The following points highlight the two main types of junctions. The types are: 1. Cell Junctions 2. Adherens Junctions.
Type # 1. Cell Junctions:
Commonly, there is a uniform intercellular space or gap of 200-300 A separating closely adjacent cells. Sometimes however, many cells in tissues are linked to each other and to the extracellular matrix at many specialised contact sites called cell junctions that permit or restrict the passage of ions and macromolecules between cells. Cell junctions are very minute structures and are not visible by light microscopy.
Cell junctions fall into three function groups:
1. Occluding or tight junction;
2. Anchoring junction;
3. Communicating junction.
The anchoring and communicating junctions can be again classified into several subtypes. So the classification of all types of cell junctions is shown in details in the chart above.
(i) Occluding or Tight Junctions:
All epithelial cells of the mammalian body have at least one important function in common: they act as selective permeability barriers that inhibit even small molecules or fluid from leaking from one side to other side. This function is successfully well maintained due to presence of many tight junctions also called the zonulae occludens.
Tight junctions are formed through fusion of plasma membranes of two adjacent cells at a series of points of contact without leaving any intercellular space.
When thin sections through a tight junction are seen in the electron microscope, the plasma membranes of the adjacent cells appear to touch each other at intervals and even’ to fuse. Tight junctions alternate with a region where plasma membranes are separated by intercellular space.
In epithelial cells of the small intestine, tight junctions are usually located just below the apical microvillar surface. Tight junctions are composed of a belt-like band of anastomosing sealing fine standards that completely encircles the cell.
But Freeze-fracture electron microscope gives a different view of the tight junction. The tight junction appears to consist, of an interlocking network of ridges on the cytoplasmic face of the plasma membrane of each of the two contacting cells. These ridges are made of trans membrane protein particles 3-4 nm in diameter.
The contact points of tight junction is formed by two rows of these particles of which one row is donated by one cell and other row is provided by the adjacent cell. The protein particles on the two cells are joined very tightly with each other to exclude the intercellular space at the point of contact. As a result, two membranes are fused at point of contact where it creates an impenetrable seal [Fig. 4.20(a) and (b)].
Tight junctions are physiologically very important. The epithelial lining of the small intestine absorbs all nutrients from lumen of gut and does not allow to flow back the same again into the lumen due to presence of the tight junctions.
They release the absorbed nutrients from the other side into the blood via extracellular fluid. Another function attributed to these junctions is their role in maintaining cell polarity by affording a physical barrier to the movement of integral proteins laterally.
(ii) Anchoring Junctions:
Anchoring junction is a type of cell junction by which a group of cells are joined together into strong structural units by connecting elements of their cytoskeleton. This type of junction is situated below the zone of tight junction and widely distributed in different tissues. They are most abundant in tissues that are subjected to severe mechanical stress, such as skin epithelium, the neck of the uterus, cardiac muscle etc.
They are found in two structurally and functionally different forms:
(a) Adherens junctions—are connection site for actin filaments (a type of contractile filament found in muscle cells),
(b) Desmosomes and hemi-desmosomes are connection sites for intermediate filaments.
All of these cell junctions are made of two classes of proteins:
(i) Intracellular attachment proteins that join the junctional complex to specific elements of the cytoskeleton (Fig. 4.21).
(ii) Trans-membrane linker glycoproteins whose intracellular part bind to one or more intracellular attachment proteins and the extracellular part bind with the same of the neighbouring cell or with the extracellular matrix (Fig. 4.22).
(iii) Communicating Junctions:
It is a type of cell junction that mediates the passage of chemical or electrical signals from one interacting cell to its partner.
Communicating junctions are of following types:
(a) Gap Junction:
These are communicating junctions composed of clusters of channel proteins that allow molecules of less than 1,500 Daltons to pass directly from the inside of one cell to the inside of the other. Almost all animal cells that come close to each other are separated by a gap of about 15 nm. But they are connected at several points by means of gap junction.
Electron microscopic and X-ray diffraction observation reveal that both membranes contain cylinders of six dumbbell-shaped connection sub-units which are constructed from trans membrane protein. Two such cylinders join in the gap between the cells to form a channel about 1.5-2.0 nm in diameter that connects the cytoplasm of the two cells [Fig. 4.25(a) and (b)].
Each sub-unit of connection contains a single major protein of about 30,000 Daltons. The protein is made of 280 amino acid residues and crosses the lipid bilayer as four-helix.
Model of a gap junction derived from electron-micrographic analysis shows that one rotation of the six connexion sub-units about a central axis mediates the transition from an open to closed state. This closes the channels that connect the cell with its neighbours and prevents the exchange of small molecules between two cells.
The channels close in presence of Ca2+ ion when its concentration rises markedly in the cytosol. Even slight increases in the level of cytosol Ca2+ ions or decreases in cytosolic pH can decrease the permeability of gap junctions.
Gap junctions allow many small molecules to pass from one cell to other cell. For example, AMP, ADP or ATP, inorganic ions, sugar, amino acids, nucleotides and vitamins can pass through gap junction, but not their macromolecules protein, nucleic acid and polysaccharide etc. Another important compound passed from cell to cell through gap junction is cyclic AMP which acts as an intracellular messenger and regulates a number of metabolic activities.
(b) Chemical Synapse:
Neurological impulses are transmitted from neurons to target cell by the synapse. There are two types of synapse—chemical and electrical which differ in both structure, and function, In chemical synapse, a narrow region, the synaptic cleft separates the plasma membranes of the presynaptic and postsynaptic cells.
