Cell junction

Cell junction is the connection between the neighboring cells or the contact between the cell and extracellular matrix. It is also called membrane junction.

Cell junctions are classified into three types:

1. Occluding junctions

2. Communicating junctions

3. Anchoring junctions.

OCCLUDING JUNCTIONS

Cell junctions which prevent intercellular exchange of substances are called occluding junctions, i.e. these junctions prevent the movement of ions and molecules from one cell to another cell. Tight junctions belong to this category.

TIGHT JUNCTION

Tight junction is the intercellular occluding junction that prevents the passage of large molecules. It is also

called zonula occludens. It is the region where the cell membranes of the adjacent cells fuse together firmly. This type of junction is present in the apical margins of epithelial and endothelial cells in intestinal mucosa, wall of renal tubule, capillary wall and choroid plexus.

Structure of Tight Junction

Tight junction is made up of a ridge which has two halves. One half of the ridge is from one cell and another half is from the other cell. Both halves of the ridge fuse with each other very tightly and occupy the space between the two cells. Each half of the ridge consists of tight junction strands.

Proteins of tight junction

Proteins involved in the formation of tight junctions are classified into two types:

1. Tight junction membrane proteins or integral membrane proteins, such as occludin, claudin and junctional adhesion molecules (JAMs)

CELL ADHESION MOLECULES

TYPES

2. Scaffold (framework or platform) proteins or peripheral membrane proteins or cytoplasmic plaque

proteins such as cingulin, symplekin and ZO1, 2, 3. Tight junction membrane protein molecules are

anchored in the strands of the ridge and attach with their counterparts of neighboring cell, so that both the cells are held together. The scaffold (platform) proteins are attached with the tight junction membrane proteins and strengthen the anchoring in the ridges.

Functions of Tight Junction

1. Strength and stability: The tight junction holds the neighboring cells of the tissues firmly and thus

provides strength and stability to the tissues.

2. Selective permeability (gate function): The tight junction forms a selective barrier for small molecules

and a total barrier for large molecules. In the epithelial and endothelial cells, tight junction is the most apical intercellular junction, which functions as selective (semipermeable) diffusion barriers between the neighboring cells. This function is called barrier or gate function. Barrier function of tight junction regulates the interchange of ions, water and varieties of macromolecules between the cells. The magnitude of this function varies in different tissues. In some epithelial cells, few substances pass through the tight junction (by diffusion or active transport). In other cells, no substance passes through the tight junction.

3. Fencing function: Tight junction prevents the lateral movement of proteins (integral membrane proteins)

and lipids in cell membrane and thus acts as a fence. The fencing function maintains the different

composition of proteins and lipids between the apical and basolateral plasma membrane domains.

Because of this function, the tight junction is sometimes referred as impermeable junction.

4. Maintenance of cell polarity: Fencing function of the tight junction maintains the cell polarity by

keep ing the proteins in the apical region of the cell membrane.

5. Blood-brain barrier: Tight junction in the brain capillaries forms the bloodbrain barrier, which prevents

the entrance of many substances from capillary blood into brain tissues. Only lipidsoluble substances like drugs and steroid hormones can pass through the bloodbrain barrier.

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