Tight junctions, a type of epithelial cell, are fusions between plasma membranes (2009 Lecture, 2016).  They are composed of proteins and enhanced with cholesterol (Junctions Between Cells, 2016).  Healthy tight junctions cannot be permeated, preventing substances from floating between cells (Marieb, 2015). Instead of floating casually by the cells, substances must enter the cells using a multi-step process, which allows for greater control over what is allowed to enter the cell (Balda & Matter, 2008). Tight junctions also control movement out of a cell, which helps the cell maintain a state of homeostasis and prevents harmful materials from leaving the cell and entering the bloodstream (Junctions Between Cells, 2016).

You can find tight junctions hard at work all over the body. You’ll find tight junctions lining the brain, intestines, liver, bladder, and many other organs and tissues (2009 Lecture, 2016).  They are there to protect the organs and tissues from harmful materials in the body and prevent harmful materials from leaving these areas and getting into other parts of the body.  For example, in the small intestine, these junctions prevent digestive enzymes from seeping into the bloodstream (Junctions Between Cells, 2016).

One example of what might happen when tight junctions fail is commonly referred to as leaky gut disease.  This happens when tight junctions do not prevent digestive enzymes, bacteria, and other potentially harmful substances from exiting.  Those substances then enter into the bloodstream causing an array of problems.  Luckily, tight junctions are extremely committed cells, often sticking with its barrier duties until well after the cell it protects has been dead for quite a while (Tight Junctions, 2016).

References

2009 Lecture. (n.d.). Retrieved October 5, 2016 from the Cell Biology Wiki:  https://cellbiology.med.unsw.edu.au/cellbiology/index.php/2009_Lecture_8#Tight_Junctions

Balda, M. S. & Matter, K. (2008). Tight junctions at a glance. Journal of Cell Science, 121, 3677-3682.

Junctions Between Cells. (n.d.). In Biology-Pages. Retrieved from http://www.biology-pages.info/J/Junctions.html

Marieb, E. N. (2015). Essentials of Human Anatomy & Physiology. Boston: Pearson.

Tight Junctions (and other cellular connections). (n.d.). Retrieved October 5, 2016 from Epithelial Cells Wiki: http://www.bio.davidson.edu/bernd/Lab/EpithelialInfoWeb/Tight%20Junctions.html