Mesentery

Mesentery Definition

The mesentery is an organ which surrounds the organs of the gut, and suspends them from the abdominal wall. The mesentery is made of mesoderm cells, the middle of the three embryonic layers. This layer ends up surrounding all internal organs, as the peritoneum. In the gut, this layer folds over on itself and provides points of attachment for the other internal organs. The mesentery used to be known as a variety of different tissues related to the mesocolon. However, recent studies have revealed the mesentery to be a single organ, which suspends the internal organs in the abdominal cavity, and allows the various vessels of the body to reach these organs.

Mesentery Anatomy

The mesentery, for many decades, was often not considered an organ because of its thin and convoluted nature. The mesentery surrounds all of the organs in the abdomen. Because it is formed during embryogenesis, the mesentery ends up getting twisted and turned as the gut develops. Therefore, the mesentery is a complex shape which surrounds the organs. The organ consists of sheet of tissue, which surrounds organs and folds back on itself. This can be seen in the image below. The mesentery is red.

Horizontal disposition of the peritoneum in the lower part of the abdomen

Notice how the mesentery surrounds the small intestine. The mesentery is very thin. Not shown in this picture are the many blood and lymph vessels which traverse the mesentery on their way to the intestines. On a microscopic level, the mesentery is similar to other connective tissues. It is comprised of several layers of cells, derived from mesoderm, attached to a matrix of connective fibers. The extracellular matrix of the cells allows for the creation of a very strong cell and fiber network, which can heal itself if damaged. Within this structure, blood and lymph vessels can carry their respective fluids to the intestines.

Function of Mesentery

Recent studies have shown that within these folds of tissue are complex arrays of lymph vessels, blood vessels, and immune cells. This suggests that the mesentery functions as a complex organ which carries nutrients away from the intestines while at the same time protecting from infection. The intestines are busy digesting food. As it travels through the intestine, nutrients are released and get absorbed by the cells of the intestine. These nutrients are transported to the blood, where they can be distributed to the body. The mesentery provides a stable and secure route for these vessels to pass. Without the mesentery, the fragile vessels would be subject to the pulling and stretching the body goes through.

The mesentery also allows lymph vessels to reach the intestines. This is important because nutrients are not the only thing that makes it through the intestines. Often, bacteria and viruses manage to squeeze their way through the intestines. The second line of defense is the immune system. White blood cells can defend the body and create antibodies to target the invaders. However, they must be able to reach the invaders. The mesentery gives the immune system access to this area of the body, even though it is contained within a cavity.

Besides these functions of directing and protecting vessels, the mesentery has an important role in development and support of the gut. During development, the expansions and contractions of the mesentery direct the shape of the gut. The colon, for instance, is pulled into place against the abdominal wall when the mesentery connecting the two shrinks in size. In fact, without the mesentery your organs would fall into a puddle in the bottom of your gut. The complex folds and attachment points hold your organs in place. They will even work when you are hanging upside-down or doing a backflip!

Still further research into the mesentery must be done. It has also been found to be a holder or excess fat, and is riddled with nerves stretching to the intestines and other organs. The mesentery is likely to have other, undiscovered functions which will be uncovered with time. Among these are likely to be immune defenses for the body and roles in development.

Mesentery and Disease

Along those lines, research has begun to show the prominent role the mesentery plays in several diseases. Because it is a line of defense from the digestive system, it is not surprising to find out that it is heavily involved in the defense against food-borne illnesses. More interesting however, is its function in the spread of other diseases, such as cancer.

Some studies coming out in recent years have shown that the mesentery may by an important pathway for metastasizing cancer cells to travel. These cells often travel through the lymph or blood vessels. Because the mesentery is lined so prolifically with them, it can become a central highway for distributing the cells. This may feature prominently in future cancer treatments.

Other diseases, such as Crohn’s disease, are caused by a malfunctioning mesentery. People with Crohn’s disease often have a swollen or hardened mesentery. This makes it difficult for the vessels contained in the mesentery to function properly. People with this condition often have a hard time digesting food, and can have compromised immune systems.

Quiz

1. Which of the following is NOT part of the mesentery?
A. The peritoneum around the heart
B. The peritoneum around the appendix
C. The peritoneum around the colon

Answer to Question #1
A is correct. The peritoneum around the heart is called the pericardium, and is specialized for that function. The heart is in the thoracic cavity. The mesentery only surrounds organs in the abdominal cavity.

2. Which of the following is NOT a function of the mesentery?
A. Digestion
B. Support
C. Access for vessels

Answer to Question #2
A is correct. The mesentery has no direct function in digestion. It simply hangs the organs in the right configuration and allows vessels to reach the intestines. Digestion is completed by the time the nutrients leave the intestines and enter the bloodstream.

3. A lancelet is a very small fish-like creature. It is one of the smallest organism with a notochord. The lancelet does not have a mesentery. Why could this be?
A. The mesentery is only in complex organisms
B. The mesentery is only needed to distribute nutrients
C. No structural support or access to the intestines are needed in lancelets

Answer to Question #3
C is correct. A lancelet is so small that most things can diffuse right out of the intestine and into the body. Further, the lancelet lives in water, and moves only short distances. The notochord is a rigid structure, much like the backbone, which gives the organism all the support it needs. Its gut is also very linear, negating a need for a mesentery to fold it about.

References

  • De luliis, G., & Pulera, D. (2007). The Dissection of Vertebrates. Amsterdam: Academic Press.
  • Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2008). Vander’s Human Physiology: The Mechanisms of Body Function (11th ed.). Boston: McGraw-Hill Higher Education.
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