cytology--junction 5
Page 1: Overview
Title: 1918 Cytology Cell-Cell Interactions 5 Lecture
Page 2: Importance of Cell-Cell Interactions
Direct interactions between cells and the extracellular matrix are essential for
Development and function of multicellular organisms.
Types of Interactions:
Transient adhesions: Cytoskeletons of adjacent cells are not linked (e.g., immune cells).
Stable adhesion junctions: Involve the cytoskeletons of adjacent cells.
Page 3: Cell-Cell Adhesion
Selective process: Cells from the same tissue specifically adhere to one another.
E.g., liver cells adhere to liver cells, not brain cells.
Mediated by transmembrane proteins called Cell Adhesion Molecules (CAMs).
Groups of CAMs:
Selectins
Integrins
Immunoglobulin superfamily (including ICAMs and N-CAMs)
Cadherins
CAMs require Ca2+ or Mg2+ for cell adhesion.
Page 4: Types of Interactions
Homophilic interactions:
Adhesion molecule on one cell binds to the same molecule on another cell.
Leads to selective adhesion between similar cell types.
Heterophilic interactions:
Adhesion molecule on one cell recognizes a different molecule on another cell.
Page 5: Specialized Cell Junctions
Occur at cell-cell and cell-matrix contact points, especially in epithelia.
Types:
Occluding junctions (Tight junctions)
Communicating junctions (Gap junctions in animals, Plasmodesmata in plants)
Anchoring junctions (Cell-cell adherens and cell-matrix adhesions)
Page 6: Occluding Junctions
Seal cells together in epithelium preventing leakage of small molecules.
Consist of transmembrane adhesion proteins: Claudins and Occludin.
Associated with ZO proteins that anchor strands to actin cytoskeleton.
Proteins occlude the intercellular space through extracellular domains.
Page 7: Communicating Junctions
Mediate passage of signals (chemical/electrical) between cells.
Types:
Gap junctions in animals
Plasmodesmata in plants
Page 8: Gap Junctions
Structure: Uniform narrow gap (2-4 nm) between adjacent cell membranes.
Channels (Connexons): Composed of connexins (six proteins).
Allow movement of inorganic ions and small molecules between cell interiors, coupling them electrically and metabolically.
Most animal cells communicate via gap junctions.
Page 9: Dynamic Nature of Gap Junctions
Channels do not remain continuously open; they can open and close.
Permeability changes with alterations in cytosolic pH or high concentrations of Ca2+.
In case of damage, gap junctions close rapidly to prevent damage spread.
Page 10: Plasmodesmata
Unique to plant cells, directly connect cytoplasms of adjacent cells.
Continuous plasma membrane connecting cells, with a cylindrical channel (20-40 nm diameter).
Cells act like a syncytium with shared cytoplasm.
Page 11: Structure of Plasmodesmata
Contains a desmotubule modeled from the smooth endoplasmic reticulum.
An annulus of cytosol allows passage of small molecules.
Function similar to gap junctions, with regulated transport.
Page 12: Anchoring Junctions
Mechanically attach cells to neighbors or extracellular matrix (ECM).
Common in tissues under mechanical stress (e.g., heart, muscle, epidermis).
Types include:
Desmosomes
Adhesion belt
Hemidesmosomes
Page 13: Adherens Junctions
Found in epithelia, forming a continuous adhesion belt (zonula adherens).
Plasma membranes held together by cadherins, anchoring actin filaments.
Actin linked to plasma membrane through anchor proteins like catenins.
Page 14: Desmosomes
Buttonlike intercellular contact points, anchoring sites for intermediate filaments.
Intermediate filaments linked through anchoring proteins; types depend on cell type (e.g., keratin in epithelial cells).
Page 15: Cell-Matrix Junctions
Some anchoring junctions connect cells to the extracellular matrix.
Transmembrane proteins involved are Integrins, distinct from cadherins.
Types: Focal adhesions and Hemidesmosomes.
Page 16: Hemidesmosomes
Connect basal surface of epithelial cells to basal lamina.
Integrins bind to laminin in the basal lamina; cells anchored through plectin to keratin filaments.
Page 17: Focal Adhesions
Enable cell attachment to ECM through integrins linking to actin filaments.
Extracellular domains bind ECM protein components; intracellular domains bind to actin indirectly using anchor proteins.
Page 18: Role of Anchoring Junctions
Anchoring proteins link cytoskeletal proteins in one cell to those in neighboring cells.
Proteins also link to ECM components, providing structural integrity.
Page 19: Overview of Junction Types
Tight junctions: Epithelial cells
Gap junctions: Various tissues
Plasmodesmata: Plant cells
Adhesion belts: Epithelial tissue
Desmosomes: Epithelial tissue, cardiac muscle
Hemidesmosomes: Epithelial tissue
Focal adhesions: Connective tissue fibroblasts
Page 20: Junction Type Summary
Tight junctions: Epithelial tissue
Gap junctions: Epithelial, smooth muscle, connective tissue
Plasmodesmata: Plant cells
Desmosomes: Epithelial tissue, cardiac muscle
Hemidesmosomes: Epithelial tissue
Focal adhesions: Connective tissue fibroblasts