Appendix (anatomy)

Theappendix(pl.:appendicesorappendixes; alsovermiform appendix;cecal(orcaecal,cæcal)appendix;vermix; orvermiform process) is a finger-like, blind-ended tube connected to thececum, from which itdevelops in the embryo.
The cecum is a pouch-like structure of thelarge intestine, located at the junction of thesmalland the large intestines. The term "vermiform" comes fromLatinand means "worm-shaped". The appendix was once considered avestigial organ, but this view has changed since the early 2000s.Research suggests that the appendix may serve an important purpose as a reservoir for beneficialgut bacteria.
The human appendix averages 9 cm (3.5 in) in length, ranging from 5 to 35 cm (2.0 to 13.8 in). The diameter of the appendix is 6 mm (0.24 in), and more than 6 mm (0.24 in) is considered a thickened or inflamed appendix. The longest appendix ever removed was 26 cm (10 in) long.The appendix is usually located in the lower rightquadrantof theabdomen, near the righthip bone. The base of the appendix is located 2 cm (0.79 in) beneath theileocecal valvethat separates the large intestine from the small intestine. Its position within the abdomen corresponds to a point on the surface known asMcBurney's point.
The appendix is connected to themesenteryin the lower region of theileum, by a short region of themesocolonknown as themesoappendix.
Some identical twins—known asmirror image twins—can have amirror-imaged anatomy, acongenital conditionwith the appendix located in the lower left quadrant of the abdomen instead of the lower right.Intestinal malrotationmay also cause displacement of the appendix to the left side.
While the base of the appendix is typically located 2 cm (0.79 in) below theileocecal valve, the tip of the appendix can be variably located—in thepelvis, outside theperitoneumor behind the cecum.The prevalence of the different positions varies amongst populations with the retrocecal position being most common inGhanaandSudan, with 67.3% and 58.3% occurrence respectively, in comparison toIranandBosniawhere the pelvic position is most common, with 55.8% and 57.7% occurrence respectively.
In very rare cases, the appendix may not be present at all (laparotomiesfor suspected appendicitis have given a frequency of 1 in 100,000).
Sometimes there is a semi-circular fold ofmucous membraneat the opening of the appendix. Thisvalve of the vermiform appendixis also calledGerlach'svalve.
Although it has been long accepted that the immune tissue surrounding the appendix and elsewhere in the gut—calledgut-associated lymphoid tissue—carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of specific importance and function as judged by an absence of side effects followingits removal.Therefore, the notion that the appendix is onlyvestigialbecame widely held.
William Parker, Randy Bollinger, and colleagues atDuke Universityproposed in 2007 that the appendix serves as a haven for usefulbacteriawhen illness flushes the bacteria from the rest of the intestines.This proposition is based on an understanding that emerged by the early 2000s of how the immune system supports the growth of beneficialintestinal bacteria,in combination with many 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 performed atWinthrop–University Hospitalshowed that individuals without an appendix were four times as likely to have a recurrence ofClostridioides difficilecolitis.The appendix, therefore, may act as a "safe house" for beneficial bacteria.This reservoir of bacteria could then serve to repopulate thegut florain thedigestive systemfollowing a bout ofdysenteryorcholeraor to boost it following a milder gastrointestinal illness.
The appendix has been identified as an important component ofmammalianmucosal immune function, particularlyB cell-mediated immune responses andextrathymicallyderivedT 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 defences that prevent deadly diseases. Additionally, it is thought that this may provide more immune defences from invading pathogens and getting the lymphatic system's B and T cells to fight the viruses and bacteria that infect that portion of the bowel and training them so that immune responses are targeted and more able to reliably and less dangerously fight off pathogens.In addition, there are different immune cells calledinnate lymphoid cellsthat function in the gut in order to help the appendix maintain digestive health.
Research also shows a positive correlation between the existence of the appendix and the concentration of cecal lymphoid tissue, which supports the suggestion that not only does the appendix evolve as a complex with the cecum but also has major immune benefits.
