Define cell junctions. Describe its types in brief 5 mark answer only Shambhulingam
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Cell Junctions — Definition and Types
Definition
Cell junctions are specialized membrane-associated structures that form points of contact between adjacent cells or between cells and the extracellular matrix (ECM). They provide mechanical strength, regulate the passage of molecules between cells, and allow direct intercellular communication. They are particularly numerous and prominent in epithelial tissues.
Types of Cell Junctions
1. Tight Junction (Zonula Occludens)
The most apical junction; it forms a continuous band completely encircling each cell. Adjacent membranes appear fused in TEM. Key proteins include claudin, occludin, and ZO proteins linked to actin filaments. Functions:
- Seals the intercellular space, forcing molecules to cross via the transcellular (not paracellular) route.
- Acts as a membrane fence, maintaining distinct apical and basolateral membrane domains.
2. Adherens Junction (Zonula Adherens)
Located just below the tight junction; also encircles the cell as a band. Adhesion is mediated by E-cadherin (Ca²⁺-dependent transmembrane glycoproteins) linked intracellularly to catenins and then to actin filaments (terminal web). Function: firmly anchors neighbouring cells together and maintains epithelial structural integrity.
3. Desmosome (Macula Adherens)
Spot-like (not band-like) junctions scattered over lateral cell surfaces. Transmembrane proteins are desmogleins and desmocollins (cadherin family), linked to intermediate filaments (tonofilaments) inside the cell via desmoplakin. Function: provides very strong mechanical attachment; critical in tissues subjected to shear stress (skin, myocardium). Loss of function → pemphigus vulgaris (autoantibodies against desmogleins).
4. Gap Junction (Nexus)
Clusters of connexon channels (each formed by 6 connexin proteins) in adjacent cell membranes that align to create a direct aqueous channel. Allow passage of small molecules (<1,000 Da) — ions, cAMP, metabolites. Function: electrical and metabolic coupling between cells (e.g., cardiac myocytes, smooth muscle). They are spot-like, with no mechanical strength.
5. Hemidesmosome
Structurally resemble half a desmosome. Link epithelial cells to the basal lamina via integrins connected to intermediate filaments. Function: anchors the epithelium to the underlying connective tissue. Mutations in collagen VII (anchoring fibrils) → dystrophic epidermolysis bullosa.
| Junction | Key Proteins | Cytoskeleton | Function |
|---|---|---|---|
| Tight (Zonula Occludens) | Claudin, Occludin | Actin | Seals intercellular space |
| Adherens (Zonula Adherens) | E-cadherin, Catenins | Actin | Strong cell-cell adhesion |
| Desmosome (Macula Adherens) | Desmogleins, Desmocollins | Intermediate filaments | Mechanical strength |
| Gap Junction (Nexus) | Connexins | — | Intercellular communication |
| Hemidesmosome | Integrins | Intermediate filaments | Cell-to-basal lamina anchorage |
— Junqueira's Basic Histology, 17e, pp. 200–208
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