2520_F24_L32_ECM and cell adhesions

Lecture Overview

  • Title: Lecture 32 – Extracellular Matrix and Cell-Cell AdhesionsCourse: BIOL 2520, Cell BiologyTerm: Fall 2024Lecturer: Sreeparna VappalaSuggested Readings: Chapter 20: pg 717–735

Learning Objectives (Page 2)

  • Review the relationship among cells, tissues, and organs.

  • Describe the distribution of cells in connective tissue.

  • Identify components providing tensile strength in connective tissues.

  • Compare structures: collagen molecules, fibrils, and fibers.

  • Outline synthesis and secretion of collagen and its assembly.

  • Explain integrins’ role in cell attachment to extracellular matrix (ECM).

  • Describe basal lamina composition.

  • Review epithelial sheet polarization in structure and function.

  • Summarize tight junction structure and function.

  • Contrast adherens junctions, desmosomes, and hemidesmosomes.

  • Summarize gap junction structure and function.

  • Review plant cell wall composition and cellulose microfibrils’ role.

  • Differentiate between primary and secondary cell walls.

  • Compare gap junctions and plasmodesmata.

Tissue Organization (Page 3)

  • Tissues: Organized assemblies of cells.

    • Held together by:

      • Cell–cell adhesions

      • Extracellular matrix (ECM)

  • Types of Tissues:

    • Connective

    • Epithelial

    • Nervous

    • Muscular

Extracellular Matrix (ECM) (Page 4)

  • Definition: Material secreted by cells surrounding themselves.

  • In plants: Supportive matrix = cell wall (thick/hard or thin/flexible).

  • In animals: ECM (e.g., bone, tendon) carries mechanical load.

    • Sparse ECM in muscle/epidermis; mechanical load supported by cytoskeleton.

ECM in Animal Connective Tissue (Page 5)

  • Function: Abundant ECM carries mechanical load.

  • Tensile Strength: Provided by fibrous proteins, primarily collagens.

  • Composition varies among tissues (e.g., elastin in arteries).

  • Cell Types:

    • Fibroblasts (skin/tendon)

    • Osteoblasts (bone)

Collagen Structure and Assembly (Page 6)

  • Hierarchy: Polypeptides -> triple-stranded collagen molecules -> fibrils -> fibers.

  • Secretion: Collagen secreted in precursor form (procollagen).

  • Maturation Process:

    • Procollagen cleaved outside the cell.

    • Additional peptide extensions present that hinder fibril assembly.

  • Removal of extensions by procollagen proteinases.

  • Collagen layers are oriented to withstand tensile stress in multiple directions.

Integrins (Page 7)

  • Function: Connect external matrix to the internal cytoskeleton.

  • Interaction: Indirectly with collagen fibers via fibronectin.

Integrin Activation (Page 8)

  • Structure: Composed of α and β subunits.

  • Mechanism:

    • Binding causes conformational change, allowing cell movement.

    • Activated integrin can link to different extracellular molecules.

  • Activates intracellular signaling pathways through kinases.

Epithelial Sheets and Basal Lamina (Page 9)

  • Basal Surface: Positioned on the basal lamina (ECM sheet).

  • Composition of Basal Lamina:

    • Contains type IV collagen and laminin.

Epithelial Cell Junctions (Page 10)

  • Collaborative Learning: Jigsaw activity to comprehend epithelial junction types.

    • Each student becomes an expert on one type.

Types of Epithelial Junctions (Page 14-17)

  • Tight Junctions:

    • Form seals to prevent leakage of molecules.

    • Composed of proteins: claudins and occludins.

    • Maintain cell polarity.

  • Adherens Junctions:

    • Form adhesion belts around epithelial cells (e.g., in the intestine).

    • Actin filament interactions contribute to shape changes.

  • Desmosomes:

    • Link keratin filaments between epithelial cells via cadherins.

  • Hemidesmosomes:

    • Anchor keratin filaments to the basal lamina using integrins.

  • Gap Junctions:

    • Provide direct intercellular communication through connexons.

    • Allow small molecules to pass between cells.

Plant Cell Walls (Page 20-22)

  • External Wall Composition: Allows for cementing plant cells together.

    • Primary Cell Wall: Flexible, allows growth.

    • Secondary Cell Wall: Rigid, forms once growth stops.

    • Cellulose Microfibrils: Provide tensile strength and resist compression.

  • Plasmodesmata:

    • Channels connecting cytoplasm of adjacent plant cells; equivalent to gap junction.