Epithelia Flashcards
What is epithelia?
Thin layer of tissue that lines body and surfaces.
What does epithelia seperate?
Internal and external environments of the body
What are examples of where there is epithelia?
Skin
Intestinal tract
Nephrons of the kidneys/urinary tract
Respiratory tract
Reproductive tract
Regions of the eye and auditory system
Salivary glands
Pancreas/Bile duct
What does epithelia control?
Controls the exchange of material between the body and the environment
What makes epithelia capable of pectoral fluid and electrolyte transport?
because epithelia is polarised/its asymmetrical cells
Both structural and functional asymmetry
How is secretion and absorption modified in epithelia?
Via channels and transporters
What hollow organs does epithelia define the composition of luminal fluid of?
Reproductive tract, respiratory tract, intestinal tract, kidney nephron, oviduct, vas deferens, salivary secretion etc
How is the function of epithelia varied?
The fucntion of epithelia is altered by a variation in the membrane proteins they express - if they change their protein expression they can change their role e.g.m from absorptive to secretory
What is the name of the two membranes of epithelia cells?
Apical and basolateral membrane
What is the transcellular pathway?
Pathway through the cell, passing through both the apical membrane and basolateral membrane.
Pathway facilitated by carrier proteins.
What is the paracellular pathway?
Pathway across an epithelium by passing through the intercellular space between the cells.
Pathway through tight junctions.
What is the apical membrane?
Membrane facing the outside world (facing the lumen / outside of the organ).
What is the basolateral membrane?
Membrane facing the internal body (interstitial fluid)
What is serosal and mucosal?
Mucous membranes line the digestive, respiratory, urinary, and reproductive tracts. They are coated with mucous gland secretions. Outer.
Serous membranes line body cavities closed to the exterior of the body: the pericardial, peritoneal and pleural cavities. Inner.
What is the two types of epithelia?
Absorptive or Secretory
What are the two types of absorptive epithelia?
Leaky and tight
What is absorption of absorptive epithelia driven by?
The active transport of Na+
What is the active transport of Na+ maintained by?
Maintained by Na/K-ATPase
Where is absorptive epithelia?
Intestine and Nephron
What direction is the absorption of absorptive epithelia?
From mucosal to serosal solution (lumen to blood)
What direction is the secretion of secretory epithelia?
From serosal to mucosal solution
Where is secretory epithelia?
Intestine, salivary glands, pancreas, sweat glands
What is secretion of secretary epithelia driven by?
Active transport of Cl-
What type of permeability does the apical membrane have?
Selective permeability
What determines the transport properties of the apical membrane epithelium?
The channels/transprters expressed
What does apical membrane epithelium not contain?
Does not contain Na+,K+-ATPase or other housekeeping transporters
What type of proteins does the basolateral membranes have?
“House keeping proteins” that maintain basic cell function
What basic cell function does the Na+/K+-ATPase maintain?
Maintains RMP/ion gradients
What basic cell function does the Ca2+-ATPase and Na+/Ca2+ exchanger maintain?
cell signalling
What basic cell function does the Na+/H+ exchanger maintain?
pH regulation
What basic cell function does the K+ channels maintain?
RMP due to high K+ permeability relative to Na+ / Cl-
What basic cell function do aquaporins maintain?
High water permability
What are the two proteins in basolateral membranes that support absorptive/secretory role of cell types?
NKCC
GLUT
What does the NKCC do?
Uptake Cl- for secretion
Moves Cl-, Na+ and K+ into the cell from interstitital fluid on basolateral membrane.
What does the GLUT do?
It is a glucose transporter and functions for glucose absorption
How does the transport of leaky absorptive epithelia and tight absorptive epithelia differ?
Leaky has bulk transepithelia transport whereas, tight have limited transepithelia transport (because it is controlled).
What is the characteristic of the transported fluid of leaky absorptive epithelia?
It is largely isosmotic
What type of absorptive epithelia is proximal?
Leaky proximal
Tight distal
Can leaky or tight epithelia maintain large transepthilial gradients?
Tight e.g., ionic and osmotic gradients - not limited to isometric fluid absorption.
How does the mechanism by which Na+ crosses the apical membrane differ between leaky and tight epithelia?
For leaky epithelia Na+ is dependent on cotransporters and/or exchangers (counter transporters).
For tight epithelia there are Na+ selective channels.
What are the electrical differences between leaky and tight epithelia?
Leaky has high permeability, low resistance, high transport rate and low transepithelial voltage.
Tight has a regulated lower permeability, high resistance, low transport rate and high transepithelial voltage.
What is transepithelial voltage?
Potential difference is the voltage across an epithelium, and is the sum of the membrane potentials for the outer and inner cell membranes
What is transepithelial resistance predominantly due to?
The differences in paracellular pathway (tight vs leaky)
What is hydraulic conductivity?
Hydraulic conductivity is a measure of how easily water can flow
What is the hydraulic conductivity of leaky absorptive epithelia?
High (due to the range of aquaporins and tight junctions)
Absorbs a large amount of isometric fluid
What is the hydraulic conductivity of tight absorptive epithelia?
Low (because under basal conditions) but increase under hormonal control.
Absorbs a variable amount of hyperosmotic fluid.
What is the responsiveness of leaky absorptive epithelia to hormones?
Low because little control and constant rate of transport
What is the responsiveness of tight absorptive epithelia to hormones?
High because ion and water transport is closely regulated by hormones.
What are the hormones that regulate tight absorptive epithelia?
Aldosterone and vasopressin
What does aldosterone regulate?
