Vermiform appendix
Encyclopedia
The appendix is a blind-ended tube connected to the cecum
(or caecum), from which it develops embryo
logically. 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.
, 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.
The human appendix is a vestigial structure
. A vestigial structure is a structure that has lost all or most of its original function through the process of evolution. The vermiform appendage is 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 no longer houses a significant amount of these bacteria, and humans are no longer capable of digesting more than a minimal amount of cellulose per day, the human appendix is considered a vestigial structure. This interpretation would stand even if it were found to have a certain use in the human body. Vestigial organs are sometimes pressed into a secondary use when their original function has been lost. See the sections below for possible functions of the appendix that may have evolved more recently after the appendix lost its original function.
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 primate
s. It may be a vestigial organ, evolutionary baggage
, of ancient humans that has degraded down 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
, supports 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 became 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. These allele
s became more frequent and the cecum continued to shrink. After thousands of years, the once-necessary cecum has degraded to be the appendix of today. On the other hand, evolutionary theorists have suggested that natural selection selects for larger appendices because smaller and thinner appendices would be more susceptible to inflammation and disease.
that are beneficial in the function of the human colon.
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 hormone
s 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
.
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. Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where diarrhea
may be prevalent. Current epidemiological
data show that diarrhea is one of the leading causes of death in developing countries
, indicating that as diarrhea flushes out the helpful bacteria the appendix helps recovery by providing a "safe house" for the bacteria.
s 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
; it is now recognized that many cases will resolve when treated non-operatively. In some cases the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relative contraindication
to surgery.
.
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
(or caecum), from which it develops embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
logically. The cecum is a pouchlike structure of the colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
. The appendix is located near the junction of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
and the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
.
The term "vermiform" comes from Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
and means "worm-shaped".
It is widely present in the Euarchontoglires
Euarchontoglires
Euarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
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 abdomenAbdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
, near the right hip bone
Iliac fossa
The iliac fossa is a large, smooth, concave surface located on the internal surface of the ilium...
. Its position within the abdomen corresponds to a point on the surface known as McBurney's point
McBurney's point
McBurney's point is the name given to the point over the right side of the abdomen that is one-third of the distance from the anterior superior iliac spine to the navel...
(see below). While the base of the appendix is at a fairly constant location, 2 cm below the ileocecal valve
Ileocecal valve
The ileocecal valve, or ileocaecal valve, is of a bilabial papilla structure with physiological sphincter muscle situated at the junction of the small intestine and the large intestine, with recent evidence indicating an anatomical sphincter may also be present in humans) Its critical function is...
, the location of the tip of the appendix can vary from being retrocecal (behind the cecum) (74%) to being in the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
to being extraperitoneal
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
. In rare individuals with situs inversus
Situs inversus
Situs inversus is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement is known as situs solitus...
, the appendix may be located in the lower left side.
Vestigiality
Vestigial structure
Vestigiality describes homologous characters of organisms that have seemingly lost all or most of their original function in a species through evolution. These may take various forms such as anatomical structures, behaviors and biochemical pathways. Some of these disappear early in embryonic...
. A vestigial structure is a structure that has lost all or most of its original function through the process of evolution. The vermiform appendage is 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 no longer houses a significant amount of these bacteria, and humans are no longer capable of digesting more than a minimal amount of cellulose per day, the human appendix is considered a vestigial structure. This interpretation would stand even if it were found to have a certain use in the human body. Vestigial organs are sometimes pressed into a secondary use when their original function has been lost. See the sections below for possible functions of the appendix that may have evolved more recently after the appendix lost its original function.
A possible scenario for the progression from a fully functional cecum to the current human appendix was put forth by Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
. He suggested that the appendix was used for digesting leaves as primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s. It may be a vestigial organ, evolutionary baggage
Evolutionary baggage
Evolutionary baggage is the part of the genome of a population that was advantageous in past individuals but is disadvantageous under the pressures exerted by natural selection today.-Origin:...
, of ancient humans that has degraded down to nearly nothing over the course of evolution. The very long cecum of some herbivorous animals, such as found in the horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
or the koala
Koala
The koala is an arboreal herbivorous marsupial native to Australia, and the only extant representative of the family Phascolarctidae....
, supports 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 became 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. These allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s became more frequent and the cecum continued to shrink. After thousands of years, the once-necessary cecum has degraded to be the appendix of today. On the other hand, evolutionary theorists have suggested that natural selection selects for larger appendices because smaller and thinner appendices would be more susceptible to inflammation and disease.
Immune function
Some scientists have recently proposed that the appendix may harbor and protect bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that are beneficial in the function of the human colon.
Loren G. Martin, a professor of physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
at Oklahoma State University, argues that the appendix has a function in fetuses and adults. Endocrine
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...
cells have been found in the appendix of 11-week-old fetuses that contribute to "biological control (homeostatic
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
) mechanisms." In adults, Martin argues that the appendix acts as a lymphatic
Lymphatic system
The lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
organ. The appendix is experimentally verified as being rich in infection-fighting lymphoid cells
Lymphoid
Lymphoid is a term used to describe lymph or the lymphatic system.In the context of lymphoid leukemia, it refers specifically to lymphocytes Lymphoid leukemias and lymphomas are now considered to be tumors of the same type of cell lineage. They are called "leukemia" when in the blood or marrow and...