Commondiseasesof the appendix (in humans) areappendicitisandcarcinoidtumors (appendiceal carcinoid).Appendix canceraccounts for about 1 in 200 of all gastrointestinal malignancies. In rare cases,adenomasare also present.
Appendicitisis a condition characterized byinflammationof the appendix. Pain often begins in the center of the abdomen, corresponding to the appendix's development as part of the embryonicmidgut. This pain is typically a dull, poorly localized,visceralpain.
As the inflammation progresses, the pain begins to localize more clearly to the right lower quadrant, as theperitoneumbecomes inflamed. This peritoneal inflammation, orperitonitis, results inrebound tenderness(pain upon removal of pressure rather than the application of pressure). In particular, it presents atMcBurney's point, 1/3 of the way along a line drawn from theanterior superior iliac spineto theumbilicus. Typically, point (skin) pain is not present until theparietal peritoneumis inflamed, as well. Fever and an immune system response are also characteristic of appendicitis.Other signs and symptoms may includenauseaand vomiting, low-grade fever that may get worse,constipationordiarrhea, abdominal bloating, orflatulence.
Appendicitis usually requires the removal of the inflamed appendix, in anappendectomyeither bylaparotomyorlaparoscopy. Untreated, the appendix may rupture, leading toperitonitis, followed byshock, and, if still untreated, death.
The surgical removal of the appendix is called anappendectomy. This removal is normally performed as an emergency procedure when the patient is suffering fromacuteappendicitis. In the absence of surgical facilities,intravenousantibioticsare used to delay or avoid the onset ofsepsis. In some cases, the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relativecontraindicationto surgery.
The appendix is also used for the construction of an efferent urinary conduit, in an operation known as theMitrofanoff procedure,in people with aneurogenic bladder.
The appendix is also used as a means to access the colon in children with paralysed bowels or major rectal sphincter problems. The appendix is brought out to the skin surface and the child/parent can then attach a catheter and easily wash out the colon (via normal defaecation) using an appropriate solution.
Charles Darwinsuggested that the appendix was mainly used by earlier hominids for digesting fibrous vegetation, then evolved to take on a new purpose over time. The very long cecum of some herbivorous animals, such as in thehorseor thekoala, 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 this system when they lived on a diet rich in foliage.
As people began to eat more easily digested foods, they may have become less reliant on cellulose-rich plants for energy. As the cecum became less necessary for digestion, mutations that were previously deleterious (and would have hindered evolutionary progress) were no longer important, so the mutations survived. It is suggested that theseallelesbecame more frequent and the cecum continued to shrink. After millions of years, the once-necessary cecum degraded to be the appendix of modern humans.
Dr. Heather F. Smith ofMidwestern Universityand colleagues explained:
In a 2013 paper, the appendix was found to have independently evolved in different animals at least 32 times (and perhaps as many as 38 times) and to have been lost no more than six times throughout history.A more recent study using similar methods on an updated database yielded similar, though less spectacular results, with at least 29 gains and at the most 12 losses (all of which were ambiguous), and this is still significantly asymmetrical.
This suggests that the cecal appendix has a selective advantage in many situations and argues strongly against its vestigial nature. Given that this organ may have a selective advantage in numerous situations, it appears to be associated with greater maximal longevity, for a given body mass.For example, in a 2023 study, the protective functions conferred against diarrhea were observed in young primates.This complex evolutionary history of the appendix, along with a great heterogeneity in its evolutionary rate in various taxa, suggests that it is a recurrent trait.
Such a function may be useful in a culture lacking modern sanitation and healthcare practice, wherediarrheamay be prevalent. Currentepidemiologicaldata on the cause of death in developing countries collected by theWorld Health Organizationin 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by theBill and Melinda Gates Foundation). Two of the other leading causes of death are expected to have exerted limited or no selection pressure.