adherens junction

Think of a row of people standing shoulder to shoulder, each one holding the next by a belt loop, and each person's belt also tied to a strong rope running inside their own body so the whole line cannot be pulled apart. Sheets of cells need exactly this kind of strong, shared hold. An adherens junction is a continuous band where neighboring cells grip each other and then anchor that grip to the tough fiber network inside each cell, so the whole sheet behaves as one connected fabric rather than a crowd of loose individuals.

In detail, an adherens junction is built from cadherin proteins. Each cadherin pokes out of the cell membrane and clasps a matching cadherin on the neighboring cell, but only when calcium is present (that calcium dependence is where cadherins get their name). On the inside of the membrane, the tail of each cadherin connects, through helper proteins called catenins, to the cell's actin microfilaments — the thin cables of the cytoskeleton. Because many cadherins line up in a belt around the cell, and because they all tie into actin, the junction links the actin networks of all the cells in a sheet into one continuous, force-bearing web.

Adherens junctions matter wherever a sheet of cells must bend, fold, and pull together as a unit — which is most of early animal development. When a flat sheet of cells folds into a tube (as the early nervous system does), it is the contraction of the actin belts tied through adherens junctions that does the folding. They also let cells feel each other's pulling, so a tissue can coordinate its shape. A common confusion is mixing them up with desmosomes: both stick cells together, but adherens junctions tie into actin filaments, while desmosomes tie into the tougher intermediate filaments.