, suggesting that it might play a role in the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
. Zahid suggests that it plays a role in both manufacturing hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s in fetal development as well as functioning to "train" the immune system, exposing the body to antigens so that it can produce antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
. 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
Urinary system
The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder and the urethra.-Kidney:...
to rebuild a sphincter
Sphincter
A sphincter is an anatomical structure, or a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning...
muscle and reconstruct a functional bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
.
Maintaining gut flora
Duke University
Duke University is a private research university located in Durham, North Carolina, United States. Founded by Methodists and Quakers in the present day town of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco industrialist James B...
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
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
, 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. Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where diarrhea
Diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
may be prevalent. Current epidemiological
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
data show that diarrhea is one of the leading causes of death in developing countries
Developing country
A developing country, also known as a less-developed country, is a nation with a low level of material well-being. Since no single definition of the term developing country is recognized internationally, the levels of development may vary widely within so-called developing countries...
, indicating that as diarrhea flushes out the helpful bacteria the appendix helps recovery by providing a "safe house" for the bacteria.
Diseases
The most common diseaseDisease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
s of the appendix (in humans) are appendicitis
Appendicitis
Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...
and carcinoid
Carcinoid
Carcinoid is a slow-growing type of neuroendocrine tumor, originating in the cells of the neuroendocrine system.In 2000, the World Health Organization redefined "carcinoid", but this new definition has not been accepted by all practitioners. This has led to some complexity in distinguishing...
tumors (appendiceal carcinoid). Appendix cancer
Appendix cancer
Appendix cancer or appendiceal cancer are rare malignancies of the vermiform appendix.-Types:The most common tumor of the appendix is carcinoid, but not all appendiceal carcinoids are malignant...
accounts for about 1 in 200 of all gastrointestinal malignancies. In rare cases, adenomas are also present.
Appendicitis
Appendicitis
Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...
(or epityphlitis) is a condition characterized by inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
of the appendix. Pain often begins in the center of the abdomen, corresponding to the appendix's development as part of the embryonic midgut
Midgut
The midgut is the portion of the embryo from which most of the intestines develop. After it bends around the superior mesenteric artery, it is called the "midgut loop"...
. This pain is typically a dull, poorly localized, visceral
Viscus
In anatomy, a viscus is an internal organ, and viscera is the plural form. The viscera, when removed from a butchered animal, are known collectively as offal...
pain.
As the inflammation progresses, the pain begins to localize more clearly to the right lower quadrant, as the peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
becomes inflamed. This peritoneal inflammation, or peritonitis
Peritonitis
Peritonitis is an inflammation of the peritoneum, the serous membrane that lines part of the abdominal cavity and viscera. Peritonitis may be localised or generalised, and may result from infection or from a non-infectious process.-Abdominal pain and tenderness:The main manifestations of...
, results in rebound tenderness
Rebound tenderness
Rebound tenderness is a clinical sign that a doctor or other health care provider may detect in physical examination of a patient's abdomen. It refers to pain upon removal of pressure rather than application of pressure to the abdomen...
(pain upon removal of pressure rather than application of pressure). In particular, it presents at McBurney's point
McBurney's point
McBurney's point is the name given to the point over the right side of the abdomen that is one-third of the distance from the anterior superior iliac spine to the navel...
, 1/3 of the way along a line drawn from the anterior superior iliac spine
Anterior superior iliac spine
The anterior superior iliac spine is an important landmark of surface anatomy. It refers to the anterior extremity of the iliac crest of the pelvis, which provides attachment for the inguinal ligament, and the sartorius muscle...
to the umbilicus
Navel
The navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...
. 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
Laparotomy
A laparotomy is a surgical procedure involving a large incision through the abdominal wall to gain access into the abdominal cavity. It is also known as coeliotomy.- Terminology :...
or laparoscopy
Laparoscopy
Laparoscopy is an operation performed in the abdomen or pelvis through small incisions with the aid of a camera...
. Untreated, the appendix may rupture, leading to peritonitis
Peritonitis
Peritonitis is an inflammation of the peritoneum, the serous membrane that lines part of the abdominal cavity and viscera. Peritonitis may be localised or generalised, and may result from infection or from a non-infectious process.-Abdominal pain and tenderness:The main manifestations of...
, 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
Sepsis
Sepsis is a potentially deadly medical condition that is characterized by a whole-body inflammatory state and the presence of a known or suspected infection. The body may develop this inflammatory response by the immune system to microbes in the blood, urine, lungs, skin, or other tissues...
; it is now recognized that many cases will resolve when treated non-operatively. In some cases the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relative contraindication
Contraindication
In medicine, a contraindication is a condition or factor that serves as a reason to withhold a certain medical treatment.Some contraindications are absolute, meaning that there are no reasonable circumstances for undertaking a course of action...
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 bladderNeurogenic bladder
Neurogenic bladder refers to dysfunction of the urinary bladder due to disease of the central nervous system or peripheral nerves involved in the control of micturition .-Causes:...
.
Gallery
 |
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
- Cho, Jinny. "Scientists refute Darwin's theory on appendix". The ChronicleThe Chronicle (Duke University)The Chronicle is a daily student newspaper at Duke University in Durham, North Carolina. The Chronicle was first published as The Trinity Chronicle on December 19, 1905. The paper's name was changed to The Chronicle when Trinity College was renamed Duke University following a donation by James...
(Duke UniversityDuke UniversityDuke University is a private research university located in Durham, North Carolina, United States. Founded by Methodists and Quakers in the present day town of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco industrialist James B...
), August 27, 2009. (News article on the above journal article.)