Does the appendix serve a purpose in any animal? (2001)

From a structure-function perspective, the human vermiform appendix is simpy the terminus of the tiniae coli: the cords of smooth muscle have to end somewhere. Similar "I gotta end somwhere" constructs are the gubernaculum and the platysma. Appendices tend to have lots of lymphoid aggregates, but the entire gut has lymphoid aggregates. The fact that more are in the appendix per square inch of epithelium, is probably due as much to its dependent location (everything in the right colon tends to fall down) as any developmental specialization. I am also not aware of any appendix-specific transcription factors (I think most pathologists would agree such a thing would be useful if it existed, see Krukenberg tumor).

At a histologic level, there is nothing special about the appendix. The cell types in the appendix are found throughout the colon: colonic epithelium, smooth muscle, lymphoid tissue, vessels, nerves.

I think the term "vestigal" is being misinterpreted by many as a suggestion that it had some more important function earlier in evolution. I rather doubt it. They assert that an appendix hasn't been described in some species. Ok, but have you positively described the absence of tiniae coli as well?

The association with the Bursa of Fabricius may hold water, because the bursa apparently develops from the avian hindgut, however, unlike birds, when we amputate the appendix of a baby human, it doesn't develop an immune deficiency. Also, the bursa seems to be further down the gut than that appendix, although I have not done a careful examination.

This question has been answered many times through numerous studies since 2001

TLDR: Yes

http://www.sciencedirect.com/science/article/pii/S1631068316...

http://dujs.dartmouth.edu/2008/04/a-bacterial-safe-house-a-n...

https://www.sciencedaily.com/releases/2007/10/071008102334.h...

http://time.com/4631305/appendicitis-appendix-gut-bacteria/

The appendix protects good bacteria in the gut. https://www.wired.com/2013/02/appendix-revolution/

It does for Koala's. There's probably better articles. This was the best of a bad lot on a quick search:

http://www.abc.net.au/news/2007-10-10/scientists-discover-tr...