Cell Junctions

Cited from Junctions Between Cells

In many animal tissues (e.g., connective tissue), each cell is separated from the next by an extracellular coating or matrix.However, in some tissues (e.g., epithelia), the plasma membranes of adjacent cells are pressed together. Four kinds of junctions occur in vertebrates:

1. Tight junctions
2. Adherens junctions
3. Gap junctions
4. Desmosomes

In many plant tissues, it turns out that the plasma membrane of each cell is continuous with that of the adjacent cells. The membranes contact each other through openings in the cell wall called

• Plasmodesmata

Tight Junctions

Epithelia are sheets of cells that provide the interface between masses of cells and a cavity or space (a lumen). The portion of the cell exposed to the lumen is called its apical surface. The rest of the cell (i.e., its sides and base) make up the basolateral surface.

Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface.

Tight junctions perform two vital functions:

• They limit the passage of molecules and ions through the space between cells. So most materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides tighter control over what substances are allowed through.
• They block the movement of integral membrane proteins between the apical and basolateral surfaces of the cell. Thus the special functions of each surface, for example
• receptor-mediated endocytosis at the apical surface
• exocytosis at the basolateral surface

can be preserved.

Adherens Junctions

1. Adherens junctions provide strong mechanical attachments between adjacent cells. 
2.  They hold cardiac muscle cells tightly together as the heart expands and contracts. 
3. They hold epithelial cells together. 
4. They seem to be responsible for contact inhibition. 
5. Some adherens junctions are present in narrow bands connecting adjacent cells. 
6. Others are present in discrete patches holding the cells together.

Gap Junctions

Gap junctions are intercellular channels some 1.5–2 nm in diameter. These permit the free passage between the cells of ions and small molecules (up to a molecular weight of about 1000 daltons).

Desmosomes

Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin). Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.

Hemidesmosomes

These are similar to desmosomes but attach epithelial cells to the basal lamina ("basement membrane") instead of to each other.