Extracellular Matrix and Cell Junctions Fv 2024

Cell Communities Chapter 20: The Extracellular Matrix and Cell Junctions

The Extracellular Matrix (ECM)

  • Definition: The ECM is a network of materials that exists beyond the immediate vicinity of the plasma membrane of cells.

  • Functions of the ECM:

    • Influences cell division, shape, and differentiation.

    • Provides structural support for tissues.

  • Components of the ECM:

    • In animals: primarily protein-based consisting mainly of collagen.

    • In plants: primarily polysaccharide-based (e.g., cellulose, hemicellulose).

  • Connection to cells: The ECM is connected to the interior of the cell via receptors that initiate intracellular signaling events.

  • Differences between plant and animal ECM:

    • Plants: ECM made mostly of polysaccharides; it can be thick and rigid as in wood or thin and flexible as in leaves.

    • Animals: ECM is composed mostly of proteins like collagen and serves various functions based on the tissue type (connective, epithelial, etc.).

ECM Composition and Function

  • The ECM fills spaces between cells and provides a scaffold for tissue structure.

  • Influence on cell behavior:

    • Binds receptors on cell surfaces, influencing activities such as division and differentiation.

    • Example: Urine collecting ducts organized as an epithelium are embedded in an ECM with other cells.

Polysaccharides in Plant Cell Walls

  • Major components:

    • Pectin: Has an outer layer, predominantly in primary cell wall (CW).

    • Cellulose: Forms the inner layer of the cell wall (secondary CW).

Cellulose Microfibrils and Their Role

  • Structure of Cellulose:

    • Comprised of beta-1,4 linkages between glucose monomers.

    • Cellulose molecules aggregate into microfibrils held together by hydrogen bonding.

  • Function: Resists tension and compression in plant cells; modified by the presence of lignin in woody tissues for additional rigidity.

ECM in Animal Cells

  • General Characteristics:

    • Found in connective tissues; varies in composition based on tissue type.

  • Key features:

    • Dermis: Soft and flexible skin; Bone: Dense tissue due to calcium phosphate; Cartilage: Provides shock absorption.

  • Components:

    • Fibrous proteins (e.g., collagen) and ground substance, consisting of glycoproteins, enzymes, and glycosaminoglycans (GAGs).

Collagen: Structure and Function

  • Composition:

    • Made of three polypeptide chains wound together.

    • Represents about 25% of the protein mass in mammals.

  • Functions:

    • Provides tensile strength (e.g., in tendons) and prevents light scattering (e.g., cornea).

  • Diversity: More than 40 different collagen genes exist, with varying expression across tissues.

Fibronectin and Integrins

  • Fibronectin: Links collagen to cellular receptors.

  • Integrins: Connect ECM to the cytoskeleton; important for cell adhesion and signaling.

    • Exists in two conformations that facilitate strong binding to the ECM and cytoskeleton.

Types of Cell Junctions

  • Epithelial Sheets and Cell Junctions:

    • Include tight junctions, adherens junctions, desmosomes, gap junctions, and hemidesmosomes.

  • Functions of Junctions:

    • Tight junctions: Prevent leakage between cells.

    • Adherens junctions: Connect actin filaments of adjacent cells; stabilize epithelial layers.

    • Desmosomes: Provide mechanical stability by anchoring intermediate filaments.

    • Gap junctions: Allow for the exchange of ions and small molecules between cells.

Cellular Structures

  • Cytoskeletal Elements:

    • Include intermediate filaments (flexible and strong), microtubules (rigid, hollow cylinders), and actin filaments (involved in cell movement and structure).

Specialized Cell Structures and Functions

  • Goblet Cells: Secrete mucus and are asymmetrically organized to facilitate secretion.

  • Types of junctions: Desmosomes and hemidesmosomes link cytoskeletal elements to extracellular matrix, providing structural integrity.

Summary of Junctions:

  • Tight Junctions:

    • Serve as barriers and restrict membrane protein mobility.

    • Sealing proteins claudins and occludins are central to their structure.

  • Adherens Junctions:

    • Draw actin filament binding to cadherins for cell adhesion.

  • Desmosomes:

    • Use cadherins to form strong intercellular connections with intermediate filaments.

  • Gap Junctions:

    • Composed of connexons allowing direct communication between adjacent cells.

  • Plasmodesmata in Plants:

    • Permit passage of molecules through cell walls, differing from gap junctions structurally.

Functionality and Cell Communication

  • Regulation of permeability in gap junctions: Can be modulated by environmental signals (e.g., light intensity impacting retinal cells).

  • Continuity of cell communication remains crucial for tissue function and response to stimuli.