Na+ transport
What does vasopressin regulate?
Water transport of aquaporin 2
Where is leaky absorptive epithelia located?
On the proximal regions of organs
Where is tight absorptive epithelia located?
On the distal regions of organs
Where on the nephron is leaky absorptive epithelia?
Proximal tubule of the nephron
Where on the small intestine is leaky absorptive epithelia?
Duodenum, jejunum and ileum
Where on the nephron is tight absorptive epithelia?
The late distal tubule and collecting duct
Where on the large intestine is tight absorptive epithelia?
Distal colon
Draw the cellular model of leaky absorptive epithelium in the small intestine:
SGLT1 and GLUT2
Draw the cellular model of leaky absorptive epithelium in the kidney:
Differs between the early proximinal tubule and the late proximinal tubule.
Early involves SGLT2 (glucose coupled to Na+) and GLUT2 (glucose into interstitial).
Late involves SGLT1 (glucose coupled to 2Na+) and GLUT2 (glucose into interstitial).
And Na+K+ATPase on basolateral.
Draw the cellular model of tight absorptive epithelium:
Should illistarte ENaC channel on the apical membrane (Na+ into the cell).
Na+K+ATPase on basolateral membrane pushing K+ into cell and Na+ out of cell against their concentration gradients.
Should show Water following the osmotic gradient into the instititial fluid but ONLY through the transcellular pathway (NOT through the paracellular pathway. Cl- paracellular into the instititial fluid.
K+ out of cell into instititial fluid.
What membrane is the SGLT located on?
On the apical membrane
What membrane is the GLUT located?
On basolateral membrane
What percentage of filtered glucose is reabsorbed by SGLT2 in early proximal tubule of the kidney?
90%
(Final 10% by the SGLT1)
What is referred to as uphill absorption in the kidney?
Secondary active uptake through apical membrane
What is referred to as downhill absorption in the kidney?
Facilitated diffusion through basolateral membrane
What are two types of epithelial Cl- channels?
cAMP-activated Cl- channel (CFTR)
Ca2+-activated Cl- channel (TMEM16A)
What Cl- channel is defective in the disease cystic fibrosis?
cAMP-activated Cl- channel
Or known as the Cystic Fibrosis transmembrane conductance regulator (CFTR)
What is the physiological function of secretory epithelia in the GI Tract?
secretion of enzymes for the digestion of food
What is the physiological function of secretory epithelia in respiratory epithelium?
Keep surfaces moistened to maintain mucocilliary clearance
What is the physiological function of secretory epithelia in the reproductive tract?
Male - maturation of sperm
Female - survival of fertilisation of gamete
What is the physiological function of secretory epithelia in sweat glands?
Thermoregulation
What is the hydraulic conductivity of secretory epithelia?
High
What is the hormonal/nervous control of secretory epithelia?
Under precise control - low basal rates of secretion increase by nervous or hormonal control as required.
Draw the cellular model for electrogenic Cl- secretion:
Downhill diffusion on apical membrane invovles Cl- moving out of the cell through a NaCl channel into the lumen (passive diffusion). As is Na+ through the paracellular pathway. Water follows into the lumen through para and transcellular pathways.
On the basolaterial membrane is a NaKATPase and K+ leak channel moving K+ back into the interstituial fluid.
Also on the basolaterial membrane is a NKCC secondary active cotransporter moving Na+, Cl- and K+ into the cell.
What happens to Cl- secretion in cystic fibrosis?
It is impaired because there is a mutation resulting in no CFTR. Therefore Cl- secretion decreased and water secretion decreased.
Results to contributing to thick viscous mucus
What are epithelia cells specialised for?
Movement of ions, water and molecules.
Are epithelia cell polarised?
Yes
What is the apical membrane of epithelia?
The membrane facing the external environment.
What is the basolateral membrane of the epithelia?
The membrane facing the internal environment and neighbouring cells.
What do epithelia cells make contact with?
Neighbouring cells (through intracellular junctions)
Extracellular matrix (ECM) on the basal, not the apical surface
fluid in the lumen.
What is the different property between the apical and basal side of epithelia membranes?
Apical (facing outside of the body) has transport proteins.
What are the four different ways that cell membranes of neighbouring cells are joint?
Tight junctions
Adhering junction
Gap junction
Desmosome
What increases absorptive capability in the epithelia of the kidney and GI system?
Increase surface area for movement of ions and molecules (glucose, Na+ and amino acids)
What is meant by electrogenic transport?
The net movement of charge contributing to a difference in electrical potential.
What is meant by “uphill” transport?
Against gradient (rewire active transport mechanisms)
What is meant by “downhill transport”?
With gradient from high to low concentration
What is the uphill transport mechanism in Cl- electrogenic transport?
NKCC = Na+, K+, 2Cl- Co-Transporter
Actively moves Na+, K+ and Cl- ions into the cell against their electrochemical gradients by using energy provided by Na+ which is maintained by the Na+K+ ATPase.
The active transport against concentration gradient = uphill
What is the downhill transport mechanism in Cl- electrogenic transport?
Cl- channel
Once Cl- ions have accumulated within the cell (so that there is higher concentration in the cell than outside the cell) it exits passively down its concentration gradient and out of the cell through a Cl- channel.
From high to low concentration = downhill
What pathway does water not diffuse through in tight absorptive epithelia?
Does not go through the paracellular pathway
What is the difference between primary and secondary active transport?
Primary = uses ATP directly
Secondary = uses a ion gradient to cotransport
