Cell Junctions

^ Specialized structures of the plasma membrane that:

• Attach and anchor cells

• Establish apical and basolateral membrane domains by sealing adjacent plasma membranes

• Provide channels for ionic and metabolic coupling

^ Not restricted to epithelial cells; cell junctions occur, however, in large number in epithelial tissues to resist the physical forces acting on the cells.

^ Types

Zonula Occludens Zonula Adherens

Zonula occluders Zonula adherens figure 3.2. Cell junctions and surface specializations.

Zonula occluders Zonula adherens figure 3.2. Cell junctions and surface specializations.

• Tight junction (zonula occludens)

♦ Belt-like, barrier junction around apex of the cell

♦ Provides close apposition of adjacent plasma membranes and occludes the intercellular space

■ Prevents diffusion of material between the intercellular space and the lumen of the organ

■ Establishes apical and basolateral membrane domains in the cell by preventing the lateral migration of proteins in plasma membrane

• Adherent junctions

♦ Attach cells to each other and anchor them to the basal lamina; no fusion of the plasma membrane

♦ Types of adherent junctions

■ Belt desmosome (zonula adherens). Belt-like junction that encircles the apex of the cell like a barrel strap and is located immediately beneath the zonula occludens; serves to attach adjacent cells together; associated with actin filaments.

■ Spot desmosome (macula adherens). Disk-like junctions scattered over the surface of the cell, which are paired with similar structures in adjacent cells; associated with intermediate filaments (e.g., keratin filaments in epithelial cells).

■ Hemidesmosome. Represent a "half desmosome"; these junctions anchor the basal surface of the cell to basal lamina.

• Junctional complex. Consists of the zonula occludens, zonula adherens, and desmosomes; because these structures cannot be resolved as separate structures at the light microscopic level, they appear as a single, bar-shaped, dark region at the apical corners of adjacent cells. The term terminal bar was used by early micro-scopists to define the zonula occludens and zonula adherens at the light microscopic level.

♦ Gap junctions consist of connexons, six transmembrane proteins clustered in a rosette that defines a central pore. Connexons from adjacent cells abut one another, forming a continuity between cells.

♦ Provides metabolic and electrical continuity (coupling) via the pores between cells

Glandular Epithelial Tissues

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