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Appendix, More Than Just a Vestigial Organ

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  • Recent study suggests that appendix serves as a “safe-house” for beneficial gut bacteria.
  • This is because mammal species with appendix have higher concentrations of lymphoid or immune tissue in their cecum.
  • The lymphatic tissue can also stimulate growth of some types of beneficial gut bacteria.

Appendix, More Than Just a Vestigial Organ

Appendix may serve as a reservoir for beneficial gut bacterial.

The human appendix is a narrow, finger-shaped pouch that extends from the cecum in the digestive system. In some cases it gets inflamed leading to appendicitis which often culminates in surgery.



Appendicitis is an inflammation of the appendix.

Appendicitis causes pain in the lower right abdomen. As inflammation worsens, the pain worsens and eventually becomes severe.

Appendicitis most often it occurs in people between the ages of 10 and 30 years.

Signs and symptoms of appendicitis may include sudden pain that begins on the right side of the lower abdomen, nausea, vomiting, loss of appetite and low grade fever.

Surgical removal of the appendix or appendicectomy is the standard treatment for this condition.

It is considered a vestigial organ with little or no function. But recent research states that the appendix may serve an important purpose.

In particular, it may serve as a reservoir for helpful gut bacteria.

Studying Appendix Evolution in Mammals

Several other mammal species also have an appendix, and studying how it evolved and functions in these species may shed light on this mysterious organ in humans.

The research team led by Heather F. Smith, Ph.D., Associate Professor, Midwestern University Arizona College of Osteopathic Medicine, is currently studying the evolution of the appendix across mammals.

The international research team gathered data on from 533 mammal specials for the presence or absence of the appendix and other gastrointestinal and environmental traits.

To identify how the appendix changes through mammalian evolution, and to determine why some species have the organ and some don't, they mapped the data onto a phylogeny (genetic tree).


Several previous hypothesis had linked appendix to dietary and environmental factors.

Researchers found that the appendix has evolved independently in several mammal lineages, over 30 separate times.

Once it has appeared, it almost never disappears from a lineage suggesting that it likely serves an adaptive purpose.

They discovered that mammal species with an appendix have higher average concentrations of lymphoid (immune) tissue in the cecum.

This finding suggests that the appendix may play an important role as a secondary immune organ.

Lymphatic tissue can also stimulate growth of some types of beneficial gut bacteria, providing further evidence that the appendix may serve as a "safe house" for helpful gut bacteria.

Animals which had tapering or spiral-shaped cecum had higher chances of having an appendix compared to animals with round or cylindrical cecum.

This suggested that the appendix evolved as part of a larger "cecoappendicular complex" including both the appendix and cecum.

Researchers collaborating with Dr. Smith on this study are William Parker, Ph.D., Department of Surgery, Duke Medical Center, Durham, North Carolina; Sanet H. Kotzé, Ph.D., Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, University of Stellenbosch, Tygerberg, South Africa; and Michel Laurin, Ph.D., from the Muséum National d'Histoire Naturelle in France. Midwestern University Senior Research Associate Brent Adrian also contributed illustrations for the study.

The study is published in Comptes Rendus Palevol.


  1. Appendicitis - (http://www.mayoclinic.org/diseases-conditions/appendicitis/basics/definition/CON-20023582?p=1)
  2. Heather F. Smith et al. Morphological evolution of the mammalian cecum and cecal appendix. Comptes Rendus Palevol; (2017) doi.org/10.1016/j.crpv.2016.06.001

Source: Medindia

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