|MadSci Network: Anatomy|
Is it a sign of the evoluation from animal to human? ---------------------------------------------------- That’s a pretty big question! First of all, let’s review what we know about appendicitis. Appendicitis is the inflammation of the appendix. From a quick search of medical literature about appendicitis, we can learn about the epidemiology of acute appendicitis and its causes. “The peak incidence of acute appendicitis is in the second and third decades of life; it is relatively rare at the extremes of age. Males and females are equally affected, except between puberty and age 25, when males predominate in a 3:2 ratio. Perforation is more common in infancy and in the aged, during which periods mortality rates are highest. The mortality rate has decreased steadily in Europe and the United States from 8.1 per 100,000 of the population in 1941 to <1 per 100,000 in 1970 and subsequently. The absolute incidence of the disease also decreased by about 40% between 1940 and 1960 but since then has remained unchanged. Although various factors such as changing dietary habits, altered intestinal flora, and better nutrition and intake of vitamins have been suggested to explain the reduced incidence, the exact reasons have not been elucidated. The overall incidence of appendicitis is much lower in underdeveloped countries, especially parts of Africa, and in lower socioeconomic groups.” The general cause of appendicitis is obstruction of the intestinal lumen by a fecalith (a solid mass of feces) most commonly, but can also be caused by a tumor. However, obstruction is only seen in 30-40% of cases. In the majority of cases, the mucosa of the lumen is ulcerated, which can be caused by infection. From Harrison's Principles of Internal Medicine Part 12: Disorders of the Gastrointestinal System SECTION 1: Disorders of the Alimentary Tract Copyright 2004 by The McGraw-Hill Companies, Inc William Silen Let's also review the appendix itself, and what its function is. Here is some information from an immunology textbook: “The peripheral lymphoid organs are specialized to trap antigen, to allow the initiation of adaptive immune responses, and to provide signals that sustain recirculating lymphocytes.” “The appendix is a gut-associated lymphoid tissue located at the beginning of the colon.” “The gut-associated lymphoid tissues (GALT), which include the tonsils, adenoids, and appendix, and specialized structures called Peyer's patches in the small intestine, collect antigen from the epithelial surfaces of the gastrointestinal tract. In Peyer's patches, which are the most important and highly organized of these tissues, the antigen is collected by specialized epithelial cells called multi-fenestrated or M cells. The lymphocytes form a follicle consisting of a large central dome of B lymphocytes surrounded by smaller numbers of T lymphocytes. Similar but more diffuse aggregates of lymphocytes protect the respiratory epithelium, where they are known as bronchial-associated lymphoid tissue (BALT), and other mucosa, where they are known simply as mucosal-associated lymphoid tissue (MALT). Collectively, the mucosal immune system is estimated to contain as many lymphocytes as all the rest of the body, and they form a specialized set of cells obeying somewhat different rules.” Immunobiology : the immune system in health and disease / Charles A. Janeway, Jr. et al.-- 5th ed. Garland Publishing, New York 2001 And here is some information on the normal flora (bacteria and archaea) that inhabit the intestine of healthy animals, including humans. “The intestinal microflora is a complex ecosystem containing over 400 bacterial species. Anaerobes outnumber facultative anaerobes. The flora is sparse in the stomach and upper intestine, but luxuriant in the lower bowel. Bacteria occur both in the lumen and attached to the mucosa, but do not normally penetrate the bowel wall.” “Concentrations of 10^9 to 10^11 bacteria/g of contents are frequently found in human colon and feces.” “Although the normal flora can inhibit pathogens, many of its members can produce disease in humans. Anaerobes in the intestinal tract are the primary agents of intra-abdominal abscesses and peritonitis. Bowel perforations produced by appendicitis, cancer, infarction, surgery, or gunshot wounds almost always seed the peritoneal cavity and adjacent organs with the normal flora.” Medical Microbiology, Samuel Baron MD, University of Texas Medical Branch at Galveston. 1996 Fourth Edition. The second part of your question is about differences between humans and other animals, and the evolution of the appendix and appendicitis. There is an entire webpage in http://www.talkorigins.org/ dedicated to this question here. http://www.talkorigins.org/faqs/vestiges/appendix.html There is also an interesting New York Times article, reprinted on this blog, which addresses a similar question. http://johnhawks.net/weblog/topics/senescence So we can see from the medical literature that the appendix has a function in immunity, but we know that people without an appendix can still survive. So its presence is not necessary for life, like the heart is, but it still serves a function. As you will read in the previous link to the talkorigins website, “vestiges are very often complex or specialized structures, in fact overly complex for their functions, and prime examples are the wing of the ostrich and the eyes of blind cavefish. A vestige can be a complex structure, in an absolute sense, while simultaneously being rudimentary or degenerate relative to the same homologous structure in other organisms.” As for the specific question about appendicitis being a sign of evolution: Some non-human primates have appendices, as do some non-primate animals. I don’t see any reason why those appendices could not also succumb to inflammation and infection. I wasn’t able to find any examples of appendicitis in my search, but perhaps a veterinarian or someone who takes care of research primates might be able to answer that question. I hope this helps!
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