Cell junctions
Cell junctions are cellular structures, made up of multiprotein complexes. Cell junctions play a crucial role in maintaining the structural integrity and functioning coordination of tissue in multicellular organisms. In humans cell junction serve as a building block of a biological connectivity, facilitating communication and cooperation between neighbouring cells. Cell junctions also helps in reducing stress on the cell. In this article we will see different types of cell junctions and their functions.
Types of cell junctions
Cell junctions can be classified as follow based on their location.
Between cells
- Tight junctions
- Adherence junctions
- Desmosomes
- Gap junctions
Between cells and matrix
- Hemidesmosomes
- Focal adhesions
Cell junctions can be classified based on their function as follows.
Occluding junctions
- Tight junction
Adhering junctions
- Adherence junctions
- Desmosomes
- Hemidesmosomes
Communicating junctions
- Gap junctions
Tight junctions
Tight junctions consist of web like strands of transmembrane proteins, occludins and cloudins. Tight junctions are also known as zonula occludens. Tight junction is a waterproof region that fuse the outer surface of adjacent plasma membranes together. This prevents the leaking of molecules and liquid from one side of tissue to the other side. For example, epithelial tissue lining in digestive tract and urinary bladder.
Functions of tight junctions
- Tight junctions prevent the passage of water and solutes between the cells.
- Tight junctions also block the entrance of microbes into a host.
- Tight junction constitutes the main barrier to paracellular diffusion.
- Tight junctions provide a selective permeability barrier across epithelial cells. For example, glucose can passively flow between cell membranes, but cannot pass across tight junctions.
- Tight junctions are also important for synchronised movement of cells, which is important for processes like wound healing and foetus development.
Adhering junctions
Adherence junction also known as zonula adherence. It is found in epithelial cells and help to connect to cells together. They are made up of different anchoring proteins in the cell membrane; catenins, cadherins, alpha-actinin, vinculin. This junction provides mechanical strength and stability to tissue. Adhesive junction (adherence junction, desmosomes and hemidesmosomes) are similar in structure and function, but they contain distinct intracellular attachment proteins and transmembrane linker proteins.
Functions of adhering junctions
Structural support: Adherens junctions provide additional structural support to tissues. They hold adjacent cells together, especially in tissues subjected to mechanical stress (e.g., skin, cardiac muscle).
Cardiac Muscle Integrity: In the heart, adherens junctions maintain tight connections between cardiac muscle cells. As the heart expands and contracts during each heartbeat, these junctions prevent cell separation.
Cell Adhesion: Adherens junctions are built primarily from cadherins. Cadherins’ extracellular segments bind to each other, while their intracellular segments bind to catenins. Catenins are connected to actin filaments, linking the junctions to the cell’s internal cytoskeleton.
Embryonic development: During embryogenesis, adherens junctions encourage adhesion between homologous cells.
Desmosomes
Desmosomes are essential intercellular junctions that play a crucial role in connecting adjacent cells within animal tissues. These button-like spots, scattered throughout cell membranes, serve as powerful adhesions, binding neighbouring cells together. Desmosomes are particularly abundant in tissues subjected to mechanical forces. Two key proteins, desmoglein and desmocollin, form the bonds between adjacent cells. These transmembrane proteins belong to the cadherin family. Intermediate filaments, desmoglein, and desmocollin are essential for proper desmosome function.
Function of Desmosomes
Cell Adhesion
- They provide robust adhesion, ensuring that cells remain tightly connected.
- Examples include the epidermis (the outer skin layer) and the myocardium (heart muscle tissue).
Intermediate Filaments Anchoring
- Beneath the cell membrane, desmosomes anchor to intermediate filaments.
- These filaments extend into the cell, providing structural stability.
- Desmosomes rely on intermediate filaments.
Gap junctions
Gap junctions are remarkable structures that physically connect adjacent cells, allowing the rapid exchange of small molecules. Gap junctions act as cellular bridges, facilitating communication and maintaining tissue integrity. These tiny channels play an essential role in nearly every system within the human body, including the nervous system. Altered gap junction function is linked to various diseases. Cardiac arrhythmias, skin disorders, and neurological conditions involve gap junction dysfunction.
Function of Gap Junctions
Cellular Communication
- Gap junctions enable direct communication between neighbouring cells.
- They allow the exchange of ions, second messengers, and small metabolites.
- Gap junction coordinates physiological processes and maintains homeostasis.
Electrical Synapses
- Gap junctions serve as electrical synapses.
- Neurons and cardiac muscle cells use gap junctions for rapid electrical coupling. Synchronized contractions in the heart rely on these connections.
Summary
Cell junctions are essential components of human body, contributing to tissue organization, homeostasis and physiological function. Tight junctions create leakproof barriers in epithelial tissues, regulating ion and molecule passage. Adherens junctions hold cells together, especially in mechanically stressed tissues, linking to the actin cytoskeleton. Desmosomes resemble button like spot, preventing cell separation and relying on intermediate filaments. Gap junctions allow direct communication between neighbouring cells, facilitating ion transport and electrical coupling. Understanding the structure and functions of cell junctions provide information to understand tissue development, disease pathogenesis and potential therapeutic interventions.
Frequently asked questions
What are the 4 different cell junctions?
The four types of cell junctions are tight junction, desmosomes, adhesion junction and gap junction.
What is the definition of the cell junction?
Cell junctions are cellular structures, made up of multiprotein complexes.
What are the gap junctions in cell?
Gap junctions are remarkable structures that physically connect adjacent cells, allowing the rapid exchange of small molecules.
What is the function of tight junctions?
Tight junctions create a continuous intracellular barrier between epithelial cells. This barrier helps to separate tissue spaces and regulate selective movement of solutes across the epithelium.
Which cell has no gap junction?
Skeletal muscle cells and circulating blood cells does not have gap junctions.
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