Page 88 - Atlas of Histology with Functional Correlations

FIGURE 2.4 ■ A junctional complex between epithelial cells. ×31,200.

FUNCTIONAL CORRELATIONS 2.5 ■ Junctional

Complex

Junctional complexes have a variety of functions, depending on their

morphology, shape, and location. In the epithelium that lines the stomach,
intestines, and urinary bladder, the zonulae occludentes or tight junctions are
the most apical junctions that prevent the passage of corrosive chemicals or

waste products between cells and into the bloodstream. The tight junctions
consist of transmembrane proteins called claudin that fuse the outer
membranes of adjacent cells. In this manner, the cells form a tight, beltlike
epithelial barrier. Similarly, the zonula adherens or adhering junctions assist
these cells in resisting separation; the transmembrane proteins attach to the

cytoskeleton proteins and bind adjacent cells. Actin filaments attach to
zonula adherens. Desmosomes are spotlike structures that are most
commonly seen in the epithelium of the skin and in cardiac muscle fibers.

Here, the cells are subjected to great mechanical stresses. In these organs,
desmosomes prevent skin cells from separating and cardiac muscle cells from
pulling apart during the powerful heart contractions. The desmosomes are
bound to intermediate filaments and form strong attachment sites between
adjacent the cells.

Other junctional complexes are hemidesmosomes and gap junctions.
Hemidesmosomes are one half of the desmosome and are present at the base
of epithelial cells where strong adhesion to the connective tissue is required
to prevent tearing of the epithelium from the underlying connective tissue

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