As my last post may have suggested, I’ve recently been taking a deeper look at the large intestine – specifically the appendix. The appendix is a small, intestinal, diverticulum (basically a little pouch) that protrudes off the cecum (the first part of the large intestine, itself a little pouch- though much bigger than the appendix). You may have heard (and indeed, at the time of writing, Wikipedia has it written) that the appendix is a vestigial structure- a now useless remnant of something that was useful to our ancestors. Darwin actually helped propagate this belief, theorizing that the appendix was a shrunken remnant of a larger cecum. Furthermore, the relatively common and apparently benign surgical removal of the appendix, the procedure known as appendectomy, seems to support the idea that the appendix is of no particular use to humans today.
But is it?
There is an increasing body of information supporting the idea that the appendix is not a vestigial structure and that it has a specific role in human health. This might get a bit lengthy, so I will approach this topic in stages- probably culminating in a few posts.
First things first- is the appendix really vestigial? As I mentioned above, Darwin believed that the appendix was vestigial. He came to this idea because of the (erroneous) belief that hominids were the only primates to possess an appendix. Other primates that eat vast quantities of leaves and fibrous material that needs to be fermented by gut microflora, have large cecums where fermentation can occur. Humans, who don’t rely on copious vegetation for nutrition, only have a small cecum. It was thus hypothesized that the appendix was the shrunken remains of our forbearers’ large cecum. What Darwin was missing, however, was the fact that a number of species, including many primates, have large cecums and ALSO have an appendix. Hmm…
Another clue that the appendix is not simply the excess baggage of our herbivorous forbearers is that according to phylogenetic analysis, the appendix has actually arisen at least twice, independently, in evolutionary history. Such research also suggests that the appendix has been maintained in mammalian evolution for 80 million years . To have evolved twice, independently, and to have been maintained for 80+ million years, suggests the appendix is not a useless remnant.
If the appendix is not vestigial, what is its function?
The dual evolution of the appendix, and the occurrence of an appendix in species with large cecums suggests that the organ plays an important role in normal physiology. Anatomically, the appendix is found at the end of the cecum, in a rather secluded corner of the intestines (if you can imagine such a thing). While the length of the appendix varies greatly from human to human, the diameter remains relatively constant. Another constant is the appendix’s association with a large amount of immune tissue known as GALT (gut-associated lymphoid tissue). While most people tend to think of immune tissue as ‘bacteria-fighting’ stuff, it turns out that some immune tissue produces substances (such as secretory IgA and mucin) that actually support bacterial growth, specifically the growth of biofilms.
Biofilms have been the focus of quite a bit of research recently, and usually not in a good way. Because people tend to think of biofilms (literally aggregates of bacteria embedded in self-produced slime) as pathogenic and problematic conglomerates, the focus of most research has been how best to disrupt and destroy them. It’s not entirely unwarranted either, Biofilms tend to be associated with unpleasant conditions, such as infections of medical implants and dental plaque. However, biofilms of commenselate bacteria (the ones we evolved with, on our skin and in our gut) are a way of safeguarding good bacteria.
When this is all put together, it appears that the appendix, with its relatively constant diameter and with the secreted products of GALT, is well adapted to facilitate and maintain communities of mutualistic intestinal flora . It has thus been theorized that the appendix can act as a source of normal microbiota that can inoculate the gut when needed.
Why would your gut need to be inoculated with normal microbiota? Isn’t that what’s already in your gut?
It has been suggested [2, 3] that the appendix acts as a ‘safe house’ for resident microbiota when a GI infection occurs. When disease-causing bacteria are flushed from the intestines by diarrhea, the normal bacteria are eliminated as well. The appendix safe-guards a population of the normal bacteria that can then repopulate the large intestine after the diarrhea has passed. This function may not seem too important today in the developed world, where we enjoy relatively good hygiene and relatively low levels of epidemic diarrhea, but in the not too distant past and in populations that still suffer from diseases such as cholera, the appendix likely plays an important role in recovering from diarrheal diseases.
While the appendix offers benefits if you live in a developing country, it is less important (though not entirely so- I’ll get to that later) in developed countries with modern hygiene practices such as water treatment and sewage systems. In fact, in the developed world, the appendix has become a bit of a liability, with a surprisingly large portion of the population developing appendicitis at some point during their life. In my next post I’ll discuss the appendix in disease and health, and probably wax poetic about how we should consider this interesting little organ in our modern environment.
1. Smith, H.F., R.E. Fisher, M.L. Everett, A.D. Thomas, R.R. Bollinger, and W. Parker, Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. J Evol Biol, 2009. 22(10): p. 1984-99.
2. Bollinger, R.R., A.S. Barbas, E.L. Bush, S.S. Lin, and W. Parker, Biofilms in the normal human large bowel: fact rather than fiction. Gut, 2007. 56(10): p. 1481-2.
3. Laurin, M., M.L. Everett, and W. Parker, The cecal appendix: one more immune component with a function disturbed by post-industrial culture. Anat Rec (Hoboken), 2011. 294(4): p. 567-79.