צמתי עיגון

Epithelial cells contain three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens junctions. Desmosomes appear as dense, disc-shaped plaques and use cadherins for cell-to-cell adhesion. Hemidesmosomes resemble half-desmosomes and link cells to the basal lamina using integrins. Adherens junctions use either cadherins or integrins depending on whether they connect cells or the extracellular matrix.

Hemidesmosomes contain a dense plaque on the cytosolic side of the plasma membrane formed by the cytolinker protein plectin. Keratin-containing intermediate filaments attach to this plaque via alpha-six-beta-four integrins and proteins such as BP230 and BP180. On the extracellular side, integrins bind laminin, a glycoprotein and major component of the basal lamina, creating a stable connection.

Adherens junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. This distinguishes adherens junctions from desmosomes and hemidesmosomes, which lack this prominent actin organization.

Hemidesmosomes transmit extracellular matrix signals, helping cells respond to mechanical forces and change shape, polarity, and direction of migration. This signaling capability arises from the integrin-mediated connection between intermediate filaments and the basal lamina, allowing cells to sense and adapt to their mechanical environment.

Focal adhesions are anchoring complexes that connect actin filaments to the basal lamina in epithelial cells. They are often formed due to cell interactions with extracellular matrix substrata, which initiate intracellular signaling affects focal adhesions through kinase cascades. Together with hemidesmosomes, focal adhesions provide stability and tissue integrity.

The three types of anchoring junctions—desmosomes, hemidesmosomes, and adherens junctions—collectively influence the shape and folding of epithelial tissue. By providing strong and flexible connections on lateral and basal cell surfaces, these junctions maintain tissue integrity and allow coordinated cellular organization essential for proper epithelial function.

The dense plaque in hemidesmosomes is formed by the cytolinker protein plectin on the cytosolic side of the plasma membrane. Keratin intermediate filaments connect to this plaque via alpha-six-beta-four integrins and adhesion proteins BP230 and BP180, creating a robust structural framework that anchors the cell to the basal lamina.