Contact-Dependent Cell Communication

Contact-dependent cell communication requires direct physical contact between neighboring cells. In juxtacrine signaling, a membrane-bound ligand on one cell binds to a membrane-bound receptor on an adjacent cell, as in the Notch–Delta signaling pathway.

In animal tissues, gap junctions are intercellular junctions that form direct communication channels between neighboring cells. Gap junctions are specialized protein channels that connect the cytoplasm of neighboring cells.

These junctions allow selective small ions and water-soluble molecules to pass through, but exclude macromolecules, such as large peptides.

Gap junctions are made of transmembrane connexin proteins. Six connexin proteins assemble to form a connexon. When one connexon on a cell membrane encounters another connexon on an adjacent cell, they create a complete continuous channel that can open and close.

Similarly, in plants, the rigid walls of adjacent cells are connected by membrane-lined channels called plasmodesmata.

Each plasmodesma allows molecules to move between cells mainly through the cytoplasmic sleeve, which is the space between the plasma membrane and the desmotubule. The desmotubule is continuous with the endoplasmic reticulum.