Integumentary System. Search for:. Cell Junctions. Learning Objective Describe the characteristics of tight junctions. Key Takeaways Key Points Tight junctions are the closely associated areas of two cells whose membranes join together to form a virtually impermeable barrier to fluid.
Tight junctions perform vital functions—such as holding cells together—and form protective and functional barriers. Tight junctions are composed of a branching network of sealing strands with each strand acting independently from the others. The major types of proteins in junctions are the claudins and the occludins. Each strand is formed from a row of transmembrane proteins embedded in both plasma membranes, with extracellular domains joining one another directly.
Key Terms blood-brain barrier : A structure in the central nervous system CNS that keeps various substances found in the bloodstream out of the brain while allowing in the substances essential to metabolic function, e. Claudins : Proteins that form the backbone of the tight junction strands.
The proteins located on the cell surface bind with other cells or with the extracellular matrix ECM. Tight junction: Diagram of tight junction components. Learning Objective Describe the characteristics of adherens junctions. Key Takeaways Key Points Adherens junctions are involved in a number of critical functions, including providing additional structural support.
For example, they hold cardiac muscle cells tightly together as the heart expands and contracts. Adherens junctions are built primarily from cadherins, whose extracellular segments bind to each other and whose intracellular segments bind to catenins.
Catenins are connected to actin filaments. Key Terms cadherin : Any of a class of transmembrane proteins important in maintaining tissue structure. Learning Objective Describe the characteristics of gap junctions.
Key Takeaways Key Points Gap junctions allow various molecules and ions to pass freely between cells. A gap junction channel is composed of two connexons, also known as hemichannels that line up across the intercellular space.
Most gap junction hemichannels are composed of a complex of six connexin proteins, each characterized by four transmembrane domains. Six connexin sub-units assemble to create one connexon, or hemichannel. Channel composition influences the function of the gap junction. Gap junctions allow for electrical communication between cells, and also allow the passage of small second messengers.
Gap junctions are expressed in virtually all tissues and cells, but most notably in cell types that are involved in direct electrical communication, such as neurons and cardiac muscle. Key Terms cytoplasm : The contents of a cell except for the nucleus. It includes cytosol, organelles, vesicles, and the cytoskeleton. A tight junction is a watertight seal between two adjacent animal cells Figure. Proteins predominantly two proteins called claudins and occludins tightly hold the cells against each other.
This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin.
For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space. Also only in animal cells are desmosomes , which act like spot welds between adjacent epithelial cells Figure.
Cadherins, short proteins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins connect two adjacent cells and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles. Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for transporting ions, nutrients, and other substances that enable cells to communicate Figure.
Structurally, however, gap junctions and plasmodesmata differ. Gap junctions develop when a set of six proteins connexins in the plasma membrane arrange themselves in an elongated donut-like configuration — a connexon.
Gap junctions are particularly important in cardiac muscle. The electrical signal for the muscle to contract passes efficiently through gap junctions, allowing the heart muscle cells to contract in tandem. To conduct a virtual microscopy lab and review the parts of a cell, work through the steps of this interactive assignment. Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions, desmosomes, and gap junctions.
Plant cells are connected and communicate with each other via plasmodesmata. Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent animal cells. However, their structures are quite different. A tight junction is a watertight seal between two adjacent cells, while a desmosome acts like a spot weld.
Diseased animal cells may produce molecules that activate death cascades to kill the cells in a controlled manner. Why would neighboring healthy cells also die? They differ because plant cell walls are rigid. Plasmodesmata, which a plant cell needs for transportation and communication, are able to allow movement of really large molecules. Gap junctions are necessary in animal cells for transportation and communication.
Neither can they form firm adherens junctions. In vivo , a paucity of integrins would inhibit cancer cells from binding and responding to fibronectin. Therefore they also have difficulty attaching firmly to an extracellular matrix, which may explain why many cancers metastasize, or spread from their original site of formation.
These differences in growth in culture between normal and cancer cells are shown below. Cell Junctions Cell junctions serve different functions in cells and tissues. Tight Junctions zonula occludens are typical in sheets of epithelial cells that line the lumens of organs e.
Zonula refers to the fact that these structures form a band encircling an entire cell, attaching it to all surrounding cells. Tight junction membrane proteins TJMPs create the waterproof barrier between cells. Desmosomes adherens junctions essentially glue adhere cells together, giving tissues their strength. Belt desmosomes zonula adherens surround entire cells, strongly binding them to adjacent cells.
In both cases, cadherins cross cell membranes from intracellular plaque proteins, spanning the intercellular space to link adjacent cell membranes together. Plaques are in turn, connected to intermediate filaments keratin of the cytoskeleton, further strengthening intercellular attachments and thus, the tissue cell layer. Gap junctions , the third cell junction, do not so much physically bind cells together as enable chemical communication between cells.
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