Retta - L4 part 2

Microfilaments and Microtubules

• Actin filaments, known as microfilaments, have a diameter of 7 nm, making them the thinnest cytoskeletal components.
• Microtubules are larger, with a diameter of 25 nm, and are composed of tubulin proteins.
• Intermediate filaments are so named because their diameter is between that of microfilaments and microtubules.
• Microfilaments are made of globular actin subunits that twist to form protofilaments, which further twist to create the filament structure.
• Microtubules are composed of dimers formed by alpha and beta tubulin subunits arranged in a hollow cylindrical structure.
• Both microfilaments and microtubules can dynamically elongate or shorten based on the addition or removal of monomeric subunits.

Cytoskeletal Filaments and Cell Morphology

• Actin filaments contribute to the structural integrity of the cell, supporting processes like the formation of microvilli.
• The distribution and dynamics of actin filaments influence cellular shape and morphology, especially during cell migration.
• Cell attachment to the extracellular matrix is mediated by focal adhesions linked to actin filaments, crucial for tissue stability.
• Actin filaments are essential in muscle contraction, forming the sarcomere structure, which is key in muscular function.

Microtubule Organization and Functions

• Microtubules originate from the Microtubule Organization Centre (MTOC) near the nucleus, extending in star-like patterns throughout the cell.
• They provide a structural framework for the intracellular transport of various cargo, including vesicles and organelles, in both directions using motor proteins.
• Kinesin moves cargo toward the cell periphery, while dyneins move it toward the cell center, reflecting the dynamic nature of microtubule function.

Intermediate Filaments

• Intermediate filaments are composed of various proteins, including lamins (forming the nuclear lamina), keratins (found in epithelial cells), and vimentin (in connective tissues).
• They provide mechanical stability through cytoskeletal integrity, helping to resist cellular stress and maintain tissue structure.
• Diseases such as epidermolysis bullosa arise from mutations in intermediate filament proteins, leading to fragile skin.

Dynamic Behavior During Cell Division

• During cell division, microtubules form the mitotic spindle that separates chromosomes, while actin filaments create a contractile ring for cytokinesis, dividing the cytoplasm of the daughter cells.
• The coordinated action of actin filaments and microtubules enables effective cell division, which includes both mitosis and the cytokinetic phase of division.

Cellular Organization and Migration

• The cytoskeleton plays a significant role in determining cellular organization and polarity, enabling cells to respond to external signals for movement.
• Migratory cells exhibit distinct leading and trailing edges; the leading edge has extensive protrusions for movement, while the rear edge contracts and detaches from the substrate.### Cytoskeleton Overview
• The cytoskeleton consists of different filament types, providing structural support and organization in epithelial cells.
• Key components include actin filaments, microtubules, and intermediate filaments; each performing unique functions.

Cellular Polarity and Attachment

• Actin filaments and microtubules exhibit polarity with distinct plus and minus ends, aiding in directionality of cellular functions.
• Intermediate filaments lack polarity, presenting a uniform structure.
• Epithelial cells utilize the cytoskeleton for anchoring to the extracellular matrix, facilitating adhesion between cells.

Dynamics of Cytoskeleton

• Migratory cells exhibit rapid assembly and disassembly of cytoskeletal components, especially actin, enabling swift responses to environmental changes.
• Actin filaments undergo processes of elongation and shortening, driven by the reassembly of globular actin subunits.

Microtubule Structure and Function

• Microtubules are composed of dimeric subunits of alpha and beta tubulin, forming protofilaments that combine into a hollow, cylindrical structure.
• Microtubules can grow or shrink quickly, responding to cellular needs by adding or removing subunits.

Intermediate Filament Characteristics

• Intermediate filaments are unique, forming from elongated proteins in a staggered manner, resulting in a rope-like structure that provides mechanical strength.
• These filaments associate both longitudinally and laterally for increased stability.

Accessory Proteins and Regulation

• Accessory proteins play a critical role in regulating the dynamics of cytoskeletal components, influencing polymerization and depolymerization.
• Microtubule organizing centers initiate the assembly of microtubules through a nucleation process.

Types of Accessory Proteins

• Nucleating proteins help form initial filaments, encouraging growth from nucleated structures.
• Monomer sequestering proteins prevent premature assembly by binding to monomers.
• Lateral cross-linking proteins stabilize bundles of filaments, enhancing structural integrity.
• Motor proteins, such as myosin, facilitate movement by enabling reciprocal sliding of filaments, contributing to cell contractility.
• Capping proteins regulate filament stability and growth at both ends of the filaments.
• Severing proteins enhance disassembly by cutting filaments into shorter segments, speeding up the process compared to gradual subunit removal.

Significance of Cytoskeletal Dynamics

• The cytoskeleton's adaptability allows cells to sense and respond to environmental signals, including chemotactic cues.
• Changes in cytoskeletal dynamics can result in cellular movements, attachment stability, and division in response to stimuli.