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:
  • Plasmodesmata

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

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.

Extracellular Matrix (ECM) Components

• Three Main Classes:
  1. Proteoglycans:
  • Form a gel-like matrix; consist of protein core and glycosaminoglycans (GAGs).
  • Attract water, provide hydration.
  1. Structural Proteins:
  • Collagen: Provides tensile strength; forms fibers from multiple polypeptide chains.
  • Elastin: Provides elasticity; stretches and recoils.
  1. 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.