Cell-cell junctions are crucial for tissue integrity and function. They come in various types, each with unique roles in connecting cells and maintaining tissue structure. These junctions are made up of specific proteins that work together to form strong bonds.

Adhesion molecules play a key role in cell interactions, with cadherins and integrins being particularly important. These junctions and molecules are vital for tissue strength, cell communication, and maintaining cellular organization, all essential for proper tissue function.

Cell-Cell Junctions

Types of cell-cell junctions

• Tight junctions create a seal between adjacent cells preventing paracellular transport and maintaining cell polarity (epithelial cells)
• Adherens junctions provide strong mechanical attachments between cells by connecting actin filaments of neighboring cells (epithelial and endothelial cells)
• Gap junctions allow direct communication between cells by permitting passage of small molecules and ions (cardiac muscle cells)
• Desmosomes provide strong mechanical attachments between cells by connecting intermediate filaments of neighboring cells (epithelial and cardiac muscle cells)

Composition of cell-cell junctions

• Tight junctions composed of claudins and occludins forming the seal between cells and ZO proteins linking the junction to the actin cytoskeleton
• Adherens junctions composed of cadherins forming homophilic interactions between cells and catenins linking cadherins to the actin cytoskeleton
• Gap junctions composed of six connexins forming a connexon (hemichannel) with two connexons from adjacent cells forming a complete gap junction channel
• Desmosomes composed of desmogleins and desmocollins forming homophilic interactions between cells and desmoplakins linking the junction to intermediate filaments

Adhesion Molecules and Cell-Cell Interactions

Adhesion molecules in cell interactions

• Cadherins are calcium-dependent adhesion molecules that form homophilic interactions between cells maintaining tissue integrity and morphogenesis (E-cadherin, N-cadherin, P-cadherin)
• Integrins are transmembrane receptors composed of $\alpha$ and $\beta$ subunits that mediate cell-cell and cell-extracellular matrix interactions by binding to specific ligands (fibronectin, laminin)

Importance of cell-cell junctions

• Cell-cell junctions provide mechanical strength to tissues preventing cells from separating under stress and maintaining tissue architecture and organization
• Tight junctions establish apical and basolateral domains maintaining distinct protein and lipid compositions in each domain essential for directional transport and secretion
• Gap junctions allow direct exchange of small molecules and ions facilitating electrical and metabolic coupling between cells important for coordinating cellular activities and tissue homeostasis