Overview
This lecture covers the main types of cell junctions, their structures, functions, and clinical significance, focusing on how cells connect to each other and to the extracellular matrix.
Types of Cell Junctions
- Cell junctions are structures connecting adjacent cells or cells to the extracellular matrix.
- Main types: tight junctions, adherens junctions, desmosomes, gap junctions, and hemidesmosomes.
Tight Junctions (Zonula Occludens)
- Composed of claudins and occludins (transmembrane), zona occludens proteins (cytosolic), and actin filaments.
- Located at the apical surface of epithelial cells.
- Function as a diffusion barrier, blocking ion and molecule passage between cells.
- Found in blood-brain barrier, gastrointestinal tract, respiratory tract; "leaky" tight junctions in kidney proximal tubules.
- Disruption can cause peptic ulcers (H. pylori) and diarrhea (C. difficile).
Adherens Junctions (Zonula Adherens)
- Made of E-cadherins (calcium-dependent), vinculin, catenin (cytosolic), and actin filaments.
- Located just basal to tight junctions.
- Provide resistance to shearing/stretch and keep cells together under stress.
- Found in GI tract, respiratory tract, urinary bladder, blood vessels, and skin.
- Mutations in cadherins can lead to cancer metastasis.
Desmosomes (Macula Adherens)
- Composed of cadherins (desmoglein, desmocollin), desmoplakin (plaque), and keratin intermediate filaments.
- Strong adhesion, especially in cardiac muscle (intercalated discs) and skin (epidermis).
- Autoantibody attack on desmoglein causes pemphigus vulgaris (blistering skin/mucosal disease).
Hemidesmosomes
- Connect cell to basal lamina (extracellular matrix) using integrins and keratin intermediate filaments.
- Anchor epithelial cells to the basement membrane.
- Loss of hemidesmosomes (autoimmune attack against integrins) causes bullous pemphigoid (subepidermal blisters).
Gap Junctions
- Made of connexons (each formed by 6 connexins) bridging adjacent cells.
- Permit direct cell-to-cell communication for ions and small signaling molecules.
- Vital in cardiac tissue, smooth muscle, and some neurons for electrical and chemical signaling.
- Help coordinate responses like apoptosis and adaptive processes.
Key Terms & Definitions
- Tight Junctions — Seal cells together to prevent leakage of molecules.
- Adherens Junctions — Provide mechanical attachment; resist shearing forces.
- Desmosomes — Very strong adhesion points, resisting mechanical stress.
- Hemidesmosomes — Attach cells to the basement membrane via integrins.
- Gap Junctions — Channels for direct communication between cells.
- Claudins/Occludins — Main transmembrane proteins in tight junctions.
- Cadherins — Calcium-dependent adhesion proteins in adherens junctions.
- Desmoglein/Desmocollin — Desmosomal cadherins.
- Integrins — Transmembrane proteins connecting cells to extracellular matrix.
- Connexons/Connexins — Protein subunits forming gap junction channels.
Action Items / Next Steps
- Review diseases linked to cell junction dysfunction (e.g., pemphigus vulgaris, bullous pemphigoid, peptic ulcer disease).
- Diagram each junction and label key proteins for better retention.
- Prepare for exam questions on structure/function/clinical links of each junction type.