Desmosome

    A desmosome, also known as macula adherens or macula adherentes (Latin: adhering spot), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes.

Desmosomes help to resist shearing forces and are found in simple and stratified squamous epithelium. The intercellular space is very wide (about 30nm). Desmosomes are also found in muscle tissue where they bind muscles cells to one another.

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Structure

Desmosomes are molecular complexes of cell adhesion proteins and linking proteins that attach the cell surface adhesion proteins to intracellular keratin cytoskeletal filaments.

The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin family of cell adhesion molecules. They are transmembrane proteins that bridge the space between adjacent epithelial cells by way of homophilic binding of their extracellular domains to other desmosomal cadherins on the adjacent cell. Both have five extracellular domains, and have calcium binding motifs.

The extracellular domain of the desmosome is called the Extracellular Core Domain (ECD) or the Desmoglea, and is bisected by an electron-dense midline where the desmoglein and desmocollin proteins bind to each other. These proteins can bind in a W, S or λ manner.

On the cytoplasmic side of the plasma membrane there are two dense structures called the Outer Dense Plaque (ODP) and the Inner Dense Plaque (IDP), spanned by the Desmoplakin protein. The Outer Dense Plaque is where the cytoplasmic domains of the cadherins attach to desmoplakin via plakoglobin and plakophillin. The Inner Dense Plaque is where desmoplakin attaches to the intermediate filaments of the cell.

Desmosomes function like rivets.

Blistering diseases

If the desmosomes connecting adjacent epithelial cells of the skin are not functioning correctly, layers of the skin can pull apart and allow abnormal movements of fluid within the skin, resulting in blisters and other tissue damage. Blistering diseases such as Pemphigus vulgaris can be due to genetic defects in desmosomal proteins or due to an autoimmune response. These patients are often found to have antibodies that bind to the desmosomal cadherins and disrupt the desmosomes.

Hemidesmosomes

When visualized by electron microscopy, hemidesmosomes are similar in appearance to desmosomes. Rather than linking two cells, hemidesmosomes attach one cell to the extracellular matrix. Rather than using cadherins, hemidesmosomes use integrin cell adhesion proteins. Hemidesmosomes are asymmetrical and are found in epithelial cells connecting the basal face to other cells.

References

• Green KJ and Jones JC. (1996). "Desmosomes and hemidesmosomes: structure and function of molecular components". The FASEB Journal 10 (8): 871-81. Full text online
• Desmosomes Connect Intermediate Filaments from Cell to Cell in Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter (2002) Published by Garland Science. ISBN 0-8153-4072-9