Epithelium:
Sheets of polarized cells.
Has discrete functional domains (apical and basal).
Connective Tissue:
Loosely organized tissue.
Cells can be attached to each other, a rigid scaffold, or both.
Specialized structures forming long-term associations between cells to create tissues/organs.
Types of Cell-Cell Junctions in Animals:
Adhesive Junctions
Adherens Junctions
Desmosomes
Tight Junctions
Gap Junctions
Plant Cells:
Plasmodesmata
Adherens Junctions:
Connect cells through cadherins (cell adhesion molecules).
Link to actin cytoskeleton via catenins.
Desmosomes:
Form strong adhesive points between cells using cadherins (e.g., desmoglein, desmocollin).
Connect to intermediate filaments, providing mechanical strength.
Form a seal between epithelial cells.
Prevent paracellular movement (movement between cells).
Integrate with actin filaments and are characterized by the presence of transmembrane proteins such as claudins.
Allow direct chemical and electrical communication between adjacent cells.
Composed of connexons (homomers or heteromers of connexins).
Facilitate the passage of ions and small molecules (up to 1.4 nm).
Important for coordinated cellular functions, such as in cardiac and smooth muscle.
Type of Junction | Main Function | Intermembrane Features | Size | Structures Involved |
---|---|---|---|---|
Adhesive Junctions | Cell-cell adhesion | Continuous zones | 20-25 nm | Actin microfilaments |
Desmosomes | Strong cell-cell adhesion | Localized points | 25-35 nm | Intermediate filaments (tonofilaments) |
Tight Junctions | Sealing spaces between cells | No space | None | Transmembrane junctional proteins, actin |
Gap Junctions | Exchange of ions and molecules between cells | Connexons with 3-nm pores | 2-3 nm | Connexins |
Function: Maintain tissue integrity by adhering cells.
Types:
E-cadherin: Found in epithelial cells.
N-cadherin: Found in neurons and cardiac muscle.
Cadherins promote cell sorting during development and tissue formation.
Composed of transmembrane proteins (desmoglein and desmocollin) and adaptor proteins (plakoglobin, plakophilin, desmoplakin).
Essential for tissues under mechanical stress (skin, heart).
Closer link is formed between adjacent cells, preventing leaks.
Composed of proteins claudin and occludin which regulate permeability.
Important for direct signaling and metabolic cooperation between cells.
Allow small molecules, ions, and electrical signals to pass directly through junctions.
Three Main Classes:
Proteoglycans:
Form a gel-like matrix; consist of protein core and glycosaminoglycans (GAGs).
Attract water, provide hydration.
Structural Proteins:
Collagen: Provides tensile strength; forms fibers from multiple polypeptide chains.
Elastin: Provides elasticity; stretches and recoils.
Adhesive Glycoproteins:
Bind cells to ECM; include fibronectin and laminins.
Fibronectin:
Functions as a bridging molecule between cells and the ECM.
Involved in cell migration and tissue development.
Laminins:
Fundamental for the structural integrity of the basal lamina.
Composed of three polypeptide chains and interacts with integrins.
Function: Links the intracellular cytoskeleton to ECM.
Binds to fibrin, fibronectin, and laminins, playing a critical role in cell signaling, migration, and attachment.
Cell Junctions and the Extracellular Matrix
Type of Junction | Main Function | Intermembrane Features | Size | Structures Involved |
---|---|---|---|---|
Adhesive Junctions | Cell-cell adhesion | Continuous zones | 20-25 nm | Actin microfilaments |
Desmosomes | Strong cell-cell adhesion | Localized points | 25-35 nm | Intermediate filaments (tonofilaments) |
Tight Junctions | Sealing spaces between cells | No space | None | Transmembrane junctional proteins, actin |
Gap Junctions | Exchange of ions and molecules between cells | Connexons with 3-nm pores | 2-3 nm | Connexins |