Cytoskeleton, Cell Junctions, and Extracellular Matrix: Structure and Function

Cytoskeleton

Overview of the Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that extends throughout the cytoplasm of eukaryotic cells. It provides structural support, facilitates cell movement, and organizes cellular components.

• Function: Maintains cell shape, enables cellular motion, anchors organelles, and assists in intracellular transport.

• Three Types of Cytoskeleton Fibers: Microfilaments (actin filaments), intermediate filaments, and microtubules.

Types of Cytoskeleton Fibers

• Microfilaments (Actin Filaments):

  • Structure: Thin, flexible fibers composed of actin monomers.

  • Functions: Support cell shape, enable muscle contraction, cell movement (amoeboid motion), and cytokinesis.

  • Example: Formation of the contractile ring during cell division.

• Intermediate Filaments:

  • Structure: Rope-like fibers made of various proteins (e.g., keratin, vimentin).

  • Functions: Provide mechanical strength, stabilize cell structure, and anchor organelles.

  • Example: Nuclear lamina supports the nuclear envelope.

• Microtubules:

  • Structure: Hollow tubes composed of tubulin dimers (α- and β-tubulin).

  • Functions: Maintain cell shape, serve as tracks for organelle movement, form mitotic spindle, and are components of cilia and flagella.

  • Example: Chromosome separation during mitosis.

Centrioles and Basal Bodies

Centrioles are cylindrical structures composed mainly of microtubules, found in animal cells. Basal bodies are structurally similar to centrioles and serve as the organizing centers for cilia and flagella.

• Centrioles: Organize microtubules during cell division, forming the centrosome.

• Basal Bodies: Anchor and organize the microtubules in cilia and flagella.

Ultrastructure and Functions of Cilia/Flagella

Cilia and flagella are motile structures composed of microtubules arranged in a "9+2" pattern. They are involved in cell movement and fluid transport across cell surfaces.

• Structure: "9+2" arrangement: nine doublet microtubules surrounding two central microtubules.

• Function: Cilia move fluid over cell surfaces; flagella propel cells (e.g., sperm).

• Example: Respiratory tract cilia clear mucus; sperm flagella enable motility.

Extracellular Matrix (ECM)

Structure and Role of ECM in Animal Cells

The extracellular matrix (ECM) is a complex network of proteins and carbohydrates outside animal cells. It provides structural support, regulates cell behavior, and facilitates communication between cells.

• Components: Collagen, proteoglycans, fibronectin, and other glycoproteins.

• Functions: Supports tissues, influences cell differentiation, and mediates cell adhesion.

• Example: Collagen fibers provide tensile strength to connective tissues.

Intercellular Junctions in Plants and Animals

Types of Cell Junctions

Cell junctions are specialized structures that connect cells to each other or to the ECM, enabling communication and maintaining tissue integrity.

• Animal Cells:

  • Tight Junctions: Seal cells together, preventing passage of molecules between cells.

  • Desmosomes: Provide mechanical strength by anchoring intermediate filaments.

  • Gap Junctions: Allow direct communication between cells via channels.

• Plant Cells:

  • Plasmodesmata: Channels that traverse cell walls, allowing transport and communication between plant cells.

Summary Table: Cytoskeleton Fibers

Type	Monomer	Structure	Main Functions	Example
Microfilaments	Actin	Thin, flexible strands	Cell shape, movement, muscle contraction	Contractile ring in cytokinesis
Intermediate Filaments	Various (e.g., keratin)	Rope-like fibers	Mechanical strength, organelle anchoring	Nuclear lamina
Microtubules	α- and β-tubulin	Hollow tubes	Cell shape, transport, mitosis, cilia/flagella	Mitotic spindle

Summary Table: Cell Junctions

Junction Type	Location	Function	Example
Tight Junction	Animal cells	Seal cells, prevent leakage	Intestinal epithelium
Desmosome	Animal cells	Mechanical strength	Skin tissue
Gap Junction	Animal cells	Cell communication	Cardiac muscle
Plasmodesmata	Plant cells	Transport and communication	Leaf cells

Key Terms: cytoskeleton, microfilaments, intermediate filaments, microtubules, actin, tubulin, centrioles, basal bodies, cilia, flagella, extracellular matrix (ECM), collagen, proteoglycan, fibronectin, tight junction, desmosome, gap junction, plasmodesmata.