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.