KN

Unit 8 Pt 1

Overview of Cell Interactions

  • Focus on cell adhesion and communication.
  • Important for multicellular organisms composed of eukaryotic cells.
  • Cell interactions influence both the cells and their external environment.

Types of Tissues in Animals

  • Animal tissues divided into two main categories:
    • Connective Tissue: Includes bones, tendons with abundant extracellular matrix (ECM).
    • ECM consists of proteins and polysaccharides; collagen is a key protein.
    • Epithelial Tissue: Consists of tightly bound sheets of cells (e.g., skin, gut lining).
    • ECM is a thin mat beneath polarized epithelial cells called the basal lamina.

Cell Adhesion

  • Understanding cell adhesion is vital for ensuring epithelial tissues operate cohesively.
  • Proteins called transmembrane adhesion proteins span the plasma membrane linking cells internally and externally.
  • Major families of adhesion proteins:
    • Cadherins: Cell-cell attachments.
    • Integrins: Cell-extracellular matrix attachments.

Types of Cell Junctions

  • Adherens Junctions and Desmosomes:
    • Both types use cadherins.
    • Differ in interaction with cytoskeleton:
    • Adherens Junctions: connect with actin filaments.
    • Desmosomes: connect with intermediate filaments.
  • Hemidesmosomes connect integrins to intermediate filaments and ECM.

Cadherins

  • Essential for cell-cell adhesion.
  • Present in multicellular animals, absent in fungi, plants, and prokaryotes.
  • Calcium-dependent adhesion: Their function is influenced by calcium, critical for maintaining strong cell adhesion.
    • Removal or absence of calcium results in cell dissociation.

Cadherin Types

  • E-cadherin: Found in epithelial cells.
  • N-cadherin: Found in nerve cells.
  • P-cadherin: Found in placental cells.
  • Homophilic interactions: Cadherin from one cell binds to same type on an adjacent cell, forming strong junctions.

Mechanism of Interaction

  • Each cadherin has:
    • A rigid domain linked by a hinge region stabilized by calcium ions.
    • Removal of calcium results in flexible hinge leading to dissociation.
  • Junctions resemble Velcro in structure: many weak bonds create strong overall attachment.

Importance of Cadherins

  • Cadherins contribute to tissue segregation by allowing cells to differentiate based on cadherin expression.
  • Experimental evidence shows cells with matching cadherins aggregate, forming tissue.

Epithelial vs. Mesenchymal Cells

  • Epithelial Cells:
    • Form sheets, connected by junctions.
    • Organized, allowing for polarity and barrier functions.
  • Mesenchymal Cells:
    • Dispersed, loosely organized, with the ability to migrate.
  • Transition from epithelial to mesenchymal (and vice versa) is essential for both development and cancer processes.

Adherence Junctions

  • Require various actin-associated proteins for assembly and maintenance.
  • Important for mechanotransduction, allowing cells to respond to mechanical stimuli (e.g., peristalsis in the intestine).

Interaction with Cytoskeleton

  • Cadherins interact with the cytoskeleton via accessory proteins known as catenins:
    • Beta-catenin binds the actin cytoskeleton.
    • Gamma-catenin (plaquoglobin) binds intermediate filaments.

Conclusion

  • Understanding cadherins and their interactions with the cytoskeleton is crucial for comprehending how cells adhere, communicate, and maintain tissue integrity.