Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments extending through the cytoplasm of all cells, including eukaryotes and some prokaryotes, providing structural support, shape, and organisation. 

1.0Cytoskeleton Introduction

The cytoskeleton is defined as a complex, filamentous network of proteins that extends throughout the cytoplasm and provides mechanical support, intracellular organization, and motility.
It is composed of microfilame nts, intermediate filaments, and microtubules, each contributing to specific cellular functions.

2.0Components of the Cytoskeleton

1. Microfilaments (Actin Filaments)

• The thinnest cytoskeletal fibers (~7 nm in diameter), composed of two intertwined strands of actin. 
• Formed by globular actin (G-actin) that polymerizes into filamentous actin (F-actin), with distinct plus (+) and minus (−) ends. 

2. Intermediate Filaments

• Intermediate in thickness (~8–12 nm), composed of various fibrous proteins like keratin, vimentin, desmin, neurofilaments, and lamins.
• Provide tensile strength, anchor organelles, and support nuclear structure via nuclear lamina. 

3. Microtubules

• The thickest filaments (~25 nm diameter), hollow tubes formed by polymerization of α‑ and β‑tubulin dimers into protofilaments (commonly 13). 
• Exhibit dynamic instability—rapidly assembling and disassembling. 

3.0Cytoskeleton Structure

The cytoskeleton consists of three main types of filaments:

1. Microfilaments (Actin Filaments)

• Composed of actin protein.
• Diameter: 7 nm.
• Found beneath the plasma membrane, forming a dense network for cell shape and movement.

2. Intermediate Filaments

• Composed of various proteins like keratin, vimentin, desmin, lamins.
• Diameter: 8–12 nm.
• Provide mechanical strength and stability to the cell.

3. Microtubules

• Composed of tubulin dimers (α and β tubulin).
• Diameter: 25 nm.
• Form spindle fibers, cilia, and flagella, and serve as tracks for intracellular transport.

4.0Cytoskeleton Functions

• Cell Shape & Mechanical Support: Maintains cell shape, resists deformation, and supports tissue stability via connections with extracellular matrix. 
• Intracellular Transport & Organization: Acts as tracks for intracellular transport: motor proteins like kinesin and dynein move organelles and vesicles along microtubules and actin filaments.
• Cell Motility & Division: Microfilaments control movements such as cytokinesis, amoeboid movement, and muscle contraction via actin–myosin interactions.
  Microtubules form mitotic spindles essential for chromosome segregation during mitosis and meiosis. 
• Structural Integrity & Organelle Positioning: Intermediate filaments anchor organelles and maintain cellular architecture, while microtubules and microfilaments resist compression and tension, respectively. 
• Cilia, Flagella & Centrosomes: Microtubules form the core of cilia, flagella (9+2 arrangement), and centrioles; dynein arms generate movement in cilia and flagella. 
• Mechanotransduction & Cellular Signaling: The cytoskeleton senses and responds to mechanical forces, influencing cell fate, migration, and signaling via focal adhesions and associated proteins. 

5.0Cytoskeleton Examples

Some common cytoskeleton examples in cells include:

1. Microfilaments (Actin Filaments) – Aid in cell shape, endocytosis, and motility.
2. Intermediate Filaments – Provide mechanical strength; examples include keratin in epithelial cells and lamins in the nucleus.
3. Microtubules – Form mitotic spindle, cilia, and flagella; examples include tubulin-based tracks for vesicle transport.

6.0Cytoskeleton in Cell Division and Motility

• During mitosis and meiosis, microtubules form the spindle apparatus, ensuring accurate chromosome segregation.
• Actin filaments contribute to the contractile ring formation during cytokinesis.
• The cytoskeleton allows cells to crawl, extend pseudopodia, and migrate, which is crucial for tissue repair and immune response.