Lecture Notes on Cytoskeleton

Overview

• Discussion on the structure and function of the cytoskeleton.
• Focus on three primary components: Microfilaments, Intermediate Filaments, and Microtubules.

Microfilaments

Structure

• Smallest of all cytoskeletal elements.
• Another name: Actin.
• Made up of G actin (monomer) and F actin (polymer).
• Forms a double helix structure.

Functions

• Cell migration: Flexibility helps in changing shape to squeeze through spaces (e.g., diapedesis - white blood cells moving through capillaries).
• Cell Division: Forms a constriction ring during cytokinesis to separate daughter cells.
• Cell Extensions: Forms microvilli (small extensions for increasing surface area in the GI tract) and stereocilia (large extensions for balance and hearing in the inner ear).
• Cell Junctions: Main components of tight junctions and adherence junctions, preventing cell separation.
• Muscle Contraction: Interacts with myosin, enabling contraction via sliding filament theory.
• Membrane Transport: Endocytosis (bringing substances into the cell) and Exocytosis (pushing substances out of the cell).

Intermediate Filaments

Structure

• Intermediate in size, tough, and resilient.
• No specific monomer/polymer structure.
• Various types based on tissue:
  • Lamins: Present in the nucleus.
  • Keratin: Found in epithelial cells.
  • Vimentin: Present in fibroblasts and connective tissue.
  • Desmin: Found in muscle cells.
  • Neurofilaments: Present in neurons.

Functions

• Tumor Markers: Can be stained to identify the origin of tumor cells in tissues.
• Shape Maintenance: Resists compressive forces, maintaining cell shape.
• Cell Junctions: Formation of desmosomes (cell to cell) and hemidesmosomes (cell to extracellular matrix).
• Reinforcing Structures: Prevent separation in high-stress tissues like cardiac muscle and epithelial layers.

Microtubules

Structure

• Largest of the cytoskeletal elements.
• Made up of alpha and beta tubulin dimers forming protofilaments.
• Combine to create a hollow tube structure.
• Have polarity: + end towards the cell periphery and - end towards the nucleus.

Functions

• Intracellular Transport: Acts as railroad tracks for motor proteins (Kinesin for anterograde transport, Dynein for retrograde transport).
• Cell Movement: Forms structures like flagella (sperm motility) and cilia (moving substances like mucus in the respiratory tract).
• Cell Division: Forms centrosomes that help in the formation of mitotic spindles for chromosome separation during mitosis.

Special Structures

• Basal Body: Found at the base of cilia and flagella with a 9x3 arrangement of microtubules.
• Axoneme: Core structure of cilia/flagella with a 9x2+2 arrangement.
• Mitotic spindles: Extend from centrosomes to chromosomes during mitosis, aiding in their separation.

Conclusion

• Detailed discussion on the structure and function of each cytoskeletal element.
• Covered roles in cell migration, division, extensions, junctions, muscle contraction, membrane transport, and structural integrity.