Vermiform appendix

Vermiform Appendix
Arteries of cecum and vermiform appendix (appendix visible at lower right, labeled as "vermiform process")
Normal location of the appendix relative to other organs of the digestive system (frontal view)
Latin	appendix vermiformis
Gray's	subject #249 1178
System	Digestive
Artery	appendicular artery
Vein	appendicular vein
Precursor	Midgut
MeSH	Appendix
Dorlands/Elsevier	Vermiform appendix

The appendix (or vermiform appendix; also cecal [or caecal] appendix; also vermix) is a blind-ended tube connected to the cecum, from which it develops embryologically. The cecum is a pouchlike structure of the colon. The appendix is located near the junction of the small intestine and the large intestine.

The term "vermiform" comes from Latin and means "worm-shaped".

It is widely present in the Euarchontoglires and has also evolved independently in the diprotodont marsupials and is highly diverse in size and shape.

Size and location in humans

The appendix averages 11 cm in length but can range from 2 to 20 cm. The diameter of the appendix is usually between 7 and 8 mm. The longest appendix ever removed measured 26 cm from a patient in Zagreb, Croatia. The appendix is located in the right lower quadrant of the abdomen, near the right hip bone. Its position within the abdomen corresponds to a point on the surface known as McBurney's point (see below). While the base of the appendix is at a fairly constant location, 2 cm below the ileocecal valve, the location of the tip of the appendix can vary from being retrocecal (behind the cecum) (74%) to being in the pelvis to being extraperitoneal. In rare individuals with situs inversus, the appendix may be located in the lower left side.

Function

Vestigiality

Vermiform appendix

The human appendix has been proposed to be a vestigial structure, a structure that has lost all or most of its original function through the process of evolution. The vermiform appendix is proposed to be the shrunken remainder of the cecum that was found in a remote ancestor of humans. Ceca, which are found in the digestive tracts of many extant herbivores, house mutualistic bacteria which help animals digest the cellulose molecules that are found in plants. As the human appendix does not house a significant number of these bacteria, humans are not capable of digesting more than a minimal amount of cellulose per day.

Immune function

Loren G. Martin, a professor of physiology at Oklahoma State University, argues that the appendix has a function in fetuses and adults. Endocrine cells have been found in the appendix of 11-week-old fetuses that contribute to "biological control (homeostatic) mechanisms." In adults, Martin argues that the appendix acts as a lymphatic organ. The appendix is experimentally verified as being rich in infection-fighting lymphoid cells, suggesting that it might play a role in the immune system. Zahid suggests that it plays a role in both manufacturing hormones in fetal development as well as functioning to "train" the immune system, exposing the body to antigens so that it can produce antibodies. He notes that doctors in the last decade have stopped removing the appendix during other surgical procedures as a routine precaution, because it can be successfully transplanted into the urinary tract to rebuild a sphincter muscle and reconstruct a functional bladder.

Maintaining gut flora

Possible function of the human vermiform appendix as a "safe house" for beneficial bacteria in the recovery from diarrhea.

Although it was long accepted that the immune tissue, called gut associated lymphoid tissue, surrounding the appendix and elsewhere in the gut carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of importance as judged by an absence of side effects following appendectomy.

William Parker, Randy Bollinger, and colleagues at Duke University proposed that the appendix serves as a haven for useful bacteria when illness flushes those bacteria from the rest of the intestines. This proposal is based on a new understanding of how the immune system supports the growth of beneficial intestinal 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 at Winthrop University-Hospital showed that individuals without an appendix were four times more likely to have a recurrence of Clostridium difficile. However, other research showed that there is a significantly greater rate of C. difficile infection among people with an appendix, with more than 80% of the infections occurring among patients with an intact appendix.

Evolution

Disuse

A possible scenario for the progression from a fully functional cecum to the current human appendix was put forth by Charles Darwin. He suggested that the appendix was used for digesting leaves as primates. It may be a vestigial organ of ancient humans that has degraded to nearly nothing over the course of evolution. The very long cecum of some herbivorous animals, such as found in the horse or the koala, appears to support this theory. 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 have survived. It's suggested that these alleles became more frequent and the cecum continued to shrink. After millions of years, the once-necessary cecum degraded to be the appendix of today.

Use

Dr. Heather F. Smith of Arizona State University explains: “recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix.” “Three morphotypes of cecal-appendices can be described among mammals based primarily on the shape of the cecum: a distinct appendix branching from a rounded or sac-like cecum (as in many primate species),an appendix located at the apex of a long and voluminous cecum (as in the rabbit, greater glider and Cape dune mole rat), and an appendix in the absence of a pronounced cecum (as in the wombat). In addition, long narrow appendix-like structures are found in mammals that either lack an apparent cecum (as in monotremes) or lack a distinct junction between the cecum and appendix- like structure (as in the koala). A cecal appendix has evolved independently at least twice, and apparently represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world. Although the appendix has apparently been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade,”

Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where diarrhea may be prevalent. Current epidemiological data on the cause of death in developed countries collected by the World Health Organization in 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by The Bill and Melinda Gates Foundation). Two of the other leading causes of death are expected to have exerted limited or no selection pressure.

Diseases

The most 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 also present.

Appendicitis (or epityphlitis) 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 peritoneal inflammation, or peritonitis, results in rebound tenderness (pain upon removal of pressure rather than 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 of appendicitis.

Appendicitis requires removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, the appendix may rupture, leading to peritonitis, followed by shock, and, if still untreated, death.

The surgical removal of the vermiform appendix is called an appendectomy, or appendicectomy. This removal is normally performed as an emergency procedure when the patient is 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 completely; more often, an inflammatory mass forms around the appendix. This is a relative contraindication to surgery.

Use as efferent urinary conduit

The appendix is used for the construction of an efferent urinary conduit, in an operation known as the Mitrofanoff procedure, in people with a neurogenic bladder.

Use as access to colon in faecal incontinence (ACE and Chait tube)

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. See http://www.healthpoint.co.nz/default,169125.sm;jsessionid=1CD66A058B10550C51041E477C8E7075

Gallery

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  Ileo-cecal valve and vermiform appendix.

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  Mucinous adenocarcinoma of the appendix tip.

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  Cross section of the appendix with Enterobius. H&E stain.

See also

• Homology (biology)
• Noncoding DNA
• Appendectomy

Further reading

• "The vestigiality of the human vermiform appendix: A Modern Reappraisal"—evolutionary biology argument that the appendix is vestigial
• Appendix May Actually Have a Purpose—2007 WebMD article
• Smith, H.F.; R.E. Fisher, M.L. Everett, A.D. Thomas, R. Randal Bollinger, and W. Parker (October 2009). "Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix". Journal of Evolutionary Biology 22 (10): 1984–99. doi:10.1111/j.1420-9101.2009.01809.x. ISSN 1420-9101. PMID 19678866. http://www3.interscience.wiley.com/journal/122544996/abstract.
• Cho, Jinny. "Scientists refute Darwin's theory on appendix". The Chronicle (Duke University), August 27, 2009. (News article on the above journal article.)

External links

• SUNY Labs 37:12-0102—"Abdominal Cavity: The Cecum and the Vermiform Appendix"