Cell and Molecular Biology: Chapter 7 Notes

Chapter 7: Interactions Between Cells and Their Environment

Overview of Cell Interactions

  • Hierarchy: Cells → Tissues → Organs
  • Animals: Composed of cells and their extracellular matrix (ECM).
  • Extracellular Matrix (ECM):
    • Unique to animal cells
    • Produced by cells and is present outside of them
    • Composed of proteins, glycoproteins, and carbohydrates (e.g., collagen protein).

Definition and Structure of ECM

  • Functions of the ECM:
    • Provides structural support.
    • Determines shape and activity of cells.
    • Influences cellular behavior through anchorage, migration, signaling, and serving as a barrier.

Types of Cells and Tissues

  • Epithelial Tissue:
    • Formed by tightly packed epithelial cells (e.g., epidermis, lining of gut).
    • Cell-Cell and Cell-ECM Adhesion: Critical for maintaining tissue integrity.
  • Connective Tissue:
    • Contains mesenchymal cells with less adhesion.

Types of Cellular Adhesion

  1. Endothelia:
    • Similar to epithelia, they line blood vessels.
  2. Epithelial Cells:
    • Found in various organs such as skin, gut, and kidneys.
  3. Mesenchymal Cells:
    • Less tightly adhered, differentiated from epithelial cells.

Extracellular Matrix Components

  • Key Components:
    • Collagen: Most abundant protein in humans (25%). Designed to resist tensile forces; present in various tissues (e.g., tendons, skin).
    • Proteoglycans: Protein-polysaccharide complexes that contribute to ECM hydration and resistance to compression.
    • Fibronectin: Dimer that binds to ECM components and integrins on cells, facilitating cell migration.
    • Laminin: A glycoprotein crucial for the migration of various cell types, including stem cells.

Proteins and Signaling in ECM

  • Integrins:
    • Principal ECM receptors on cells, functioning as transmembrane linkers between ECM and cytoskeleton.
    • Heterodimers that mediate outside-in and inside-out signaling for cell behavior changes.

Cell Migration and Adhesion Mechanisms

  • Focal Adhesions: Sites of integrin-mediated adhesion that link to the actin cytoskeleton.
  • Hemidesmosomes: Anchoring junctions attaching epithelial cells to the basement membrane via integrins and intermediate filaments (e.g., keratin).
  • Cadherins: Critical for cell-cell adhesion in tissues, requiring calcium ions for function.

Specialized Cell-Cell Interactions

  • Selectins: Glycoproteins that mediate adhesive interactions between circulating leukocytes and blood vessel walls.
  • Immunoglobulin Superfamily (IgSF): Mediators of homotypic cell adhesion and involved in immune responses.

Role of ECM in Cancer and Inflammation

  • Cellular Changes in Cancer:
    • Malignant cells can metastasize by altering their adhesion properties and degrading ECM barriers.
  • Inflammation Responses:
    • Leukocytes traverse endothelial layers via specific adhesion molecules and interactions with ECM components.

Intercellular Junctions

  1. Tight Junctions: Prevent solute movement between cells, maintaining distinct compartments.
  2. Gap Junctions: Allow direct communication between adjacent cells through connexin channels.
  3. Plasmodesmata (in plants): Cytoplasmic channels connecting adjacent plant cells, allowing transport of larger molecules.

Conclusion

  • Understanding cell-ECM interactions is crucial for studying tissue structure, function, and response to injury, as well as implications in diseases such as cancer and inflammatory conditions.

Study Tips

  • Pay attention to the structure and function of different cell types and their interactions with ECM.
  • Focus on how various proteins facilitate cell adhesion and signaling.
  • Review the mechanisms of cell migration in the context of both normal physiology and disease states.