Cell Junctions

What Are Cell Junctions?

Cell junctions are intercellular structures that maintain cells adjacent to each other in tissue, providing the means for communication and adhesion to form a functional unit. They give structure to tissues, allowing cells to attach with other cells and the extracellular matrix, bringing form and function to tissues.

They range from holding tissues together to providing the means whereby intercellular communication can take place, controlling permeability. Cell junctions are under active investigation for correct embryonic development, wound healing, and immune response. In this sense, they bind cells together to offer coordinated function and stability to multicellular organisms.

Types Of Cell Junctions

Cell junctions hold cells together, enabling communication between them. Some major ones include tight junctions, adherens junctions, and gap junctions. In these different types, distinct structures and functions perform an essential function in ensuring integrity and functionality in the tissue.

Tight Junctions

The intercellular space is occluded by the tight junctions, sealing adjacent cells to each other and preventing the intercellular passage of molecules and ions. They consist of claudins, occludins, and JAMs. These junctions regulate the paracellular transport and maintain the barrier function of the epithelial layers.

Adherens Junctions

Adherens junctions are cell adhesion structures based on cadherin, and through catenins, they are associated with actin filaments. The junctions provide mechanical linkage between neighbouring cells, and each cytoskeleton participates in signal transduction, influencing cell behaviour and tissue pattern.

Gap Junctions

Gap junctions are composed of connexins which form connexons, channels across the intercellular space between two cells. Intercellular junctions allow direct communication between cells in that ions and small molecules can diffuse between cells to coordinate activities such as the contraction of the heart muscle and neuronal signalling.

Comparison Of Cell Junctions

Tight junctions are belike with occluding proteins, while adherens junctions are attached or anchored to actin filaments, and gap junctions are composed of channel-forming connexons.

Function

The tight junction acts as a barrier; adherens junctions provide mechanical support and signal transduction; and gap junctions for cell-to-cell communication.

Tissue Location in the Body

The location of these tissues in the body is in the intestinal epithelium and the blood-brain barrier for the tight junction; in cardiac muscle and epithelial cells for the adherens junctions; and cardiac muscle and neuronal cells for the gap junctions.

Tight Junctions

The tight junctions are explained below-

Structure And Composition

These are formed by the association of several transmembrane proteins: claudins, occludins, and JAMs. The association of these proteins forms a continuous kinesin-like belt around the cells and seals the interstice between two adjacent cells.

• Claudins: Found to be essential for the formation of strands of tight junctions and more importantly, modulate their permeability

• Occludins: Implicated in both stabilizing the structure of the tight junction and regulating the barrier function.

• Junctional Adhesion Molecules: Mediate the formation of the first cell-cell contacts and be involved in tight junction stability and function.

Functions

The tight junctions provide the barrier function of the epithelial layers by preventing the passive molecular and ionic flux through the intercellular space. Moreover, they are engaged in the control of paracellular transport, which means the modulation of the traffic of substances taking place in the intercellular space.

• Barrier Function: It prevents the passage of pathogens and toxins.

• Regulation of Paracellular Transport: Allows for the passage of ions and small molecules across cells.

Examples In The Human Body

• Blood-Brain Barrier: The tight junctions of the endothelial cells of the blood vessels in the brain block the passage of most of the substances to the brain, thus protecting it from most of the blood's toxic substances.

• Intestinal Epithelium: The tight junctions of the epithelial lining of the intestine regulate the passage of nutrients and prevent the entry of pathogens.

Adherens Junctions

The adherens junctions are explained below-

Structure And Composition

Adherens junctions are cadherin-based, calcium-dependent adhesion proteins coupled with actin filaments via catenins. This complex provides a strong adhesive belt between cells.

• Cadherins: They are transmembrane proteins involved in the process of cell-cell adhesion.

• Catenins: They couple cadherins to actin filaments and participate in the process of signal transduction.

• Actin Filaments: They provide structural support and participate in processes related to cell movement and stability.

Functions

They also provide mechanical support by linking the cytoskeletons of neighbouring Adherenceren junctions are sites of signal transduction, modulating cell behaviour and tissue pattern.

• Mechanical Support: Tissue integrity is maintained.

• Signal Transduction: Affects cell differentiation, proliferation, and migration.

Examples In The Human Body

• Cardiac muscle: The adherens junctions in cardiac muscle cells have to provide the strong adhesion necessary for the mechanical force of heart contractions.

• Epithelial Cells: Adherens junctions in epithelial cells hold tissue together and provide for signal transduction.

Gap Junctions

The gap junctions are explained below-

Structure And Composition

Gap junctions are made of connexins which assemble as connexons and form channels between adjacent cells. Structurally, these channels provide direct communication between cells.

• Connexins are the transmembrane proteins which make up the building blocks of connexons.

• These intercellular channels or connexons are constructed from six connexin proteins and allow the passage of ions and small molecules from one cell to another.

Functions

Gap junctions allow ions and small molecules to directly pass from one cell into an adjacent cell, enabling the communicating cells to share these materials. This intercellular communication is essential for various integrated activities, as diverse as the contraction of muscles and the transmission of neural signals.

Cell–Cell Communication: Direct transfer of many-second messengers and ions

Ion and Small Molecule Transport: Transfer of ions, metabolites, and other small molecules from one cell to another

Examples In The Human Body

• Cardiac Muscle: Gap junctions in cardiac muscle cells coordinate contractions; that is, the rapid spread of electrical signals is allowed to proceed by gap junctions.

Neuronal Cells: Gap junctions in neurons help to communicate and coordinate the responses speedily.

Recommended video on "Cell Junctions"