Cell Junctions and the Extracellular Matrix
Cell Junctions and the Extracellular Matrix
Overview of Tissue Types
- Epithelium:
- Sheets of polarized cells.
- Has discrete functional domains (apical and basal).
- Connective Tissue:
- Loosely organized tissue.
- Cells can be attached to each other, a rigid scaffold, or both.
Cell-Cell Junctions
- Specialized structures forming long-term associations between cells to create tissues/organs.
- Types of Cell-Cell Junctions in Animals:
- Adhesive Junctions
- Adherens Junctions
- Desmosomes
- Tight Junctions
- Gap Junctions
- Plant Cells:
Types of Cell-Cell Junctions in Detail
1. Adhesive Junctions
- Adherens Junctions:
- Connect cells through cadherins (cell adhesion molecules).
- Link to actin cytoskeleton via catenins.
- Desmosomes:
- Form strong adhesive points between cells using cadherins (e.g., desmoglein, desmocollin).
- Connect to intermediate filaments, providing mechanical strength.
2. Tight Junctions
- Form a seal between epithelial cells.
- Prevent paracellular movement (movement between cells).
- Integrate with actin filaments and are characterized by the presence of transmembrane proteins such as claudins.
3. Gap Junctions
- Allow direct chemical and electrical communication between adjacent cells.
- Composed of connexons (homomers or heteromers of connexins).
- Facilitate the passage of ions and small molecules (up to 1.4 nm).
- Important for coordinated cellular functions, such as in cardiac and smooth muscle.
Summary of Junction Characteristics
| Type of Junction | Main Function | Intermembrane Features | Size | Structures Involved |
|---|
| Adhesive Junctions | Cell-cell adhesion | Continuous zones | 20-25 nm | Actin microfilaments |
| Desmosomes | Strong cell-cell adhesion | Localized points | 25-35 nm | Intermediate filaments (tonofilaments) |
| Tight Junctions | Sealing spaces between cells | No space | None | Transmembrane junctional proteins, actin |
| Gap Junctions | Exchange of ions and molecules between cells | Connexons with 3-nm pores | 2-3 nm | Connexins |
Cadherins
- Function: Maintain tissue integrity by adhering cells.
- Types:
- E-cadherin: Found in epithelial cells.
- N-cadherin: Found in neurons and cardiac muscle.
- Cadherins promote cell sorting during development and tissue formation.
Desmosomes
- Composed of transmembrane proteins (desmoglein and desmocollin) and adaptor proteins (plakoglobin, plakophilin, desmoplakin).
- Essential for tissues under mechanical stress (skin, heart).
Tight Junctions Features
- Closer link is formed between adjacent cells, preventing leaks.
- Composed of proteins claudin and occludin which regulate permeability.
Gap Junctions Functions
- Important for direct signaling and metabolic cooperation between cells.
- Allow small molecules, ions, and electrical signals to pass directly through junctions.
- Three Main Classes:
- Proteoglycans:
- Form a gel-like matrix; consist of protein core and glycosaminoglycans (GAGs).
- Attract water, provide hydration.
- Structural Proteins:
- Collagen: Provides tensile strength; forms fibers from multiple polypeptide chains.
- Elastin: Provides elasticity; stretches and recoils.
- Adhesive Glycoproteins:
- Bind cells to ECM; include fibronectin and laminins.
Fibronectin and Laminins
- Fibronectin:
- Functions as a bridging molecule between cells and the ECM.
- Involved in cell migration and tissue development.
- Laminins:
- Fundamental for the structural integrity of the basal lamina.
- Composed of three polypeptide chains and interacts with integrins.
Integrins
- Function: Links the intracellular cytoskeleton to ECM.
- Binds to fibrin, fibronectin, and laminins, playing a critical role in cell signaling, migration, and attachment.
