Gap Junction vs Tight Junction
Cell junctions are specialized sites of cell membrane with specific functions and are found in multicellular organisms. There are three types of cell junction; namely, tight junctions, gap junctions, and adhering (anchoring) junctions. These junctions are important to maintain cell-to-cell communication, facilitate molecular transport among cells, make impermeable boundaries in order to prevent diffusion, and hold cells together by binding them tightly etc.
Tight junctions are characterized by fusion of adjacent cell membranes and are only found in epithelial tissue. Tight junctions have several functions including, sealing the intercellular space in epithelial and endothelial cell layers and preventing free paracellular passage of substances. Also, these junctions determine the polarity of epithelial cells by forming a boundary between the apical domain of the plasma membrane and the basolateral domain and preventing diffusion of protein and lipid between the cells. The permeability of tight junctions depends on the molecule’s charge and shape. Also, depending on the location of tight junction, the permeability to ions and water-soluble molecules of low molecular weight varies. The barrier property of tight junction is determined by the number of parallel tight junction strands. The strands are formed mainly by aggregating the claudin and occluding proteins, and associated zonula proteins.
Gap junctions are basically responsible to provide cell to cell communication by transport of ions and small molecules up to approximately 1 kDa. Also, they are allowing chemical and electrical coupling of adjacent cells that are needed for heart and smooth muscle cell action and regular embryogenesis. Gap junction in smooth muscle is called a nexus while, in cardiac muscle, it contributes to make a part of intercalated disc. Gap junctions are formed by integral membrane proteins called connexins. Six connexins assemble to form a structure called connexon. These connexons align with adjacent connexons of neighboring cell membranes to form hydrophilic channels.
What is the difference between Gap Junctions and Tight Junctions?
• Unlike the gap junctions, tight junctions are found only in epithelial cells. Gap junctions are widespread in distribution.
• Tight junctions make barriers and prevent or reduce transport of substances in the extracellular space between cells while gap junctions make passages which allow passing molecules between cells.
• At gap junctions, there is approximately 2nm gap between adjacent cells. At tight junctions, there is no gap between adjacent cells.
• Unlike the gap junctions, tight junctions regulate cell polarity via protein complexes (CRB3 and Par3 complexes).
• Tight junctions are visible under an electron microscope as continuous, anastomosing strands of particles, forming bands or complex network, while gap junctions are visible as aggregated particles organized in spots or large areas.
• Gap junctions consist of connexin proteins while, tight junctions consist of aggregation of claudin and occluding proteins, and associated zonula proteins.