Tight junction

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Structure

Functions

• They hold cells together
• They block the movement of integral membrane protein
  Integral membrane protein
  
  An Integral Membrane Protein is a protein molecule that is permanently attached to the biological membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes Denaturation agents....
  s between the apical and basolateral surfaces of the cell, allowing the specialized functions of each surface (for example receptor-mediated endocytosis
  Endocytosis
  
  Endocytosis is the process by which cell s absorb material from outside the cell by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large Chemical polarity molecules that cannot pass through the hydrophobic plasma membrane or cell membrane....
   at the apical
  Apical
  
  Apical, from the Latin apex meaning to be at the apex or tip, may refer to:*Apical consonant, a consonant produced with the tip of the tongue...
   surface and exocytosis
  Exocytosis
  
  Exocytosis is the durable process by which a cell directs the contents of secretory Vesicle_ out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane....
   at the basolateral surface) to be preserved. This aims to preserve the transcellular transport.
• They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion
  Diffusion
  
  Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
   or active transport
  Active transport
  
  Active transport is the mediated process of moving particles across a biological membrane against a Concentration_gradient#In_biology . If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport....
  ) in order to pass through the tissue. This pathway provides control over what substances are allowed through. (Tight junctions play this role in maintaining the blood-brain barrier
  Blood-brain barrier
  
  The blood-brain barrier is a metabolic or cellular structure in the central nervous system that restricts the passage of various chemical substances and microscopic objects between the bloodstream and the neural tissue itself, while still allowing the passage of substances essential to metabolism function ....
  .) At the present time, it is still unclear whether the control is active or passive and how these pathways are formed. In one study for paracellular transport across the tight junction in kidney proximal tubule, a dual pathway model is proposed: large slit breaks formed by infrequent discontinuities

Classification

• Tight epithelia have tight junctions that prevent most movement between cells. An example of a tight epithelium is the distal convoluted tubule
  Distal convoluted tubule
  
  The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system....
  , part of the nephron
  Nephron
  
  Nephron is the basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine....
   in the kidney
  Kidney
  
  The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine....
  .

• Leaky epithelia do not have these tight junctions, or have less complex tight junctions. For instance, the tight junction in the kidney proximal tubule, a very leaky epithelium, has only two to three junctional strands, and these strands exhibit infrequent large slit breaks.

See also

• Cadherin
  Cadherin
  
  Cadherins are a class of type-1 transmembrane proteins. They play important roles in cell adhesion, ensuring that cells within tissues are bound together....
  
• Zonulin
  Zonulin
  
  Zonulin is a protein that participates in tight junctions between cell of the wall of the digestive tract. Initially discovered in 2000 as the target of zonula occludens toxin, secreted by cholera pathogen Vibrio cholerae, it has been implicated in the pathogenesis of coeliac disease and diabetes mellitus type 1....
  
• Gap junction
  Gap junction
  
  A gap junction or nexus is a specialized intercellular connection between certain animal cell -types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells....
  

External links