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The foregut is the anterior part of the
alimentary canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
, from the
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
to the
duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
at the entrance of the
bile duct A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. ...
. Beyond the stomach, the foregut is attached to the abdominal walls by
mesentery The mesentery is an organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intesti ...
. The foregut arises from the
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
, developing from the folding primitive gut, and is developmentally distinct from the midgut and
hindgut The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal ju ...
. Although the term “foregut” is typically used in reference to the anterior section of the primitive gut, components of the adult gut can also be described with this designation. Pain in the epigastric region, just below the intersection of the ribs, typically refers to structures in the adult foregut.


Adult foregut


Components

*
Esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
*
Respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to ...
(lower respiratory tract) * Stomach *
Duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
(up to ampulla of vater) *
Liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
* Gallbladder *
Pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
*
Spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
– The spleen arises from the
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Emb ...
al dorsal mesentery (the foregut arises from the endoderm not mesoderm). But the spleen shares the same blood supply as many of the mature structures that arise from the foregut (i.e. the celiac artery)


Nerve supply

The enteric nervous system is one of the major divisions of the nervous system derived from neural crest cells. In mammals, it is composed of large number of interconnected ganglia that are arranged into two concentric rings embedded throughout the gut wall, beginning in the esophagus and ending in the anus. The main function of the ENS is to control the secretory activity of the gastrointestinal glands and peristalsis of the gastrointestinal wall. A large number of organs derived from the developing foregut also receive input from the
vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
, which also works in tandem with the ENS to control gastrointestinal function.


Development

The foregut develops from a cranial region of
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
created after the initial cephalocaudal folding of the embryo. Starting at the
stomodeum The stomodeum, also called stomatodeum or stomatodaeum, is a depression between the brain and the pericardium in an embryo, and is the precursor to the mouth and the anterior lobe of the pituitary gland. Development The mouth is developed par ...
, a rapid expansion of the primitive gut forms the
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
, from which the respiratory bud branches off.Sadler, T.W, (2011) Langman’s Medical Embryology (12th edition), LWW, Baltimore, MD During early foregut development, the esophagus lengthens considerably, reaching its proportional postnatal size. Simultaneously, the stomach begins to expand in width dorsally and ventrally in an asymmetric manner. This asymmetric expansion creates two curvatures, with the ventral side creating the lesser curvature and the dorsal side creating the greater curvature. The expanding dorsal stomach wall then rotates the on its transverse plane, pulling its caudal portion upward and forcing the upper duodenum into a C shape. This rotation positions the left
vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
anteriorly and right vagus nerve posteriorly. While the hindgut and midgut are only attached dorsally to the body wall by a fold of
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
, the foregut also has a ventral attachment. Its two attachments are commonly referred to as the dorsal mesogastrium and the ventral mesogastrium. As the stomach rotates during early development, the dorsal and ventral mesentery rotate with it; this rotation produces a space anterior to the expanding stomach called the greater sac, and a space posterior to the stomach called the lesser sac. After this rotation the dorsal mesentery thins and forms the greater omentum, which is attached to the greater curvature of the stomach. The ventral mesentery forms the lesser omentum, and is attached to the developing
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
. In the adult, these connective structures of omentum and mesentery form the
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
, and act as an insulating and protective layer while also supplying organs with
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
and lymph vessels as well as nerves. Arterial supply to all these structures is from the
celiac trunk The celiac () artery (also spelled ''coeliac''), also known as the celiac trunk or truncus coeliacus, is the first major branch of the abdominal aorta. It is about 1.25 cm in length. Branching from the aorta at thoracic vertebra 12 (T12) in ...
, and venous drainage is by the
portal venous system In the circulatory system of animals, a portal venous system occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart. Both capillary beds and the blood vessels that connect them are con ...
. Lymph from these organs is drained to the prevertebral celiac nodes at the origin of the celiac artery from the aorta.


Signalling pathways

In vertebrates, functional differentiation continues even after birth, with the transformation into the adult phenotype occurring through epithelial–mesenchymal transition.Sanderson, I. Walker, W.A., (1998) Development of the Gastrointestinal Tract. B.C. Decker Inc, Lewiston, NY Patterning events that determine tissue differentiation in vertebrates rely on several hox genes, the
morphogen A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the vario ...
sonic hedgehog, and transcription factors such as sox2 and
sox9 Transcription factor SOX-9 is a protein that in humans is encoded by the ''SOX9'' gene. Function SOX-9 recognizes the sequence CCTTGAG along with other members of the HMG-box class DNA-binding proteins. It is expressed by proliferating but n ...
.Faure, S. De Santa Barbara, P. (2011) Molecular Embryology of the Foregut, Faure S, De Santa Barbara P. Molecular embryology of the foregut. Journal of Pediatric Gastroenterology and Nutrition. 52.Suppl 1: S2–3. PMC. Web. 3 Mar. 2015. Recent research has suggested that most foregut malformations are due to defects in these signalling pathways, with sonic hedgehog
gene knockout A gene knockout (abbreviation: KO) is a genetic technique in which one of an organism's genes is made inoperative ("knocked out" of the organism). However, KO can also refer to the gene that is knocked out or the organism that carries the gene kno ...
mice showing phenotypes similar to those seen in patients with esophageal atresia/stenosis, tracheo-esophageal fistula, and respiratory tract anomalies.Litingtung, Y. Lei, L. Westphal, H. Chiang, C. (1998) Sonic Hedgehog is Essential to Foregut Development. Nature Genetics 20, 58–61 doi:10.1038/1717


Clinical significance

* Esophageal atresia is a congenital defect of the digestive system in which the continuity of the esophageal wall is interrupted. In most cases, the upper esophagus fails to connect with the lower esophagus and stomach.Pansky, B. (1982). Chapter 87: Congenital Malformations of the Digestive System: Foregut Malformations. In Review of medical embryology. New York: Macmillan. *
Esophageal stricture A benign esophageal stricture, or peptic stricture, is a narrowing or tightening of the esophagus that causes swallowing difficulties. Signs and symptoms Symptoms of esophageal strictures include heartburn, bitter or acid taste in the mouth, chok ...
is the narrowing of the esophagus resulting in swallowing difficulties. * Pyloric stenosis is the thickening ( hypertrophy) of the muscle that forms the pyloric sphincter, obstructing the passage of food. *
Biliary atresia Biliary atresia, also known as extrahepatic ductopenia and progressive obliterative cholangiopathy, is a childhood disease of the liver in which one or more bile ducts are abnormally narrow, blocked, or absent. It can be congenital or acquired. I ...
is a congenital defect where the common bile duct, which connects the small intestine to the liver, is obstructed or absent. *
Pancreatic disease Pancreatic diseases are diseases that affect the pancreas, an organ in most vertebrates and in humans and other mammals located in the abdomen. The pancreas plays a role in the digestive and endocrine system, producing enzymes which aid the digesti ...
exist as both congenital and acquired diseases. Two of the well known types of congenital defect are: Pancreatic divisum, where the pancreatic duct fails to form, and
Annular pancreas Annular pancreas is a rare condition in which the second part of the duodenum is surrounded by a ring of pancreatic tissue continuous with the head of the pancreas. This portion of the pancreas can constrict the duodenum and block or impair the fl ...
, where extra pancreatic tissue grows and wraps around the
duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
leading to obstruction by constriction.


References


External links


med.nyu.edu – embryology, Bogart, B




{{Authority control Embryology of digestive system