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DMH Part 1 > GI Development > Flashcards

GI Development Flashcards

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Parietal Peritoneum

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peritoneum associated with the body wall

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Visceral Peritoneum

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peritoneum on the surface of abdominal organs

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Greater Sac

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The greater sac is the space that you enter as soon as you penetrate the anterior body wall.

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27
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Lesser Sac

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The lesser sac (omental bursa) is the space behind the stomach. The lesser sac forms as a result of rotation of the stomach, which is a cul-de-sac of the greater sac.

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Peritoneal Organs

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organs of the abdominal cavity that are encased in and suspended by a mesentery

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Retroperitoneal Organs

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organs that have mesentery only on the anterior surface

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Formation of the Gut Tube

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  • At 3 weeks of development, folding of the embryo results in formation of a gut tube lined by endoderm. The head and tail folds, together with lateral folding, result in the formation of the foregut and hindgut. The yolk sac remains outside the embryo, which remains open to the midgut.
  • The points of transition between open midgut and closed foregut and hindgut are called the anterior and posterior intestinal portals.
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Dorsal and Ventral Mesentary

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  • embryonic structures
  • During development the gut is suspended from the dorsal wall by a dorsal mesentery that conveys vessels and nerves to the gut. In addition, the stomach (but not the midgut and hindgut) has a ventral mesentery. These embryonic mesenteries will form the peritoneal ligaments and mesenteries (described later) in the adult.
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Foregut

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  • The buccopharyngeal membrane, a structure that separates the future mouth from the pharynx, temporarily seals the anterior portion of the foregut. The foregut forms the pharynx, esophagus, stomach, and the upper duodenum. In addition, diverticula (outpocketings) of the foregut form the glands of the pharynx, the respiratory tract, liver and gall bladder, and pancreas.
  • The respiratory diverticulum (lung bud) branches off the foregut at 4 weeks of development. This diverticulum divides the esophagus and trachea Formation of a fistula between the esophagus and trachea may result in a blind-ended esophagus that does not permit the passage of food and instead food enters the trachea (Tracheoesophageal fistula)
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Esophagus

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•The esophagus forms distal to the lung bud. The muscle layers of the esophagus are composed of both skeletal and smooth muscle and these cells are derived from splanchnic mesoderm. Through mechanisms that are not understood, sometimes the connection between the esophagus and stomach is lost. This results in a blind-ended esophagus. (Esophageal atresia)

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Stomach

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  • The stomach initially appears as a fusiform dilation of the foregut. It is suspended from the posterior abdominal wall by the dorsal mesentery and the anterior abdominal wall by the ventral mesentery.
  • Stomach Rotation
  1. 90° rotation about the craniocaudal axis such that the dorsal convex surface faces left and the ventral concave border face right (seen by innervation of vagus – the right vagal trunk becomes the posterior vagal trunk, the left vagal trunk becomes the anterior vagal trunk)
  2. The second rotation is a tipping of the caudal (pyloric) end of the stomach in a cranial direction
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Greater and Lesser Omentum

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  • The rotation of the stomach also results in the expansion of the dorsal mesentery to form the greater omentum.
  • The lesser omentum is formed from the ventral mesentery and extends from the stomach to the liver. After this expansion of the dorsal mesentery to form the greater omentum, the layers of the lower portion of the greater omentum fuse, obliterating the lower portion of the lesser sac.
  • In the adult, the omental bursa (lesser sac) is the space directly dorsal to (behind) the stomach.
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Duodenum

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•The duodenum forms at the junction of foregut and midgut distal to the liver bud (see below). The duodenum shifts with stomach rotation to become “C” shaped. In addition, it moves against the posterior abdominal wall and loses its mesentery.

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Liver

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  • During the third week of gestation the liver appears as a hepatic diverticulum of the gut tube. Endodermal strands penetrate the septum transversum and form hepatic cords. As the liver grows, it protrudes into the ventral mesentery. This protrusion of the liver into the ventral mesentery divides the mesentery into the falciform ligament (between the liver and the anterior body wall) and the lesser omentum (between the stomach and the liver). The umbilical vein lies in the free edge of the falciform ligament.
  • The endodermal cells derived from the gut endoderm form the parenchyma of the liver whereas cells derived from the septum transversum (mesoderm) form the sinusoids and Kupffer cells. By the tenth week of development, the liver forms 10% of the total body weight. During fetal development, the liver has major hematopoietic function that largely disappears by birth.
  • The entire endoderm of the developing foregut has the potential to differentiate into liver, but is repressed by factors secreted by the surrounding tissues. However, secretion of fibroblast growth factors (FGFs) by the cardiac mesoderm results in the blockade of these inhibitors in the area of the presumptive liver and allows that region to differentiate into hepatocytes and biliary cells
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Gall Bladder

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•The gall bladder forms as an outgrowth of the hepatic diverticulum. The bile duct rotates together with the ventral pancreatic duct to its final position behind the duodenum.

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Pancreas

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  • The pancreas develops from 2 buds off the foregut - dorsal and ventral. Rotation of the duodenum brings the ventral pancreas around the gut, and the ventral and dorsal pancreas fuse. The ventral pancreatic duct forms the main pancreatic duct that joins with the bile duct and enters the duodenum at the main duodenal papilla. The dorsal pancreatic duct forms the accessory pancreatic duct, which enters the duodenum at the minor duodenal papilla.
  • The molecular regulation of pancreas development is incompletely understood, but it is known that fibroblast growth factor (FGF) and activin produced by the notochord repress Sonic Hedge Hog (SHH) expression in the presumptive pancreatic endoderm. This results in the activation of the pancreatic and duodenal homeobox 1 (PDX gene) that then turns on pancreas specific genes, resulting in the formation of both the endocrine and exocrine pancreas.
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Spleen

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  • The spleen forms from a mesodermal primordium in the greater omentum. Due to the rotation of the stomach it ends up posterior to the stomach on the left side of the body suspended in the greater omentum.
  • The foregut and associated organs are supplied by the celiac artery, a major branch of the abdominal aorta.
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Midgut

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  • The midgut is open to the yolk sac up until the fourth week; however, the gut tube is largely closed by the end of the fourth week. The vitelline duct is the remnant of the yolk stalk that was attached to the midgut.
  • The midgut forms the lower duodenum, jejunum, ileum, cecum, ascending colon, and proximal half of transverse colon.
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Rapid Elongation of Midgut

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Rapid elongation of midgut produces a physiological herniation into the extraembryonic coelom (umbilical cord) at the 6th week.

