L7 Flashcards
what are epithelia
they are barriers
they separate body compartments (organs)
what do epithelia act as
to act as gatekeepers
they facilitate movement of ions and molecules between external and internal environments
what are some areas where epithelia separate internal and external environments
Skin
Intestinal tract
Nephrons of the kidneys/urinary tract
Respiratory tract, reproductive tract
Regions of the eye and auditory system
what are the properties of epithelia
Separate internal and external environments
Control exchange of material between body and the
environment
Define the composition of the luminal fluid of hollow
organs
what is an example of where epithelia control exchange of material between body and the
environment
Intestinal tract and nephron/urinary tract
what are some examples of where epithelia define the composition of the luminal fluid of hollow
organs
Reproductive tract, respiratory tract, intestinal tract, kidney nephron, oviduct, and vas deferens, etc.
what is the basic structure of epithelial tissue
it has 1 or more layers of epithelial cells, a basement membrane and supporting tissues (extracellular matrix, cells)
the structure of epithelial tissue depends on what
where it is located in the body and its function
what is an example of serosal/interstitial fluid
blood
what is another name for the intERcellular space
the basolateral space
what membrane of the epithelium faces the mucosal solution
apical membrane
what does the basement membrane face
serosal solution (blood)
what are the 2 ways that substances can cross the epithelium
Paracellular = substances can travel between the epithelial cells
Transcellular = through the cells
what is the difference between gap and tight junctions
Gap = communication they are a passage and tight junctions are barrier proteins
describe the apical membrane
Faces the lumen (outside)
of the organ
it can be called the apical, mucosal, luminal
membrane or brush border
often has microvilli (MV)
describe the basolateral membrane
Faces the interstitial fluid
(inside) ‘internal body’
it can be called the basolateral membrane,
serosal membrane
what are the 2 parts of the basolateral membrane
Basal membrane (BM)
Membrane lining the lateral intercellular space (LIS)
where does the basolateral membrane spann from
tight junction to tight junction
what is vectorial transport
transport in a specific diffection
It involves active and passive transport of ions & solutes and depends on active transport (e.g. Na/K-ATPase) and osmosis
how do we sweat
Sweat glands are specialised parts of the skin epithelium. What these glands do is they secrete salt. water follows salt which is how we sweat
what is the natural environment made up of
electrical and chemical
why are epithelial cells capable of fluid and electrolyte transport
because they are polarised/asymmetrical
cells
they have structural and functional asymmetry
what is meant by structural asymmetry
the apical and basolateral membranes are different which makes it polar (because it is different on either side)
why do epithelial cells have functional asymmetry
because apical and basolateral membranes are
functionally different
They contain different set of channels and transporters which have properties distinct from each other
what are some common characteristics of the basolateral membrane
High K+ permeability relative to Na+, Cl
High water permeability
it also contains housekeeping transporters
Attached to the basement membrane
what is the reason that the basolateral membrane his a high K+ permeability relative to Na+, Cl
because it contains lots of K+ channels
why is the basolateral membrane so permeable to water
because it contains lots of aquaporins
what are some examples of housekeeping transporters located in the basolateral membrane
Na+/K+ ATPase
Ca2+ ATPase and Na+/Ca2+ exchanger
Na+/H+ exchanger (NHE-1)
what are some common characteristics of the apical membrane
Selectively permeable to various ions
no housekeeping transporters
variable water permeability
the apical membranes permeability to ions depends on what….
with examples
Permeability depends on the type of epithelium and what channels and transporters that membrane has
Na+ absorbing epithelia, high Na + permeability
– Range of transporters and channels
Secretory epithelia, high Cl- permeability
– Cl- channels
Determines the transport properties of the epithelium
Typical functional features of the basolateral membrane of
an epithelial cell are:
A. faces the external environment, does not have K+ channels, does
not contain aquaporins.
B. has microvilli, faces the interstitial fluid, touches neighbouring cells.
C. contains Na+ channels, has K+ channels, is permeable to water.
D. faces the interstitial fluid, has Na+-K+-ATPase, is permeable to K+.
D
there are 2 types of absorptive epithelia
lecky and tight
what are the characteristics of absorptive epithelium
Net transport is from mucosal to serosal solution
Absorption driven by the active transport of Na+
Na/K-ATPase
what are some examples of where you can find absorptive epithelium
Intestine and nephron
what are the characteristics of secretory epithelium
Net transport from serosal to mucosal solution
Secretion is driven by the active transport of Cl-
NKCC
what are some examples of secretory epithelium
Salivary glands, pancreas, sweat glands
what are some examples of epithelia that is both secretory and absorptive
Small and large intestine, kidney nephron,
airways
what is the difference between tight and leaky absorptive epithelia
(5 points)
They are separated on basis of….
