Study Notes on Cell Junctions

Definition: Cell junctions are connections between cells and also between cells and the extracellular matrix.
Learning Outcome: Be able to explain the composition and function of the six types of cellular junctions.

Epithelial Cells:
- Characteristics: Strongly connected, organized into closely adherent sheets, nonmotile, polarized (apical-basal orientation).
- Role: Protects the body (e.g., lining gastrointestinal tract).
Mesenchymal Cells:
- Characteristics: Motile, less connected than epithelial cells, found within the extracellular matrix, not polarized.

Focus: The lecture emphasizes epithelial cells due to their numerous junction types.
Epithelial Examples: Cells lining the gastrointestinal tract protect against bacteria and absorb nutrients.
Six Types of Cellular Junctions:
  1. Tight Junctions
  2. Adherence Junctions
  3. Desmosomes
  4. Gap Junctions
  5. Focal Adhesions
  6. Hemidesmosomes

Function: Prevent movement between cells, acting as a barrier to passage of substances (e.g., bacteria).
Location: Most apical junction in epithelial cells.
Mechanism:
- Outer leaflets of plasma membranes are close together, preventing passage.
- Impedes movement of membrane proteins and things between cells.
Experimental Evidence:
- Infusion of tracer molecules shows no diffusion through tight junctions when applied at the lumen.
- Injections from the basal surface may reach tight junctions but cannot pass upward.

Function: Provide adhesion between adjacent cells, connecting the cytoskeletons (specifically actin microfilaments).
Location: Just below tight junctions.
Composition:
  - Actin microfilaments insert into plaques.
  - Cell transmembrane adhesion proteins are cadherins, binding intercellularly.
  - Proteins in plaques include myosin and tropomyosin, contributing to contraction.

Function: Similar to adherence junctions; hold cells tightly together.
Types:
- Belt Desmosome (extensive coverage)
- Spot Desmosome (localized)
Composition:
- Plaque-like thickening on adjacent cell membranes, attaching to intermediate filaments (e.g., keratin).
- Transmembrane proteins (cadherins) bind extracellularly, linking intermediate filaments intracellularly.

Function: Allow communication between adjacent cells through small channels.
Size: Channels 1.5 to 2 nanometers in diameter, permitting ions and small molecules.
Types of Cells: Present in various cell types including epithelial, smooth muscle, and cardiac muscle cells.
Structure:
- Formed from six connexin proteins assembling to create a connexon.
- Connexons from adjacent cells interconnect, forming open channels.
Importance in Cardiac Muscle: Enable synchronized contraction of heart muscle cells via electrical coupling.
Abnormalities: Mutations can lead to congenital heart defects.

Function: Connect cells to extracellular matrix; similar to adherence junctions.
Mechanism:
- Integrins act as transmembrane proteins binding cytoskeletal actin filaments to extracellular matrix proteins.

Function: Connect the basal surface of epithelial cells to the basal lamina (extracellular matrix).
Description:
- Named as “half a desmosome” connecting to the basal lamina.
- Utilize integrins that bind to laminin in the basal lamina, linking to intermediate filaments intracellularly.

Review of Structural Importance:
- Desmosomes and hemidesmosomes reinforce epithelial structures.
- Keratin intermediate filaments and adhesion proteins maintain integrity and connection to the extracellular matrix.