Cell Junctions and Cell-to-Cell Adhesions

Cell junctions occur at many points in cell-to-cell contact, but they are particularly plentiful and important in epi­thelial tissue. Four basic types of intercellular junctions are observed: tight junctions, adhering junctions, gap junc­tions, and hemidesmosomes (Fig. 4-27). Often, the cells in epithelial tissue are joined by all four types of junctions.

Tight Junctions. Continuous tight or occluding junctions (i.e., zona occludens), which are found only in epithelial tissue, seal the surface membranes of adjacent cells to­gether. This type of intercellular junction prevents mate­rials such as macromolecules present in the intestinal contents from entering the intercellular space.

Adhering Junctions. Adhering junctions represent a site of strong adhesion between cells. The primary role of adher­ing junctions may be that of preventing cell separation. Adhering junctions are not restricted to epithelial tissue; they provide adherence between adjacent cardiac muscle cells as well. Adhering junctions are found as continuous, beltlike adhesive junctions (i.e., zonula adherens), or scattered, spotlike adhesive junctions called desmosomes (i.e., macula adherens). A special feature of the adhesion belt junction is that it provides a site for anchorage of microfilaments to the cell membrane. In epithelial desmosomes, bundles of keratin-containing intermediate filaments (i.e., tonofilaments) are anchored to the junction on the cytoplasmic area of the cell membrane. A primary disease of desmosomes is pemphigus. This disease is caused by a buildup of antibodies to the proteins of the

Continuous R..--;C"L---'tight junction :. л---:-«ч (zonula occludens) Microfilament bundle (marginal band) Adhesion belt (zonula adherens) Tonofilament bundle Desmosome (macula adherens) Gap junction

Microvilli on luminal border

Hemidesmosome

Basement membrane

RE 4-27 The chief types of intercellular junctions found in epi­thelial tissue. (From Cormack D.H. [1993]. Essential histology. Philadelphia: J.B. Lippincott)

desmosomes. Persons affected have skin and mucous membrane blistering.

Gap Junctions. Gap junctions, or nexus junctions, involve the close adherence of adjoining cell membranes with the formation of channels that link the cytoplasm of the two cells. Gap junctions are not unique to epithelial tissue; they play an essential role in many types of cell-to-cell commu­nication. Because they are low-resistance channels, gap junctions are important in cell-to-cell conduction of elec­trical signals (e.g., between cells in sheets of smooth muscle or between adjacent cardiac muscle cells, where they func­tion as electrical synapses). These multiple communication channels also enable ions and small molecules to pass di­rectly from one cell to another. Gap junctions are associated with a variety of diseases, among which are inner ear deaf­ness, cardiac arrhythmias, and Charcot-Marie-Tooth disease (the most common inherited peripheral neuropathy).

Hemidesmosomes. Hemidesmosomes are another type of junction. They are found at the base of epithelial cells and help attach the epithelial cell to the underlying connective tissue. They resemble half a desmosome, hence their name.