Terminal bar (histology)

Terminal bar (histology) refers to a junctional complex at the apical end of the lateral membranes of epithelial cells. This complex is crucial for maintaining the structural integrity and function of epithelial tissues. It consists of three main components: the zonula occludens (tight junction), the zonula adherens, and the macula adherens (desmosome). Each of these components plays a vital role in cell adhesion, permeability control, and communication between cells.

Components

The terminal bar is composed of three distinct junctional complexes:

• Zonula Occludens (Tight Junction): This is the most apical component of the terminal bar. It encircles the cell and acts as a barrier to prevent the passage of molecules and ions through the space between cells. It is crucial for maintaining the polarity of epithelial cells and creating a boundary between the apical and basolateral surfaces of the cell.

• Zonula Adherens: Located just below the tight junction, the zonula adherens provides strong adhesion between neighboring cells. It is formed by cadherin molecules that interact with catenins, which are linked to the actin cytoskeleton. This structure is important for maintaining tissue integrity and facilitating cell signaling.

• Macula Adherens (Desmosome): The desmosome is a spot-like adhesion junction found in various epithelial tissues. It provides strong adhesion by linking intermediate filaments of one cell to those of a neighboring cell. Desmosomes are particularly important in tissues that experience significant mechanical stress, such as the skin and heart.

Function

The terminal bar plays a critical role in maintaining the structure and function of epithelial tissues. By forming a barrier to the passage of substances between cells, it helps to regulate the internal environment of tissues. Additionally, the terminal bar facilitates cell-to-cell communication and signaling, which is essential for tissue development, repair, and function.

Clinical Significance

Alterations in the structure or function of the terminal bar can lead to various diseases. For example, disruptions in tight junctions can result in increased permeability, leading to conditions such as inflammatory bowel disease and certain types of allergies. Similarly, abnormalities in desmosomes are associated with skin disorders and cardiomyopathies.

See Also

• Epithelium
• Cell junction
• Tight junction
• Adherens junction
• Desmosome