📝 STANDARD DESCRIPTION:
4 Major Types of Cell Junction:
1. Tight Junction (Zonula Occludens) — seals apical edge of epithelial cells; prevents paracellular leak.
2. Adherens Junction (Zonula Adherens) — belt around cell; cadherins + actin.
3. Desmosome (Macula Adherens) — 'spot weld'; cadherins + intermediate filaments. Strongest mechanical link.
4. Gap Junction (Nexus) — channels of connexins; allow ion/small molecule passage between adjacent cells (electrical coupling in cardiac muscle, smooth muscle).
5. Hemidesmosome — anchors cell to basement membrane (integrins).
Together they form the 'junctional complex' at the apical pole of epithelial cells.
📚 DETAILED DESCRIPTION:
CELL JUNCTIONS — KEEPING TISSUES TOGETHER
Cell junctions are specialised regions of plasma membrane that anchor cells to each other or to the extracellular matrix, seal sheets of epithelium against leakage, and allow direct communication between adjacent cells.
1. TIGHT JUNCTION (Zonula Occludens) — 'the seal':
• A continuous belt at the most apical part of epithelial cells.
• Made of claudins + occludin proteins linking adjacent membranes directly (no gap).
• Functions: Block paracellular passage of molecules → forms a barrier (blood-brain barrier, gut epithelium, kidney tubules); polarizes the cell (separates apical from basolateral membrane).
• Defective in some diarrhoeal diseases.
2. ADHERENS JUNCTION (Zonula Adherens) — 'the belt':
• Located just below the tight junction.
• E-cadherin in adjacent membranes binds homophilically (like Velcro) in presence of Ca²⁺.
• Cadherins anchor to actin filaments via catenins.
• Provides mechanical strength + signaling.
• Loss of E-cadherin → tumour invasion.
3. DESMOSOME (Macula Adherens) — 'spot welds':
• Disc-shaped patches scattered along lateral membranes.
• Desmoglein + Desmocollin (cadherins) link adjacent cells.
• Anchor to intermediate filaments (keratin in epithelium).
• Especially abundant in tissues subjected to mechanical stress — skin, oesophagus, cardiac muscle (intercalated discs).
• Auto-antibodies against desmoglein → pemphigus foliaceus / pemphigus vulgaris.
4. GAP JUNCTION (Nexus / Communicating Junction) — 'the wire':
• Built of connexin proteins; 6 connexins form a connexon (hemichannel); two connexons (one in each cell) form a complete channel.
• Allow direct passage of ions, second messengers (cAMP, IP₃), and small metabolites (<1 kDa) between adjacent cells.
• Critical in cardiac muscle (intercalated discs) → coordinated contraction; in smooth muscle (gut, uterus) → peristalsis & labour; in osteocytes via canaliculi; in liver (cell metabolic coupling).
5. HEMIDESMOSOME — 'cell to mat':
• Anchors basal cell to underlying basement membrane.
• Uses integrins (α6β4) instead of cadherins.
• Linked to keratin filaments inside the cell.
• Defective in epidermolysis bullosa — skin sloughs at slightest touch.
6. FOCAL ADHESIONS — 'cell to mat' for movement:
• Smaller integrin-based attachments to extracellular matrix.
• Linked to actin filaments — important in migrating cells (fibroblasts, leukocytes, tumour cells).
JUNCTIONAL COMPLEX (Apical to basal):
Tight Junction → Adherens Junction → Desmosome — visible as a 'tri-layered' band on TEM at the apical pole of every absorptive epithelial cell.
// COMPARATIVE ANALYSIS
| CARDIAC MUSCLE |
Intercalated discs are a unique combination — desmosomes (mechanical anchorage) + gap junctions (electrical coupling) — letting heart contract as a syncytium. |
| LIVER |
Hepatocytes connected by gap junctions for metabolic synchronisation across the lobule. |
CLINICAL RELEVANCE
Pemphigus Foliaceus (Dog/Cat): Auto-antibodies attack desmoglein-1 → upper-epidermal cells fall apart → crusts on face/ears/footpads.
Bullous Pemphigoid: Antibodies against hemidesmosomes → epidermis lifts off → blisters.
Cancer Metastasis: Loss of E-cadherin in adherens junctions allows tumour cells to detach and spread.
