The cells of epithelial and other tissues are held together by various types of cell junctions, that can be categorized into 3- main groups –
(I) Adhesive junctions
The cells of epithelial tissue and even cardiac muscles are difficult to separate due to the presence of adhesive junctions. Such junctions are of 2-types
(a) Adherens junctions
(a) Adherens junctions (Zonulae adherens) – Such junctions are common in the epithelial living of intestine where they form a ‘belt’ encircling the apical portion of cells and binding them to surrounding cells. Such junctions are formed by calcium -dependant-linkages of cadherin (protein) molecules and cement the cells for providing a pathway for signals from cell exterior to the cytoplasm.
(b) Desmosomes (Maculae adherens) – These are disc-shaped adhesive junctions found in varieties of tissues, like skin, uterine cervix and cardiac muscles, which are subjected to mechanical stress. Desmosomes also contain cadherins for binding the cells.
(II) Tight junctions (Zonulae occludens)
In simple /single-layered epithelium the cells are adhered tightly to one another to form a thin cellular sheet. Such junctions seal the extracellular space and block the diffusion of solutes through the paracellular pathway. The adjoining membrane makes contact at certain points rather than being fused over a large surface area. The tight junctions, serve as a barrier to the free diffusion of water and solutes from the extracellular compartment on one side of an epithelial sheet to that on the other side. The tight junctions, also act as ‘fences’ to block the diffusion of integral proteins. The proteins that form the structural component of tight junctions are Claudin and Occludin. The tight junction also makes the skin impermeable to water. The junction between endothelial cells of blood capillaries also forms the blood-brain barrier which prevents the substances from passing from the blood into the brain. Such blood-brain -barrier also prevents the access of many drugs to CNS.
(III) Gap junctions –
Such junctions are specialized for intercellular communication and are composed of integral membrane protein – Connexin, forming molecular pipelines between adjacent cells. Such gap junction channels can be opened or closed like gated channels. The stimulation of cardiac or smooth muscle cells involves gap junctions. The impulse from SAN of the heart also flows from one cardiac muscle cell to another through gap junctions causing the cells to contract in synchrony. The similar gap junctions are responsible for peristalsis wave in smooth muscles. The gap junctions allow the passage for ATPs / Cyclic AMP / Coenzymes / Sugars / Phosphates and amino acid etc.