Appendix (anatomy)

Structure

The human appendix averages 9 mm in length, ranging from 5 to. The diameter of the appendix is 6 mm, any larger is considered a thickened or inflamed appendix. The appendix is usually located in the lower right quadrant of the abdomen, near the right hip bone. The base of the appendix is located 2 cm beneath the ileocecal valve that separates the large intestine from the small intestine. Its position within the abdomen corresponds to a point on the surface known as McBurney's point.

The appendix is connected to the mesentery in the lower region of the ileum, by a short region of the mesocolon known as the mesoappendix.

Variation

Some identical twins–known as mirror image twins–can have a mirror-imaged anatomy, a congenital condition with the appendix located in the lower left quadrant of the abdomen instead of the lower right. Intestinal malrotation may also cause displacement of the appendix to the left side.

While the base of the appendix is typically located 2 cm below the ileocecal valve, the tip of the appendix can be variably located–in the pelvis, outside the peritoneum or behind the cecum. The prevalence of the different positions varies amongst populations for example, the retrocecal position is most common in Ghana and Sudan, with 67.3% and 58.3% occurrence respectively, in comparison to Iran and Bosnia where the pelvic position is most common, with 55.8% and 57.7% occurrence respectively.

In rare cases, the appendix may not be present at all (laparotomies for suspected appendicitis have given a frequency of 1 in 100,000).

Sometimes a semi-circular fold of mucous membrane appears at the opening of the appendix. This valve of the vermiform appendix is also called Gerlach's valve.

Functions

Maintaining gut flora

Gut-associated lymphoid tissue is immune tissue surrounding the appendix and appearing elsewhere in the gut. It carries out several important functions. Initially, due to an absence of side effects following its removal, the appendix was deemed to lack any specific function.

William Parker, Randy Bollinger, and colleagues at Duke University proposed in 2007 that the appendix serves as a haven for useful bacteria when illness or antibiotics flushes the bacteria from the intestines. This proposition is based on an understanding of how the immune system supports the growth of beneficial intestinal bacteria, in combination with other well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue.

Research in 2012 reported that individuals without an appendix were twice as likely to have a recurrence of Clostridioides difficile colitis. The appendix, therefore, may act as a reservoir for beneficial bacteria. This reservoir could repopulate the gut flora following a bout of gastrointestinal illness.

Immune and lymphatic systems

The appendix is an important component of mammalian mucosal immune function, particularly B cell-mediated immune responses and extrathymically derived T cells. This structure helps in the proper movement and removal of waste matter in the digestive system, contains lymphatic vessels that regulate pathogens, and lastly, might even produce early defence against disease. Additionally, this may provide immune defence from invading pathogens and stimulating B and T cells to fight viruses and bacteria that infect that portion of the bowel and training them so that immune responses are targeted and more able to fight pathogens. In addition, immune cells called innate lymphoid cells help the appendix maintain digestive health.

A 2016 study reported a positive correlation between the presence of the appendix and the concentration of cecal lymphoid tissue, which supports the suggestion that the appendix has major immune benefits.

Disease

Common diseases of the appendix (in humans) are appendicitis and carcinoid tumors (appendiceal carcinoid). Appendix cancer accounts for about 1 in 200 of all gastrointestinal malignancies. In rare cases, adenomas are present.

Appendicitis

Main article: Appendicitis

Appendicitis is a condition characterized by inflammation of the appendix. Pain often begins in the center of the abdomen, corresponding to the appendix's development as part of the embryonic midgut. This pain is typically a dull, poorly localized, visceral pain.

As the inflammation progresses, the pain begins to localize more clearly to the right lower quadrant, as the peritoneum becomes inflamed. This inflammation, or peritonitis, results in rebound tenderness (pain upon removal of pressure rather than the application of pressure). In particular, it presents at McBurney's point, 1/3 of the way along a line drawn from the anterior superior iliac spine to the umbilicus. Typically, point (skin) pain is not present until the parietal peritoneum is inflamed as well. Fever and an immune system response are also characteristic. Other signs and symptoms include nausea and vomiting, fever, constipation or diarrhea, abdominal bloating, or flatulence.

Untreated, the appendix may rupture, leading to peritonitis, followed by shock, and, if still untreated, death.

Appendectomy is the surgical removal of the appendix. This removal is typically performed as an emergency procedure in a patient suffering from acute appendicitis. In the absence of surgical facilities, intravenous antibiotics are used to delay or avoid the onset of sepsis. In some cases, the appendicitis resolves; otherwise, an inflammatory mass forms around the appendix. This is a relative contraindication to surgery.

Uses

The appendix can be used to construct an efferent urinary conduit, in an operation known as the Mitrofanoff procedure, in people with a neurogenic bladder.

The appendix can also be used to access the colon in children with paralysed bowels or major sphincter dysfunction. The appendix is lifted from the abdomen and a catheter can be attached which allows the colon to be irrigated (via normal defecation).

History

Charles Darwin suggested that the appendix was mainly used by earlier hominids for digesting fibrous vegetation, before evolving. The large cecum of some herbivorous animals, such as the horse, rodents or the koala, appears to support this hypothesis. The koala's cecum enables it to host bacteria that specifically help to break down cellulose. Human ancestors may have also relied upon to digest a diet rich in foliage.

As people shifted to eat more easily digested foods, the cecum became less necessary for digestion. Mutations that were previously deleterious lost their salience, which allowed them to survive. These alleles became more frequent and the cecum shrank. After millions of years, the human cecum became the appendix.

