cell junctions by dr roomi
DESCRIPTION
Amna inayat medical college UHS uploaded by class representative,TRANSCRIPT
CELL JUNCTIONS
Types of intercellular junctions :1. Tight junctions/zonula occludens2. zonula adherens3. Desmosomes/macula adherens4. hemidesmosome5. Gap junctions/nexus
Cell Junctions
Tight Junctions (Zonula Occludens)
"Zonula“ means band "occludens" = that close off the space between
the cells. are the most apical of the junctions Seal cells of an epithelial sheet to create a
permeability barrier (MCQ)
junctional complex= zonula occludens (ZO), zonula adherens (ZA), and desmosome (D).
Tight Junctions (Zonula Occludens)
Molecular CompositionClaudin: Transmembrane proteinOccludin: Transmembrane protein
In this way, zonulae occludens in sheets of epithelial cells help form two functional compartments: an apical compartment and a basal compartment.
The number of these fusion sites is inversely correlated with the leakiness of the epithelium
Blood brain barrier
Anchoring Junctions
Stabilize cells against mechanical stress
Mechanically attach cells and their cytoskeleton to their neighbors or to the extracellular matrix
These include: zonula adherens, desmosomes and hemidesmosomes.
ZONULA ADHERENS
Join an actin bundle in one cell to actin bundle in adjacent cell
Form adhesive belt just below tight junction
Cadherins: these are transmembrane adhesion proteins
Anchor (plaque) proteins anchor cadherins to contractile actin (in the terminal web).
Cadherins Mediate Calcium-Dependent Binding Between Cells
desmosomes“macula adherens”
this junctional type resembles a single "spot-weld" and does not form a belt around the cell
Cadherins: transmembrane proteins.
bind intermediate filaments rather than actin filaments
Hemidesmosomes“Half-Desmosomes”
Adhere epithelial cells to the basal membrane
Integrins are the transmembrane proteins.
Anchor proteins link integrins to intermediate filaments
GAP JUNCTIONS (COMMUNICATING JUNCTIONS)
Adjacent plasma membrane are closely opposed. intercellular space narrows upto 2-3 nm.
STRUCTURE: Gap junctions are made up of large protein units
called CONNEXONS Each connexon has 6 subunits called CONNEXIN
surrounding a channel The diameter of the channel is 2 nm and allows
sugars, Amino Acids and small ions, cAMP to pass through.
GAP JUNCTIONS
FUNCTIONS: Rapid propagation of electrical activity Metabolic coupling Exchange of various chemical messangers
between the cells Not associated with any cytoskeletal filament Loaction: Numerous in liver and pancreatic
cells, heart and smooth muscles.