Cell junctions are specialized multiprotein complexes that form contacts and provide adhesion between neighboring cells, or between a cell and the surrounding extracellular matrix in animal tissues. These structures are particularly abundant in epithelial tissues, where they contribute to the organization and stability of cellular networks. They ensure cells remain connected and also regulate the passage of substances between cells. Cell junctions are fundamental to the existence and proper functioning of multicellular organisms, allowing cells to form cohesive tissues and organs.
Holding Cells Together
Cells within tissues require strong attachments to withstand mechanical forces and maintain their structural integrity. Adherens junctions and desmosomes are two types of cell junctions that provide this mechanical strength by anchoring cells to each other or to the extracellular matrix. These junctions physically link the cytoskeletons of adjacent cells, creating a robust network that resists pulling and stretching.
Adherens junctions are protein complexes that initiate and stabilize cell-cell adhesion, and they also help regulate the actin cytoskeleton. They are composed of transmembrane glycoproteins called cadherins, such as E-cadherin, which connect to intracellular proteins like catenins (p120-catenin, β-catenin, and α-catenin). These catenins then link the cadherin complex to the actin filaments within the cell, forming adhesion belts that encircle the cell and connect to neighboring cells’ actin networks. This continuous network allows forces from cell movement and contraction to be transmitted across tissues, as seen in epithelial layers.
Desmosomes are spot-like junctions that provide strong adhesion between cells in tissues that experience significant mechanical stress, such as the skin and heart muscle. Unlike adherens junctions, desmosomes connect to intermediate filaments, which are thicker and more resilient than actin filaments. Transmembrane proteins called desmoglein and desmocollin, which are types of cadherins, extend from the cell membrane and link to similar proteins on an adjacent cell. Inside the cell, these cadherins bind to a dense plaque of proteins, including plakoglobin, plakophilin, and desmoplakin, which in turn connect to the intermediate filaments, such as keratin filaments in skin cells. This arrangement allows desmosomes to distribute mechanical stress evenly throughout the tissue, preventing individual cells from tearing apart under tension.
Creating Barriers Between Cells
Beyond simply holding cells together, some cell junctions form impermeable seals, creating barriers that regulate the passage of substances between cells. Tight junctions, also known as occluding junctions, serve this barrier function by fusing the plasma membranes of adjacent cells, preventing the free diffusion of molecules through the intercellular space. These junctions are located near the apical surface of epithelial cells, forming a continuous seal around them.
Tight junctions are composed of transmembrane proteins like occludin and claudins, which interact with their counterparts on neighboring cells. These proteins create a series of closely associated strands that effectively “stitch” the cells together, forming a selective barrier. This barrier is particularly important in organs like the intestines, where tight junctions control nutrient absorption while blocking harmful substances from entering the bloodstream. In the bladder, tight junctions prevent urine from leaking, and in the brain, they form the blood-brain barrier, which regulates what enters the nervous tissue from the bloodstream.
Allowing Cell-to-Cell Communication
Another function of cell junctions is to facilitate direct communication between neighboring cells. Gap junctions are specialized channels that allow small molecules and ions to pass directly from the cytoplasm of one cell to another, enabling rapid electrical and chemical signaling. These junctions are widespread in solid tissues and are composed of protein complexes called connexons.
Each connexon is a hexamer of six protein subunits called connexins, forming a cylinder with a central pore. Two connexons, one from each adjacent cell, align and dock across the intercellular space, creating a continuous channel between the cells. The size and selectivity of these pores can vary depending on the specific connexin proteins involved, allowing for regulated passage of substances up to about 1,100 daltons. This direct cytoplasmic connection is especially important in tissues where rapid and synchronized activity is required, such as in heart muscle cells, where gap junctions enable synchronized contractions. In some nerve cells, gap junctions also form electrical synapses, allowing for very fast signal transmission between neurons.
Significance in Tissues and Organs
The collective functions of cell junctions are fundamental to the organization and function of multicellular organisms. By holding cells together, creating selective barriers, and enabling direct communication, these structures ensure the integrity and coordinated activity of tissues and organs. The mechanical strength from adherens junctions and desmosomes allows tissues like skin and muscle to withstand constant physical stress without disintegrating. This ensures the body’s structural stability and protects internal organs.
Tight junctions establish and maintain physiological barriers, which are essential for regulating the internal environment of organs and the body. This selective permeability is crucial for processes such as nutrient absorption in the gut and protecting the brain from circulating toxins. Gap junctions facilitate rapid and synchronized cellular responses by allowing direct exchange of signals and small molecules. This coordinated activity is observable in processes like heart muscle contraction and tissue development during embryonic growth. The combined actions of different cell junction types are indispensable for maintaining tissue homeostasis, enabling physiological processes, and supporting overall health.