Dr. Heather F. Smith and colleagues stated:

A 2013 study estimated that the appendix independently evolved in different animals 32-38 times but had been lost no more than six times. A 2017 study of an updated database yielded similar results, with estimates of at least 29 gains and at most 12 losses (all of which were ambiguous). This suggests that the cecal appendix has a selection advantage and is not vestigial. It appears to be associated with greater longevity. For example, a 2023 study reported protective functions against diarrhea in young primates.

That function may be particularly useful in the absence of sanitation and healthcare, where diarrhea may be prevalent. Epidemiological data on the cause of death in developing countries collected by the World Health Organization in 2001 reported that acute diarrhea was the fourth leading cause of disease-related death in developing countries. Two of the other leading causes of death were claimed to have exerted limited or no selection pressure.

Gallery

File:Ultrasonography of a normal appendix, annotated.jpg|Abdominal ultrasound showing a normal appendix between the external iliac artery and the abdominal wall File:Blausen 0043 Appendix Child.png|Illustration depicting the location of the appendix in a child File:Stomach colon rectum diagram-en.svg|Normal location of the appendix relative to other organs of the digestive system (frontal view) File:Appendix vermiformis.jpg|Vermiform appendix File:Appendixcancer.jpg|Mucinous adenocarcinoma of the appendix tip File:Enterobius - very low mag.jpg|Cross section of the appendix with Enterobius with H&E stain File:Micrograph of entry point of appendicular arteries.jpg|Micrograph of entry point of appendicular arteries (arrows at level of inner muscular layer), not to be confused with a perforation.

References

References

1. (October 2016). "The immunology of the vermiform appendix: a review of the literature". [[Clinical and Experimental Immunology]].
2. (2009). "Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix". Journal of Evolutionary Biology.
3. (2003). "Anatomical Variations Of Vermiform Appendix In South-East Caspian Sea (Gorgan-IRAN)". J. Anat. Soc. India.
4. "Unusual Types of Twins". Multiples of America.
5. (1984). "Situs viscerum specularis in monozygotic twins". Acta Geneticae Medicae et Gemellologiae.
6. Paterson-Brown, S.. (2007). "Principles and practice of surgery". Churchill Livingstone.
7. (December 2006). "Position and susceptibility to inflammation of vermiform appendix in Accra, Ghana". East African Medical Journal.
8. (June 1999). "Anomalies of the vermiform appendix and prevalence of acute appendicitis in Khartoum". East African Medical Journal.
9. (2014). "Variation in Anatomical Position of Vermiform Appendix among Iranian Population: An Old Issue Which Has Not Lost Its Importance". [[Anatomy Research International]].
10. (2009). "[Variations in position and place of formation of appendix vermiformis found in the course of open appendectomy]". Medicinski Arhiv.
11. (2009). "Congenital absence of the cecal appendix. Case report". Cirugia y Cirujanos.
12. (1989). "Robbins' pathologic basis of disease". Saunders.
13. (5 October 2007). "Scientists may have found appendix's purpose". [[NBC News]].
14. (December 2007). "Biofilms in the large bowel suggest an apparent function of the human vermiform appendix". Journal of Theoretical Biology.
15. (June 2004). "Getting a grip on things: how do communities of bacterial symbionts become established in our intestine?". [[Nature Immunology]].
16. (2004). "Immune exclusion and immune inclusion: a new model of host-bacterial interactions in the gut". Clinical and Applied Immunology Reviews.
17. Dunn, Rob. (January 2, 2012). "Your Appendix Could Save Your Life". [[Scientific American]].
18. (April 2004). "The vermiform appendix: not a useless organ". Journal of the College of Physicians and Surgeons--Pakistan.
19. (February 2016). "Complementarity and redundancy of IL-22-producing innate lymphoid cells". [[Nature Immunology]].
20. (September 2016). "Morphological evolution of the mammalian cecum and cecal appendix". Comptes Rendus Palevol.
21. . (2004). "Miscellaneous conditions of the appendix". *[[National Institutes of Health]]*.
22. "Statistics about Appendix disorder". rightdiagnosis.com.
23. Miller R., Kenneth. (2002). "Biology". [[Prentice Hall]].
24. . ["Appendicitis - Symptoms and causes - Mayo Clinic"](https://www.mayoclinic.org/diseases-conditions/appendicitis/symptoms-causes/syc-20369543). *[[Mayo Clinic]]*.
25. (2003). "Surgical management of the neurogenic bladder and bowel". International Braz J Urol.
26. "Wellington Children's Hospital: Caring for an ACE or Chait Tube: Healthpoint".
27. Darwin, Charles. (1871). "The Descent of Man, and Selection in Relation to Sex". John Murray.
28. (October 2009). "Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix". Journal of Evolutionary Biology.
29. (2013). "Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors". Comptes Rendus Palevol.
30. (2017). "Morphological evolution of the mammalian cecum and cecal appendix". Comptes Rendus Palevol.
31. (November 2021). "The cecal appendix is correlated with greater maximal longevity in mammals". Journal of Anatomy.
32. (2023-09-23). "Correlation between the presence of a cecal appendix and reduced diarrhea severity in primates: new insights into the presumed function of the appendix". Scientific Reports.
33. (April 2011). "The cecal appendix: one more immune component with a function disturbed by post-industrial culture". Anatomical Record.
34. (21 August 2009). "Evolution of the Appendix: A Biological 'Remnant' No More".