Junctionen

Junctionen are specialized cell–cell and cell–matrix contact structures that organize tissues by mediating adhesion, communication, and barrier function. Found throughout multicellular organisms, they coordinate mechanical stability, signaling, and polarity, enabling tissues to develop and respond to their environment. The term is commonly used in German biology, and in English literature the equivalent term is cell junctions.

Major classes include tight junctions, adherens junctions, desmosomes, gap junctions, and hemidesmosomes. Tight junctions seal the space between epithelial cells and regulate paracellular transport. Adherens junctions and desmosomes provide strong mechanical links through cadherin family proteins, with different cytoplasmic adapters connecting to actin filaments or intermediate filaments. Gap junctions form intercellular channels from connexins that allow ions and small metabolites to pass directly between cells. Hemidesmosomes anchor epithelial cells to the basement membrane via integrins and linker proteins, while focal adhesions connect cells to the extracellular matrix.

Structure commonly involves transmembrane adhesion molecules coupled to cytoplasmic scaffolding proteins that regulate assembly and signaling. Examples include claudins and occludins in tight junctions; E- and N-cadherin in adherens junctions; desmogleins and desmocollins in desmosomes; connexins in gap junctions; and integrins in hemidesmosomes and focal adhesions. Dynamics are regulated by phosphorylation, trafficking, and mechanical cues, allowing junctions to remodel during development, wound healing, and tissue homeostasis.

Disruption of junctions is implicated in diseases such as cancer metastasis, inflammatory disorders, blistering skin diseases, and sensory defects. Studying junctions informs understanding of tissue organization, cancer biology, and regenerative medicine, and junctional components are targets for therapies and tissue engineering approaches.