Cell Adhesion and Extracellular Matrix

Cell Adhesion through Junctions

• Types of Cell Junctions:
  • Gap Junctions: Allow communication between cells.
  • Tight Junctions: Form impermeable layers to prevent substance movement between cells.
  • Adherens Junctions: Involve cadherins for cell-to-cell adhesion.
  • Desmosomes: Use cadherins for strong adhesion between cells, connected to intermediate filaments.
  • Hemidesmosomes: Connect cells to the extracellular matrix (ECM) using integrins.

Extracellular Matrix

• Components: Includes collagen, elastin, laminins, and polysaccharides.
  • Collagen: Major protein in ECM, provides structural support, synthesized by fibroblasts.
  • >20 types with varying roles in different tissues (skin, ligaments, tendons, bone).
  • Elastin: Provides elasticity to tissues.
  • Proteoglycans: Form a gel-like network, binding water and providing cushioning.

Cell Sorting Experiments

• Sponges: Demonstrated that cells can sort themselves based on species-specific cell adhesion molecules (CAMs).
• Amphibian embryos: Cells sorted based on tissue types (neural vs. epidermal) indicating the existence of tissue-specific CAMs.
• Cadherins: Cell-to-cell adhesion molecules responsible for sorting and linking cells,
  • Different types exist for species and tissue specificity.
• Integrins: Cell-ECM adhesion molecules that attach cells to the ECM providing structural integrity under stress.

Types of Cellular Junctions and Their Functions

• Anchoring Junctions: Include adherens junctions and desmosomes.
• Barrier Junctions: Tight junctions that prevent leakage between cells, critical in epithelial tissues (e.g., intestines).
• Communicating Junctions: Gap junctions that allow for intercellular signaling and material exchange, especially in muscle tissues (e.g., cardiac muscle).

Importance of Extracellular Matrix

• Basal Lamina: Specialized ECM under epithelial tissues provides support; connects epithelial layers to the underlying connective tissue.
• Integrins within hemidesmosomes anchor epithelial cells to the basal lamina.
  • Critical for maintaining tissue integrity.

Cancer and Cell Adhesion

• Tumor Formation: Results from uncontrolled cell division, where normal regulations fail.
  • Benign Tumors: Stay localized, can cause issues by pressure.
  • Malignant Tumors: Involve loss of cell adhesion leading to metastasis, where cancer cells invade other tissues/organs.
• Metastasis: Cancer cells that detach from the primary tumor, travel through the bloodstream, and establish new tumors.
• Integrins' Role: Many metastatic cells express aberrant integrins, enhancing mobility and invasion capabilities.

Key Points Summary

• Cell-to-Cell Adhesion: Primarily mediated by cadherins.
• Cell-ECM Adhesion: Mediated by integrins; vital for structural integrity and communication.
• Tissue-Specific CAMs: Ensure proper cell sorting and development during embryogenesis and healing processes.
• Junction Functions: Critical in maintaining tissue architecture and regulating intercellular communication.