Cytoskeleton, Cell Movement, and Cell Junctions

The Cytoskeleton

• The cytoplasm of a cell is crisscrossed by several types of protein fibers, collectively called the cytoskeleton.

• Discovered using a high-powered electron microscope.

• Functions:

  • Helps maintain cell shape.

  • Anchors organelles.

  • Assists in the movement of organelles or the cell itself.

Microtubules

• Structure:

  • Cylindrical structures.

  • Composed of rows of a protein called tubulin.

  • Assembly is regulated by a microtubule organizing center (MTOC) known as the centrosome.

• Functions:

  • Help maintain the shape of the cell.

  • Act as tracks along which organelles move within the cell.

  • During cell division, microtubules form spindle fibers which are essential for the movement of chromosomes.

Actin Filaments (Microfilaments)

• Structure:

  • Cytoskeletal filaments of eukaryotic cells.

  • Composed of the protein actin.

  • Also referred to as the thin filaments of muscle cells.

  • Long, extremely thin fibers that typically occur in bundles or other groupings.

• Functions:

  • Primarily involved in cell movement.

  • Found in microvilli, which are projections from certain cells that can shorten and extend, indicating their role in dynamic processes.

Intermediate Filaments

• Structure:

  • Rope-like assemblies of fibrous polypeptides.

  • So named because their size is intermediate between actin filaments (thin) and microtubules (thick).

• Functions:

  • Provide support and strength to cells, acting as a structural backbone.

  • Their specific structures and functions can vary depending on the type of cell.

Cilia and Flagella

• Cilia:

  • Short, hair-like projections from the plasma membrane.

  • Involved in movement.

• Flagella:

  • Long extensions of the cytoplasm, containing microtubules.

  • Serve to propel a cell through a fluid medium.

• Overall Functions:

  • Involved in the movement of materials along the plasma membrane.

  • Involved in cellular movement.

• Examples:

  • Respiratory Tract: Ciliated cells lining the respiratory tract sweep debris, trapped within mucus, up the throat to keep the lungs clean.

  • Uterine Tube: Ciliated cells move an egg along the uterine tube, facilitating fertilization by a sperm cell.

• Movement Mechanism:

  • Motor molecules, powered by ATP, allow the microtubules within cilia and flagella to interact, bend, and thereby produce movement.

• Clinical Relevance: Ciliary Dyskinesia:

  • A genetic, recessive disorder illustrating the importance of normal cilia and flagella.

  • Caused by a gene not forming correctly, which is associated with the production of a protein found in the microtubules of cilia and flagella.

  • Result: Cilia and flagella that cannot bend or function properly.

  • Symptoms:

    • Recurrent and severe respiratory infections due to the failure of ciliated cells to clear debris from the lungs.

    • Inability to reproduce naturally: lack of ciliary action to move the egg in females and lack of flagellar action by sperm in males.

Extracellular Matrix (ECM)

• Definition: A meshwork of polysaccharides and proteins that provides support for an animal cell and regulates the movement of materials into the cell.

• A protective meshwork in close association with the cells that produce it.

• Key Structural Proteins:

  • Collagen: Resists stretching, providing tensile strength to the ECM.

  • Elastin: Gives the ECM resilience and elasticity.

• Adhesive Protein:

  • Fibronectin: Binds to integrin, a protein in the plasma membrane.

• Integrins:

  • Integral membrane proteins.

  • Connect to fibronectin externally (in the ECM) and to the actin cytoskeleton internally.

  • Play a crucial role in cell signaling, allowing the ECM to influence the activities of the cytoskeleton and, consequently, the shape and activities of the cell.

• Proteoglycans:

  • Combinations of polysaccharides and proteins.

  • Interact with other polysaccharides in the ECM to resist compression.

  • Influence cell signaling by regulating the passage of molecules through the ECM to the plasma membrane, where receptors are located.

Junctions Between Cells

• Human tissues possess various junctions between their cells, enabling them to function in a coordinated manner.

• Three main types of cell junctions:

  1. Adhesion Junctions (Desmosomes):

     • Mechanism: Mechanically attach adjacent cells by interconnecting the cytoskeletons of two neighboring cells.

     • Example Location: Common between skin cells, where they provide strong adhesion and resist pulling forces.

  2. Tight Junctions:

     • Mechanism: Connections between the plasma membrane proteins of neighboring cells create a zipper-like barrier, sealing off intercellular space.

     • Function: Prevent leakage of fluids and other substances between cells.

     • Example Location: Common in epithelia of the digestive system and the kidney, where it is essential to contain fluids (e.g., digestive juices, urine) within specific areas and prevent their passage into surrounding tissues.

  3. Gap Junctions:

     • Mechanism: Serve as communication portals between cells. Channel proteins of the plasma membrane fuse, creating direct cytoplasmic connections.

     • Function: Allow easy and rapid movement of small molecules and ions between adjacent cells, facilitating intercellular communication and coordination.