L33. Epithelia (1) Flashcards
(16 cards)
What are the functions of epithelia?
–> Protection (skin)
–> Absorption (kidney, GIT)
–> Secretion (glands)
–> Filtration (kidney)
–> Sensation (skin, eye)
What are some examples of epithelia?
- Airway epithelium (lungs) - ciliated, cleaning the lungs, gas exchange, moistens air, disease: cystic fibrosis, lavage to get rid of sticky mucus
- Blood brain barrier - protect the brain, secretion of metabolites from the brain, problem: treatment of brain diseases e.g. Alzheimer’s disease
- Tubule in the kidney and gastrointestinal system - absorption and secretion of ions, metabolites, solutes such as glucose, amino acids, urea
Epithelial cells and epithelial tissue?
- Border between internal and external environment
- Varied shapes e.g. cuboidal, columnar, squamous, or ciliated depending on function
Key structures:
- Basement membrane = barrier and anchor
- Basolateral membrane = closest to blood
- Apical membrane = facing the ‘outside world’/lumen
- Tight junctions are selective barriers
- Apical and BL membranes have different structures, protein populations and functions (polarity) that allow epithelia to absorb and secrete substances and ions
How must epithelia function?
For epithelia to function correctly they must restrict movement of ions and molecules between the internal and external environments
What are tight junctions in epithelia?
- Connects two adjacent plasma membranes of two different cells; does NOT connect the cytosol of these cells
- Continuous band under apical surface that pulls cells so close together there is no intercellular space between the cells
- Selective barrier: tight junctions may allow selective movement of ions and/or organic substances between cells
What are the 3 names referring to the apical membrane/side?
Apical = mucosal solution = luminal = brush-border membrane = urine side in the kidney
- Side or membrane of the cell that faces the OUTside
What are the 3 names referring to the basolateral membrane/side?
Basolateral = serosal solution = interstitial = blood side in the kidney
- Side or membrane of the cell that faces the INside
What are the 3 main functions of tight junctions in epithelia?
- Barrier - limits passage of ions and molecules between cells in the paracellular pathway
- Gate - can allow certain solutes to flow through the paracellular pathway
- Fence - prevent movement of proteins between apical and basolateral domains
Can eplithelia be ‘leaky’?
Epithelia can be ‘leaky’ or ‘tight’ depending on how tight the tight junctions are
- Defines properties of the paracellular pathway and constrains diffusion of solutes and fluids
What is the polarity of epithelia on the apical membrane?
- Contains microvilli in highly re-absorptive or secretory epithelial cells
- Properties are very variable depending on the cell type (e.g. water permeability)
- Does NOT contain the Na+/K+-ATPase
What is the polarity of the epithelia on the basolateral membrane?
- Properties are more similar between different BLMs (high water and K+ permeability)
- Does NOT contain microvilli, but membrane infoldings
- Expresses Na+/K+-ATPase
What are key features of epithelia?
- Cells have polarity or asymmetry
- Proteins may be located in only the apical or only in the basolateral membrane
- Epithelia regulated differently e.g. by different hormones
What are the 3 driving forces for ion/solute/water movement across epithelia?
- Chemical driving force e.g. sodium gradient, SGLT1
- Electrical driving force e.g. charge difference between inside and outside the cell
- Osmotic force/gradient e.g. concentration of osmotically active substances such as sodium or glucose
Example of leaky epithelia: proximal tubule of kidney nephrons: sodium and water reabsorbed through tight junction?
- The tight junction of the renal proximal tubule (PCT) is leaky
- Paracellular transport/reabsorption of sodium and water
- The tight junction still forces solutes (e.g. glucose) to be reabsorbed through the cell (transcellular)
- The movement of water is dependent on net solute movement across the epithelia, and water will flow across the epithelia in the same direction as the movement of solute Na+
- Water may move through the tight junction in leaky epithelial (paracellular) and/or through water specific channels called aquaporins (transcellular)
What are desmosome junctions in epithelia?
- Desmosomal cadherins hold two cells together at single dense spots (not continuous)
- Anchor point for intermediate filaments (e.g. keratin in epithelia) to form
- Epithelia exposed to physical stress are often connected via many desmosomes (skin, lung)
What are gap junctions in epithelia?
- Connect cells through tiny channels between cells made of connexin proteins
- Allow regulated passage of ions and small proteins between cells
- Allow transmission of electrical activity or calcium between cells e.g. in the heart, but also in epithelia
