Anchoring Cell Junctions

The cells that form tissues in multicellular organisms are attached together with anchoring junctions (Fig. 18.15). Integral membrane proteins called cell adhesion molecules, of which cadherin is an example, extend out from each cell and bind tightly together, while their cy-tosolic domains attach to the cytoskeleton. There are two basic types of anchoring junction. In adherens junctions the cell adhesion molecules are linked to actin microfilaments by linking proteins such as catenin. In desmosomes the cell adhesion molecules are linked to intermediate filaments. Tissues that need to be mechanically strong, such as the epithelial cells of the gut (page 14) and cardiac muscle, have many anchoring junctions linking the cytoskeletons of the individual cells. Anchoring junctions are one of the three types of cell junctions, the others being tight junctions (page 55) and gap junctions (page 55).

The epidermis of the skin is made up of a layer of living cells called keratinocytes covered by a protective layer of their dead bodies. Dead keratinocytes form a good protective layer because while alive they generate a dense internal cytoskeleton of the intermediate filament keratin, with adjacent cells being linked by desmosomes. When the cells die, the keratin fibers remain because intermediate filaments are stable. Since the intermediate filaments were joined by desmosomes, the resulting protective fibers do not stop at the edge of the now dead cell, but are strongly connected with the fibers in the next cell, and the next, and so on, forming an extremely strong network of fibers.

Protected by the Dead

Protected by the Dead

Figure 18.15. Anchoring junctions attach the cytoskeletons of adjacent cells.

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