The Gap junctions which are also known as a nexus, are the communicating junctions which occur frequently between the epithelial cells. They are also found between many other cells of the body which are
In skeletal muscles, gap junctions are not present.
Each gap junction is a spot like structure where the plasma membrane of the adjoining cells is closely apposed with an intercellular gap of the only 2nm. Gap junctions contain a closely packed array of numerous intercommunicating channels. Each of the intercommunicating channels is composed of two connexons, one belonging to the plasmalemma of each of two cells participating in the formation of the gap junction. Connexons have cylindrical walls which are formed by six transmembrane proteins called connexins which from one plasma membrane projects into the intercellular gap to fuse with connexons from the opposing cell membrane. and form a tubular channel that directly interconnects the two cells.
The lumens of the average intercommunicating channels of gap junctions have an average diameter of 1.5 nm. These intercommunicating channels allow the free passage of ions, cyclic AMP and GMP, amino acids, sugars, and other small molecules from one cell to the other. Different connexons specifically allow the passage of different ions and molecules. This specificity depends on the type of connexin protein forming the connexon wall.
The permeability of the gap junctions is regulated, i.e, the connexons can be opened or closed depending on the intracellular concentration of calcium ions and the cytosolic pH. Normally the cytosolic calcium remains below the extracellular calcium level. At such low calcium concentration, the connexons remain open. whenever the intracellular calcium concentration rises, the connexons become closed. similarly, a decrease in the cytosolic pH also results in the closure of the connexons.
In the cardiac and smooth muscle, the gap junctions function to provide electrical coupling of the adjacent cells, so that the waves of electrical excitation can spread unimpeded and synchronous contraction of the muscles can take place.
Gap junctions are very perfectly seen in the embryonic cells where they play an important role in the exchange of informational molecules between the developing cells.
Types of cell junctions
There are three main types of cell junctions that are
1. Adherens junctions
2. Gap junctions (communicating junctions)
3. Tight junctions
These are also known as anchoring junctions. Cells in tissues and organs must be anchored to each other and to the extracellular matrix. To do this cell developed various junctional complexes in which anchoring proteins extend through the cell membrane to link cytoskeletal proteins of one cell to the other and as well as to the proteins in the extracellular matrix. There are three types of anchoring junctions that are
3. Adherens junctions
These junctions allow direct chemical communication between cell cytoplasm through the process of diffusion without contact with extracellular fluid. This junction becomes possible due to six proteins that form a cylinder with a pore in the center called connexons. The connexions of the cells interact and make a junction. The pore in the connexin very in size and polarity.
Tight junctions are types of junctions that act as a barrier and regulate the movement of water and solutes between the epithelial layers of the vertebrates. Tight junctions do not have directional discrimination but the movement of solute depends on the size and charge.
Up to now, forty proteins are identified that are involved in tight junctions. These proteins are classified into four classes that are
1. Scaffolding proteins
2. Signaling proteins
3. Regulation proteins
4. Transmembrane proteins.
These proteins organize transmembrane proteins and couple it to other cytoplasmic proteins and to actin filaments.
These proteins are responsible for junctions assembly, barrier regulation, and transcription.
These proteins regulate membrane vesicle targeting.
These proteins contain adhesion molecules occludin and claudin. Clad in is responsible for selective permeability.
Gap junctions occurrence and distribution
Gap junction is the place where cell contacts with each other have been observed in various animal organs and tissues. From 1950 to 1970 they were discovered in the
1. Crayfish nerves
2. Rat pancreas
4. Adrenal Cortex
8. Daphnia hepatic caecum
9. Hydra muscle
10. Monkey retina
11. Rabbit Cornea
12. Fish blastoderm
13. Frog embryos
14. Rabbit ovary
15. Rabbit skin
16. Chick embryos
17. The human islet of Langerhans
18. Lamprey and tunicate heart
19. Rat seminiferous tubules
21. Eye lens
Since 1970 these junctions continued to be found in all animal cells. The great discoveries about the gap junctions are made in the 1990s with the help of new technologies such as confocal microscopy. These junctions are found in all organs and tissues with some exceptions such as adult skeletal muscle. But if the gap junctions are present in the skeletal muscles they might propagate the contraction in an arbitrary way among muscle making cells.
Functions of gap junctions
Gap junctions work as a direct cell to cell pathway for electrical current, and small molecules. The control over this communication greatly affect the multicellular organisms.
Organ and tissue development
In the 1980s it was discovered that gap junction communication could be stopped by adding anti connexin antibodies to the embryonic cells. Area of the embryo where the gap junctions are blocked failed to develop normally. The mechanism of blockage of the gap junction is unknown. Studies show that gap junctions are the keys to the development of cell polarity. Some studies show that gap junctions are responsible for the transmission of signals required for drugs to create an effect on the body.
When the cell starts dying messages are transmitted to the neighbouring cells that are connected to the dying cells through gap junctions. These signals cause healthy cells also to die. This phenomenon is important to consider in diseased cells.
Cells death and their matrix are required for a tissue to reach its final configuration. Besides these, gap junctions are also required for the tissue to reach to its final configuration.
In many locations, the lateral surface of adjacent columnar epithelial cells is bound to each other by a series of cell junctions, which are collectively known as a junctional complex.
From the apical to the basal side a junctional complex is seen to consist of three components which are,