Cellular level of organization: Cell Junctions.
Introduction:
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., epithelium), the plasma membranes of adjacent cells are pressed together.
Cell junctions primarily act as communicators between cells while also helping in anchoring the cell to the basement membrane.
There are five different types of cell junctions:
Gap Junctions,
Adherens Junctions,
Hemidesmosomes,
Desmosomes, and
Tight Junctions.
Tight Junction.
Tight junctions are areas where the membranes of two adjacent cells join together to form a barrier.
The cell membranes are connected by strands of transmembrane proteins.
The cytoskeletons of the individual cells are linked through the tight junctions.
The junctions are so tight that nothing can pass through them hence substances have to cross the cell itself.
They are often present in the epithelial cells, and on the apical surface of the cell.
Tight junctions bind cells together, prevent molecules from passing in between the cells, and also help to maintain the polarity of cells.
Adherens Junctions
Also called “zonula adherens, intermediate junction, or belt desmosome".
These are protein complexes that occur at cell–cell junctions in epithelial and endothelial tissues.
They are more basal to the tight junctions and completely encircle the cell.
The primary function of the Adherens Junction is to stick to an adjacent cell or surface and provide strong mechanical attachments between adjacent cells.
Desmosomes.
Also known as a macula adherens.
Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin and Heart).
Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.
They are hyper adhesive in nature.
Hemidesmosomes.
Hemidesmosomes are found in epithelial cells connecting the basal epithelial cells to the basal lamina.
Hemidesmosomes are also involved in signaling pathways.
They are similar to the desmosomes but are at the base only allowing cells to anchor to the basement membrane.
Gap Junctions
They are specialized connections between cells which connect cytoplasms of two cells.
They act as a tunnel between two cells which helps transport important cell signals and micronutrients.
They are present in almost all tissues except in some mobile cells like RBCs while stationary mature skeletal muscle cells.
Commonly Asked Questions.
Write in short about different types of the cell junctions.