Transport properties derived from electrical
characteristics
Apical membrane transport pathways
Hydraulic conductivity (relative water flow)
Responsiveness to hormones
Location within organs
what is transepithelial voltage
Voltage that develops across an epithelium due to active transport of ions
This is talking about going across the whole cell not just across the membrane (thats a mp)
what is the transepithelial voltage range for leaky absorptive epithelium
0 - 5 mV
what is the transepithelial voltage range for tight absorptive epithelium
20 - 100 mV
what are the electrical characteristics of absorptive epithelium
transepithelial voltage and transepithelial resistance
what is transepithelial resistance
Electrical resistance to flow of ions across an epithelium
Unit: ohms/cm^-2
what is the transepithelial resistance range for tight absorptive epithelium
Leaky absorptive epithelia
20 - 100 ohms/cm^-2
therefore it is ‘Easy’ for ions to move across epithelium
Tight absorptive epithelia
300 - 10,000 ohms/cm^-2
therefore it is difficult for ions to move across epithelium
how many x larger is the resistance for tight absorptive epithelia compeered to lecky
15 – 100 x higher
what is the reason for the difference in transepithelial resistance between tight and leaky absorptive epithelia
Leaky epithelia have lots of paracellular transport because they have less tight junctions
The number and the type of tight junctions in the epithelia determines the electrical resistance
electrical resistance is a measure of what
Electrical resistance is a measure of how much substances and solutes can pass the epithelium through the paracellular pathway vs the transcellular pathway
Why do we have leaky and tight epithelia within the same tissue within the same organ?
with example
The distal part of the kidney we have tight epithelium
200L of fluid goes through the kidneys per day and the majority of that is reabsorbed. To reabsorb this we have leaky and tight absorptive epithelium
leaky does bulk (to reduce time and make it more efficient)
tight is for specific things for fine tuning of homeostasis
if you have a high resistance what does that mean for the flow of ions
that there is a slow rate of transport (flow)
how does Na cross the apical membrane in leaky absorptive epithelia
Leaky epithelia - Na+ dependent cotransporters
and/or exchangers (counter transporters)
how does Na cross the apical membrane in tight absorptive epithelia
Tight epithelia - Na+ selective channels (e.g.
ENaC: epithelial Na+ channel)
describe hydraulic conductivity in leaky absorptive epithelia
what kind of fluid will it absorb
high hydraulic conductivity
as it has a range of aquaporins and tight junctions (number and types)
it absorb large amounts of isosmotic fluid therefore it is efficient and effective
describe hydraulic conductivity in tight absorptive epithelia
low hydraulic conductivity under basal conditions
increases under hormonal control
absorb variable amounts of hyperosmotic fluid because tight can produce large osmotic gradients as it stops the water from moving back
describe the effect of hormones on leaky epithelium
has very little effect as leaky have a constant rate of transport
describe the effect of hormones on tight absorptive epithelium
very responsive to hormones
ion and water transport is closely regulated by hormone
what are the 2 hormones which have an effect on tight absorptive epithelia
aldosterone and vasopressin
what is the effect of aldosterone on tight absorptive epithelia
aldosterone, regulates Na+ transport through ENaC
what is the effect of vasopressin on tight absorptive epithelia
vasopressin also called antidiuretic hormone (ADH)
regulates water transport Aquaporin 2 (AQP2)
why do tight and leaky epithelia respond to hormones
to regulate blood pressure
where is leaky epithelia usually located within organs? why?
proximal regions of organs
in these regions because this is where ‘bulk’ transport occurs
where is leaky epithelia usually located within the nephron
proximal tubule of the nephron
where is leaky epithelia usually located within the small intestine
duodenum, jejunum and ileum of small intestine
where is tight absorptive epithelia usually located within organs? why?
distal regions of organs
as this is where ‘fine tuning’ (highly controlled absorption) takes place
where is tight absorptive epithelia usually located within the nephron
late distal tubule and collecting duct of nephron
where is tight absorptive epithelia usually located within the large intestine
distal colon of the large intestine
Leaky absorptive epithelia
A. have a high transepithelial resistance.
B. can mainly be found in distal parts of organs.
C. do not have tight junctions.
D. have a low hydraulic conductivity.
E. possess mainly Na+ dependent co-transporters/exchangers in their apical membrane.
E