The axon terminal of the presynaptical cell is filled with a particular neurotransmitter substance such as epinephrine or acetylcholine. The postsynaptic cells may be a dendrite, the cell body, the axon of another neuron or muscle or gland cell. When the postsynaptic cell is a muscle cell, the synapse is called neuromuscular junction or motor end plate.
When a nerve impulse reaches the axon terminal, some of the synaptic vesicles fuse with the membrane and are exocytosed and discharge its neurotransmitter contents into the cleft. The transmitter diffuses across the cleft and, after a lag period of about 0.5 millisecond binds to receptors on the postsynaptic cells.
The receptors fall into two categories: channel-linked receptors and non-channel linked receptors. The channel linked receptors, upon binding neurotransmitter, promptly change their conformation to create an open channel for specific ions to cross the membrane.
Therefore, they alter the membrane permeability. In case of non-channel receptors, the neurotransmitter-binding site is functionally coupled to an enzyme which catalyzes the production of an intracellular messenger, such as cyclic AMP, in presence of neurotransmitter. The intracellular messenger, in turn, causes changes in the postsynaptic cell, including modifications of the ion channels in its membrane.
Neurons communicating by an electric synapse are connected by gap junctions across which electric impulse can pass directly from the presynaptic cell to the postsynaptic one. Electric synapse allow an action potential to be generated in the postsynaptic cell with greater certainty than chemical synapses and without a lag period.
(c) Plasmodesmata:
Except few types of specialised cells, every living cell in a higher plant is interconnected to its living neighbours by fine cytoplasmic channels—each of which is called plasmodesma—that passes through the intervening cell walls.
Like gap junctions, plasmodesmata provide intercellular channels for molecules of about 1,000 molecular weight, including a variety of metabolic and signalling compounds. Depending on the plant type, the density of plasmodesmata varies from 1 to 10 per /µm2, and even the smallest meristematic cells have more than 1,000 interconnections with their neighbours.
Electron micrographs of plasmodesmata show that it is a roughly cylindrical, membrane-lined channel with a diameter of 20 to 60 nm and it traverses cell walls up to 90 nm thick. Running from cell to cell through the centre of most plasmodesmata is a narrower cylindrical structure which is called desmotubule.
Electron micrographs of desmotubule show that is continuous with elements of endoplasmic reticulum membrane of each of the connected cells (Fig. 4.26).
Between the outside of the desmotubule and the inner face of the cylindrical plasma membrane is an annulus of cytosol. It often appears to be constricted at each end of the plasmodesmata. These constrictions may regulate the flux of molecules through the annuals that joins the two cytosols.
Many evidences indicate that plasmodesmata are, in fact, needed in cell-cell communication. Fluorescent water-soluble dye microinjected into plant cells spread to the cytoplasm of adjacent cells but not into the cell wall. Similarly, if pulses of electrical current are injected, through an electrode into one cell, a measuring electrode in an adjacent cell will detect the same pulses.
Many normal metabolic products such as sucrose, spread from cell to cell. As with gap junctions, movement of molecules through plasmodesmata is reversibly inhibited by an increase in cytosolic Ca2+. Certain plant viruses and viriods can enlarge plasmodesmata in order to use this route to pass from cell to cell.
Type # 2. Adherens Junctions:
Adherens junctions connect bundles of actin filaments from cell to cell or from cell to extracellular matrix.
(i) Cell to Cell Adherens Junctions:
They are generally found at the interface between lateral plasma membranes of adjacent columnar epithelial cells, just below the region of the tight junctions. In the junctional zone, the intercellular space is filled with fine filaments.
They are connected with actin filaments and form a continuous band that girdles the inner surface of the plasma membrane of the connecting cells. This band is known as adhesion belt or zonula adherens and is’ made of a web of 6 nm actin microfilaments.
The actin bundles attach to plasma membranes through a complex of intracellular attachment proteins containing vinculin. It is thought that the contractile actin filament bundles play an important role in animal morphogenesis. They help in rolling up of the epithelial sheet into tube or other related structures (Fig. 4.23).
(ii) Cell to Matrix Adherens Junctions:
The bundle of actin filaments within each cell comes out partly as trans membrane linker through some discrete sites of plasma membrane at the intracellular space and adhere the cell tightly with the extracellular matrix. In the junctional zone the specialised regions of plasma membrane are called focal contacts or adhesion plaques.
(iii) Desmosomes and Hemi Desmosomes:
Desmosomes are button-like points of intercellular contact that rivet cells together. They are connected with intermediate filaments (a type of cytoplasmic filament 8-12 nm in diameter). The particular type of intermediate filaments joined to the desmosome depends on the cell type.
They are keratin filaments in most epithelial cell, desmin filaments in heart muscle cells and vimentin filaments found in some of the cells covering the surface of the brain.
The structure of desmosome is very complex. On the cytoplasmic surface of each interacting plasma membrane there is a dense disc-shaped plaque (0.5 µm. in diameter) composed of mixture of intracellular attachment proteins called desmoplakins.
Each plaque is connected with a thick network of intermediate filaments which pass along the surface of the plaque. Trans membrane linker glycoproteins called desmogleins bind to the plaque and interact through their extracellular part to hold the adjacent membrane [Fig. 4.24 (a) and (b)].
Hemi desmosomes or half-desmosomes are more or less morphologically similar to desmosomes but they are distinct from each other. Instead of joining adjacent cell membranes, hemi desmosome bind the basal surface of the cell to the underlying basal lamina. Both desmosomes and hemi desmosomes act as rivets to distribute tensile or shearing forces through an epithelium and its underlying connective tissue.