Cells adhere through specialized junction proteins: tight junctions seal tissues and prevent paracellular passage; desmosomes anchor cytoskeletons of adjacent cells for mechanical strength; adherens junctions form contacting belts; hemidesmosomes anchor cells to the extracellular matrix. Each junction type involves specific protein families (claudins, cadherins, integrins) linked to cytoskeletal elements. Loss of junction integrity is a hallmark of cancer metastasis.
Examine electron micrographs of junction structures. Identify protein components and their cytoskeletal links. Explain how tight junctions create selective barriers in epithelial tissues.
All junctions are identical—each has distinct structure and function. Tight junctions are absolutely impermeable—they are selectively permeable. Cell adhesion is a weakness—it provides essential mechanical integrity and tumor suppression.
You know from the fluid mosaic model that the cell membrane is a dynamic lipid bilayer studded with proteins, and from your study of the cytoskeleton that cells have internal structural networks of actin, intermediate filaments, and microtubules. Cell junctions are where these two systems meet: they are specialized protein complexes that physically connect neighboring cells (or anchor cells to the extracellular matrix), linking the membranes and cytoskeletons of adjacent cells into a mechanically and functionally integrated tissue.
Tight junctions (also called zonula occludens) form a continuous seal near the apical surface of epithelial cells. Transmembrane proteins called claudins and occludins from adjacent cells interlock like the teeth of a zipper, creating a barrier that controls what can pass between cells. Think of the epithelial lining of your intestine: tight junctions prevent stomach acid and digestive enzymes from leaking between cells into the bloodstream. However, tight junctions are not absolute seals — different claudin isoforms create junctions of varying "tightness," allowing selective paracellular transport of ions and small molecules. The kidney tubule exploits this by expressing different claudins in different segments to fine-tune ion reabsorption.
Anchoring junctions provide mechanical strength, and there are two main types. Desmosomes (macula adherens) connect the intermediate filament networks of adjacent cells via cadherin family proteins called desmogleins and desmocollins. Picture two cells riveted together at spot-welds, with each rivet anchored deep into the cell's internal cable network — that is a desmosome. They are abundant in tissues under mechanical stress, such as skin and cardiac muscle. Adherens junctions (zonula adherens) form continuous belts around cells using classical cadherins linked to the actin cytoskeleton, coordinating cell shape changes during development and wound healing. Hemidesmosomes anchor the basal surface of epithelial cells to the underlying extracellular matrix via integrins rather than cadherins, connecting to intermediate filaments inside the cell and to laminin in the basement membrane outside.
Gap junctions serve communication rather than adhesion. Six connexin proteins assemble into a channel called a connexon, and connexons from adjacent cells dock to form a continuous pore between the two cytoplasms. These channels allow ions, second messengers (like cAMP and Ca²⁺), and small metabolites (up to ~1 kDa) to pass directly between cells, electrically and metabolically coupling them. This is how cardiac muscle cells synchronize their contractions — an action potential spreads from cell to cell through gap junctions without requiring synaptic neurotransmission. The clinical relevance of junctions is profound: autoimmune diseases like pemphigus target desmosomal cadherins, causing skin blistering, and the loss of E-cadherin function in adherens junctions is one of the hallmarks of epithelial cancers transitioning to invasive, metastatic behavior.