Epithelial Tissue Junction Systems

• Junction systems ensure cohesion and communication within epithelial tissue.
• Junction systems divide into two main categories: intercellular junction systems and cell-basal lamina junction systems.

Intercellular Junction Systems

• These are abundant in epithelial tissue, though present in most tissues.
• The cytoskeleton of adjacent cells is linked via cytoplasmic and transmembrane proteins.
• There are three types of intercellular junctions: tight junctions, anchoring junctions, and communicating junctions.
• Zonula refers to junction systems that form a belt or ring around the plasma membrane.
• Macula refers to junction systems that are small oval regions scattered on cell lateral faces.

Tight Junctions (Occlusive)

• Tight junctions are also called zonula occludens (ZO).
• They form a ring encircling the apical pole of the cell, known as the zonula.
• Tight junctions provide complete occlusion of the intercellular space.
• Tight junctions are also referred to as impermeable junctions or tight junctions.
• At the zonula occludens, adjacent cell cytoplasmic membranes fuse at short distances forming crests.
• These junctional areas are called fusion points. Superposition of these fusion points leads to formation of closure lines.
• Closure lines intersect and form networks that encircle epithelial cells.
• The zonula occludens forms a physical and functional border separating apical and baso-lateral domains.
• Tight junctions consist of transmembrane proteins, including occludin and claudin family members.
• Transmembrane protiens are associated with other proteins like ZO-1, ZO-2 and ZO-3.
• ZO-1 interacts with spectrin, which is attached to actin microfilaments of the cytoskeleton

Function of Tight Junctions

• Tight junctions in epithelia oppose passage of large molecules.
• They prevent fluid diffusion into cavity lumens which allows for compartment property maintenance.
• These properties include high acidity (pH 1-2) inside the stomach.

Anchoring Junctions (Adhesions)

• Anchoring junctions ensure intercellular adhesion and maintain epithelial cell shape.
• Desmosomes attach the cell to neighboring cell's cytoskeleton.
• There are two types of desmosomes: macular desmosomes and zonular desmosomes.
• Macular desmosomes (macula adherens) appear as rounded densifications on epithelial cell lateral faces.
• Macular desmosomes do not fuse, but adhere and are common in the spinous layer of epidermis.
• The intercellular space in macular desmosomes is 25-35 nm wide and occupied by a dense line.
• The dense line corresponds to juxtaposition of the transmembrane glycoproteins of the two adjacent cells.
• These glycoproteins are cadherins, desmogleins, and desmocollins.
• The internal leaflet of the plasma membrane is thickened with a cytoplasmic plate that is 15-25 nm thick.
• The cytoplasmic plate links transmembrane glycoproteins and intermediate filaments of the cytoskeleton.
• Zonular desmosomes or zonula adherens form an adhesion belt encircling the apical end of an epithelial cell.
• The desmosomes links epithelial cells to neighboring cells.
• Zonular desmosomes is located below a tight junction and actin filaments attach to it allowing contractibility.
• This junction is present in intestinal epithelium near the apical pole just below the tight junctions.

Communicating Junctions (Nexus)

• Communicating junctions, also called gap junctions, are intercellular junctions that connect the cytoplasm of two neighboring cells.
• Communicating junctions facilitate passage of chemical or electrical signals between adjacent cells.
• These junctions are present in most body tissues.
• They are small tubular channels through the two cell membranes allowing molecule passage between adjacent cells.
• Each channel has two connexons.
• One connexon per membrane.
• One connexon is made of six transmembrane proteins, called connexins.
• Connexins create a hydrophilic channel of 2 nm in diameter.
• All water-soluble molecules below this size can pass through.