Cell Junctions — Definition & Types

Definition

Types of Cell Junctions

1. Occluding Junctions — Tight Junction (Zonula Occludens)

• Located at the most apical end of the lateral cell membrane.
• The term zonula indicates it forms a continuous belt encircling the entire cell.
• Composed of transmembrane proteins — occludins, claudins, and ZO proteins — that fuse adjacent cell membranes into anastomosing strands, virtually obliterating the intercellular space.
• Function: Seals adjacent cells to prevent paracellular passage of molecules; maintains distinct apical and basolateral membrane domains (fence function).
• The number of fused sealing strands determines permeability: fewer strands = leakier epithelium (e.g., proximal renal tubule); more strands = tighter epithelium (e.g., urinary bladder).
• Linked intracellularly to actin filaments.
• Clinical relevance: Clostridium perfringens enterotoxin binds claudins to disrupt tight junctions, causing food-poisoning diarrhoea. Defective occludins may compromise the fetal blood–brain barrier.

2. Anchoring Junctions

a) Adherens Junction (Zonula Adherens)

• Located just below the tight junction, also forming a belt around the cell.
• Transmembrane proteins: E-cadherin (Ca²⁺-dependent); intracellularly linked to catenins → actin filaments (part of the "terminal web").
• Function: Firmly anchors adjacent cells; stabilises and strengthens nearby tight junctions.
• Clinical relevance: Loss of E-cadherin in epithelial tumours (carcinomas) promotes invasion and malignant transformation.

b) Desmosome (Macula Adherens)

• Spot-like "spot-weld" junctions — does not encircle the cell (macula = spot).
• Transmembrane proteins: desmogleins and desmocollins (cadherin family). Intracellularly linked via plakoglobin → desmoplakins to intermediate filaments (cytokeratins / tonofilaments).
• Function: Strong attachment points that resist tensile and shear forces; maintain tissue integrity.
• Clinical relevance: Autoimmune destruction of desmoglein in pemphigus vulgaris leads to loss of epidermal cell cohesion and blistering.

c) Hemidesmosome

• Half-desmosome structure anchoring the basal surface of epithelial cells to the basal lamina (not cell-to-cell).
• Transmembrane proteins: integrins; linked intracellularly to intermediate filaments.
• Clinical relevance: Mutations in integrin-β4 cause certain forms of epidermolysis bullosa (skin blistering).

3. Communicating Junctions — Gap Junction (Nexus)

• Circular patches of aggregated transmembrane protein complexes distributed laterally across the cell membrane.
• Each complex is a connexon — a hexameric ring of six connexin proteins forming a central pore (~1.5 nm diameter).
• Paired connexons from adjacent cells dock to form a continuous hydrophilic channel.
• Function: Allow direct transfer of small molecules, ions, and second messengers (< ~1 kDa) between cells, enabling metabolic cooperation and electrical coupling.
• Provide no mechanical adhesive strength.
• Clinical relevance: Mutations in connexin genes are linked to certain types of hereditary deafness and peripheral neuropathy.

Summary Table