acts on receptors on duct cells, stimulating HCO3- secretion
- protein and lipid breakdown products stimulate I cells in duodenum to secrete CCK, which acts on receptors on acinar cells, stimulating enzyme secretion
What are potent stimulators of Cl- secretion
hormone CCK and the cholinergic neurotransmitter acetylcholine
Ion transport in the duct cells
- H+ + HCO3- made from water and CO2.
- H+ out of cell on basolateral membrane Via proton transporter
and Na+ H+ exchanger - Na+ and HcO3- in via cotransporter
- Na/K ATPase in basolateral membrane 3 Na+ out 2K+ in : sets up ion gradient for movement of Na to power everything
- K+ out via K+ pump
- Ca+ triggers Cl- release into apical membrane
- CFTR receptor activated by cAMP to get HCO3- and Cl- into apical membrane
- Cl-/ HCO3- antitansporter in apical membrane.
- passive diffusion of Na and H2O across tight junctions to balance charge
Protein secretion in the acinar cells
draw
secretin, CCK and ACh trigger various mechanisms to release Ca+ from the ER and cause insertion of vesicles into the apical membrane and thus protein secretion. (exocytosis of granules)
ACh–> muscarinic receptor –> G-protein coupled receptor –> PIP2–> IP3 –> Ca+ release from ER
Protein secretion in the acinar cells
draw
secretin, CCK and ACh trigger various mechanisms to release Ca+ from the ER and cause insertion of vesicles into the apical membrane and thus protein secretion. (exocytosis of granules)
ACh–> muscarinic receptor –> G-protein coupled receptor –> PIP2–> IP3 –> Ca+ release from ER
what is temporal calcium signalling
???
Ca2+ sensitive processes are tuned to respond to these transient changes in Ca2+ generated by the on/off mechanisms
respond to changes- both high + low.
What is Spatial Ca+ signalling
Ca2+ signalling components are organised into macromolecular complexes in which Ca2+ signalling functions within highly localised environments
Describe calcium signalling
A highly versatile intracellular signal that can regulate a range of cellular functions
- rapid highly localised Ca+ spikes regulate fast responses
- slower responses can be controlled by repetitive global or intracellular Ca+ oscillations
Ca+ intracellular level determined by ‘on’ reactions and ‘off’ reactions where Ca+ removed by pumps, transporters
Describe calcium signalling
A highly versatile intracellular signal that can regulate a range of cellular functions
- rapid highly localised Ca+ spikes regulate fast responses
- slower responses can be controlled by repetitive global or intracellular Ca+ oscillations
Ca+ intracellular level determined by ‘on’ reactions and ‘off’ reactions where Ca+ removed by pumps, transporters
Calcium signalling ‘on’ reactions
External Ca2+ entry or second messengers that release internal Ca2+ (stored in the ER)
- can bind buffers or effectors
Calcium working
calcium signalling ‘off reactions
Ca2+ leaves effectors and buffers
removed from cell by transporters and pumps
- Na+/ Ca+ transporter extrude Ca2+ to the outside
- SERCA pumps Ca2+ back into the ER
Mitochondria can sequester Ca2+
stopping Ca+ activation
Describe the ‘on reaction’ Ca2+ entry mechanism
- entry of Ca+ is driven by the presence of a large electrochemical gradient across the plasma membrane
- Voltage-operated ion channels allow for rapid Ca2+ influx
- Receptor operated channels respond to external stimuli to allow Ca2+ influx
- second messenger operated channels are controlled by internal messengers like cyclic nucleotides
- Store operated channels are plasma membrane ion channels and can operate over longer time scales e.g. smooth- muscle contraction, cell proliferation
In= all channels
What is entry of Ca+ driven by
The presence of a large electrochemical gradient across the plasma membrane - created by the Ca-H pump and the Na-Ca exhanger that keep intracellular [Ca2+] four orders of magnitude lower than extracellular [Ca2+].
Internal Ca2+ stores are primarily located in the
ER/SR
also small amount in mitochondria
What primarily regulates the release of calcium from internal stores
IP3 or ryanodine receptor (RYRs)
RYRs associate with large complexes
What are the four different pumping mechanisms responsible for the off reaction of Ca+
- Plasma-membrane Ca2+ ATPase (PMCA)
- Na+/ Ca2+ Exchanger
- Sarcoendoplasmic reticulum calcium transport ATPase (SERCA)
- Mitochondrial
uniporter
These pumping mechanisms have different thresholds for activity which enables cells to select the combination of off reactions that exactly meets their Ca2+ signalling requirements
what are the important homeostatic functions of the Ca+ pumps and exchangers
- maintain the resting level of Ca2+ at ~100nM
- ensure that the internal stores are kept loaded
Ca2+ signalling in skin epithelia
Ca2+ plays a prominent role in all of the cutaneous strata
(expression of junctional complexes that hold the keratinocytes in position and regulate their proliferation- skin structure)
* Orai 1: subunit of store operated ca2+ channel
- negatively controls differentiation
- controlling sustained proliferation
- migrating of keratinocytes
Ca2+ is important in maintaining the organisation of the skin. Controls how stratified epithelia is organised and maintains homeostasis.
Ca2+ signalling in wound repair
- cues triggering damage detection - Ca2+ signals to get to cut and cover it.= detecting stress mediated damage
- initiation of ROS production or mitochondria permeability
- oxidation reactions by binding directly to enzymes
(allows perpetuation of inflammatory signals)
- mediates release of paracrine mediators via exocytosis and plasma membrane channels. - generation and spartial transmission of biochemical signals
- early protective responses
- Tissue repaired
more
Darier Disease
- What gene
- characterised by?
- results in?
- Autosomal dominant inherited skin disorder.
- less adhesion between epidermal cells and abnormal keratinisation
- Loss of function mutation in gene encoding SERCA2
- SR/ER Ca2+ ATPase
- SERCA one of the pumps that remove Ca2+ in the ‘off’ reaction
- SERCA pumps replenish depleted Ca2+ ER stores
- individuals with Darier Disease have lower Ca2+ concentrations in the ER in keratinocytes
- results in loss of tight junction and desmosome
Pancreatitis
Characterised by inflammation that may cause auto-digestion of the organs by its own enzymes
pathogenesis appears multifactoralal
- enzyme activation, pancreas malfunction, inflammation, calcium overload
CALCIUM OVERLOAD
??function of pancreas is highly controlled by Ca2+.
- increased Ca+ entry through store operated calcium channels
- increased Calcium from reticulum and normal classic ‘on’ signal (via RYR and IP3)
- SERCA doesn’t work - therefore calcium can’t get back in to ER
- because SERCA isn’t working mitochondria tries to get more into it- create electrical gradient–> impairs ATP function
SERCA relies on ATP (as it’s a pump– therefore impaired ATP function will result in less Ca+ in cytoplasm
