There are 4 types of intercellular junctions
Tight Junction
Also known as zonula occludens/occluding junction
- They are the most apical of the cell junctions
- An annular junction (forming a ring)
- It consists of integral membrane proteins (claudins and occludins) that bind to similar proteins in the adjacent cell membrane
- The outer leaflet of the opposing cell membrane are fused to become a single leaflet with a thickness equal to that of one outer leaflet
Functions
- To form a seal that prevents flow of materials between epithelial cells in either direction
- To prevent integral membrane proteins of the apical surface from being transferred to the basolateral surface and vice versa
Location
- Around the apices of epithelial cells e.g. GIT (intestinal epithelium)
- Between endothelial cells e.g. in capillaries, brain, spinal cord
- The blood-brain barrier
Intermediate Junction
Also known as zonula adherens/adherent junction
- This junction also encircles the cell, usually immediately below the zonula occludens
- Adhesion is mediated by transmembrane glycoproteins of each cell, the cadherins
- Cadherins bind the protein catenin which is linked by means of actin-binding proteins to actin filaments
- All these produce an electron-dense plaque of material on the cytoplasmic surfaces of adherent junctions
Function
- They provide for the firm adhesion of one cell to its neighbors
Location
- Apices of epithelial cells
- Between smooth muscle cells
- Intercalated discs of cardiac muscle cells
Desmosome
Also known as macula adherens/attachment plaque
- They are button-like points between adjacent cells
- A plaque-like structure found in any region of the cell surface where the membrane is coated on its inner side with a layer of dense protein
- The intercellular space is about 25nm, filled with thin filaments
Functions
- It forms anchorage points between cells particularly where there is need for strong cohesion
- It prevents mechanical stress
Location
- Epidermal cells of the;
- Skin
- Mouth
- Vagina
- The neck of the uterus
- Oesophagus
Gap Junction
Also known as maculae communicantes/communicating junction/nexus
- Can occur anywhere along the lateral membranes of epithelial cells
- Composed of proteins that form channels that allows small molecules to pass
- These proteins called connexins, form hexameric complexes called connexons
- Each connexons have a central hydrophilic pore of about 1.5nm in diameter
- When two cells attach, connexins in the adjacent cell membranes move laterally and align to form connexons between the two cells
Functions
- They form channels for the diffusion of ions and larger particles from one cell to the next
- They allow for exchange of electrical and chemical signals
Location
- Liver
- Epidermis
- Connective tissues
- Cardiac muscle cells
Hemidesmosomes
- They are found in the contact area between epithelial cells and the subjacent basal lamina
- Resembles a half-desmosome and binds cells to the basal lamina
- While in desmosomes, the attachment plaques contain cadherins, in hemidesmosomes, the plaques contain abundant integrins
- Integrins are transmembrane proteins that are receptor sites for the extracellular macromolecules laminin and collagen type IV
