Epithelium Tissue: Structure, Functions, and Cell-Cell Adhesion

Epithelium Tissue: Structure, Functions, and Cell-Cell Adhesion

Epithelia are a type of tissue found in multicellular organisms, lining the external surfaces of organs and tissues. They form a continuous sheet of tightly linked cells that act as a barrier between the internal and external environments. The properties of epithelia, such as robustness and barrier function, are achieved through various types of cell-cell junctions that link epithelial cells together into a structural and functional continuum.

Cellular Structure and Cell-Cell Adhesion

Epithelial tissue is composed of a single layer or multiple layers of cells called epithelial cells. These cells are connected to each other via specialized cell-cell junctions, which maintain tissue cohesion and ensure the proper functioning of the tissue. The three main types of cell-cell junctions in epithelia are tight junctions, adherens junctions, and desmosomes.

• Tight junctions are located at the most apical region of the cell and are composed of transmembrane proteins and adaptor proteins that link to the underlying actin cytoskeleton. They form a seal between cells that is so strong that not even small molecules can pass through.
• Adherens junctions occur in various forms and are best known for their continuous adhesion belt (or zonula adherens) just below the tight junctions. They directly attach to the plasma membranes of adjacent cells through cadherins and have contractile actin bundles that help mediate the folding of epithelial cell sheets.
• Desmosomes are also known as maculae adherentes and are visualized as rivets through the plasma membrane of adjacent cells. They tie to intermediate filaments in the cytoplasm and use cadherins for cell-cell adhesion.

Tissue Functions

Epithelial tissue has several important functions that ensure the proper functioning of organs and tissues. These functions include:

• Barrier function: Epithelial tissue forms a barrier between the internal and external environments, preventing the entry of pathogens and maintaining the proper internal environment.
• Protection: Epithelial cells provide a protective layer that protects underlying tissues from external damage.
• Absorption: In some cases, epithelial tissue facilitates the absorption of nutrients and water from the external environment.
• Secretion: Certain epithelial tissues, such as the epidermis and the lining of the digestive tract, secrete substances that protect and lubricate the exposed surfaces.

Cell Junction Types and Functions

Tight Junctions

Tight junctions are responsible for sealing cells together, preventing even small molecules from leaking from one side of the epithelium to the other. They are located at the most apical region of the cell and are composed of transmembrane proteins and adaptor proteins that link to the underlying actin cytoskeleton. Tight junctions play a crucial role in maintaining tissue integrity and preventing the uncontrolled movement of molecules across the epithelium.

Adherens Junctions

Adherens junctions occur in various forms and are best known for their continuous adhesion belt (or zonula adherens) just below the tight junctions. They directly attach to the plasma membranes of adjacent cells through cadherins and have contractile actin bundles that help mediate the folding of epithelial cell sheets. Adherens junctions are essential for maintaining tissue cohesion and ensuring the proper functioning of epithelial tissue.

Desmosomes

Desmosomes are specialized cell-cell junctions that use cadherins for cell-cell adhesion and tie to intermediate filaments in the cytoplasm. They provide mechanical support and help maintain tissue integrity. Desmosomes are particularly abundant in tissues subjected to constant mechanical stress, such as skin and the heart.

In summary, epithelial tissue is a critical component of multicellular organisms, responsible for lining the external surfaces of organs and tissues. The proper functioning of epithelial tissue is achieved through specialized cell-cell junctions, including tight junctions, adherens junctions, and desmosomes, which maintain tissue cohesion and barrier function. Defects in these junctions can lead to a wide range of tissue abnormalities, disrupting homeostasis and being common in genetic abnormalities and cancers.