  1. Herniation is accompanied by a 90° counterclockwise rotation.
  2. Cephalic limb of loop will form duodenum, jejunum, and 2/3 of ileum.
  3. The apex of the loop is the vitelline duct which provides the axis of rotation (can persist after birth as Meckel’s diverticulum)
  4. Caudal loop will form distal ileum, cecum, ascending colon and 2/3 of transverse colon.

•During weeks 10-12, growth of fetus allows the intestine to return to the abdomen.

  1. Extra 180° rotation is seen with return.
  2. Descent of cecum and appendix results in final orientation of gut.

In the final arrangement, the transverse colon is anterior to the duodenum

The midgut forms the bulk of the small intestine as well as approximately one-half of the large intestine. It receives its blood supply from the superior mesenteric artery, a branch of the abdominal aorta just inferior to the celiac trunk

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Hindgut

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  • The hindgut extends from posterior intestinal portal to the cloaca. The cloacal (proctodeal) membrane temporarily seals the cloaca from the exterior. The proctodeum is the external depression of the cloacal membrane.
  • The hindgut forms the distal half of the transverse colon, descending colon, sigmoid colon, rectum, and the superior part of the anal canal.
  • The hindgut is supplied by the inferior mesenteric artery, a branch of the abdominal aorta.
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Partitioning of the Cloaca

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  1. Initially, the urinary system and the gastrointestinal system open into the common cloaca. However, during development, the urorectal septum divides the cloaca into the rectum (anal canal) and urogenital sinus.
  2. Once the urorectal septum partitions the cloaca, the cloacal membrane is called the anal membrane and urogenital membrane. Failure of the anal membrane to break down by the end of the eighth week results in the congenital abnormality of imperforate anus.
  3. The anal canal is formed by invagination of proctodeum. The upper 2/3 is of hindgut origin and lined with endoderm-derived cells, but the lower 1/3 is ectodermal in origin. The boundary between these areas is the pectinate line, an important landmark in the anal canal.
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Allantois

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  • The allantois is a blind-ended diverticulum of the hindgut that extends into the umbilical cord.
  • Although it is vestigial in humans, cells from the allantois form the blood vessels of the placenta and it also contributes to the bladder and urachus.
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Peritoneum

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•The changes in the arrangement of the abdominal structures during development give rise to the final adult distribution. The adult structures are the peritoneal membranes. The peritoneal membranes are derived from the embryological dorsal and ventral mesentery and function to

1) convey vessels and nerves to the organs
2) provide support for the organs in the abdominal cavity
3) allow for some freedom of movement/position/size of the gastrointestinal tract during digestion.

•The space between the parietal and visceral peritoneum is a potential space, but there is a thin film of fluid between them to allow free movement of the abdominal contents during general body movement and motility of the alimentary canal.

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Parietal Peritoneum

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  • the peritoneum associated with the body wall. Sensory innervation of parietal peritoneum is derived from the somatic nerves that supply the overlying body wall and muscles
  • Irritation of parietal peritoneum is felt as sharp, localized pain. Irritation of visceral peritoneum is felt as diffuse pain, often referred to dermatomes supplied by the same nerve segments
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Visceral peritoneum

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•peritoneum associated with abdominal organs Sensory innervation of visceral peritoneum travels in the nerves that supply the underlying organ.

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The peritoneal cavity is divided into:

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  • Greater peritoneal sac, which is the space that is entered when the anterior abdominal wall is opened
  • The omental bursa (lesser peritoneal sac) lies behind the stomach.

The peritoneal cavity is a closed sac in the male, but in the female is open to the exterior via the uterine tubes.

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Peritoneal vs. Retroperitoneal Organs

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  • Peritoneal organs are those organs that are encased in and suspended by peritoneum whereas retroperitoneal organs have peritoneum only on the anterior surface.
  • Embryologically, the entire gastrointestinal tract is suspended by a mesentery and is therefore entirely peritoneal. However, during development various parts of the tract are pushed against the posterior abdominal wall, lose their mesenteries, and become retroperitoneal.
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Peritoneal Organs

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•Peritoneal organs are suspended by a mesentery. These include the stomach, liver, small intestine, transverse colon, sigmoid colon and spleen.

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Retroperitoneal Organs

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•Retroperitoneal organs have a mesentery only on the anterior surface. These include the duodenum, pancreas, ascending and descending colon, and kidneys.

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Peritoneal ligaments associated with foregut structures: Stomach and Duodenum

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  1. Lesser omentum
    a. Hepatogastric ligament
    b. Hepatoduodenal ligament
  2. Greater omentum
    a. Gastrocolic ligament
    b. Gastrosplenic ligament
    c. Gastrophrenic ligament
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Peritoneal ligaments associated with foregut structures: Liver

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  1. Falciform ligament
  2. Coronary ligament
  3. Left triangular ligament
  4. Right triangular ligament
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Peritoneal ligaments associated with foregut structures: Spleen

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  1. Gastrosplenic ligament
  2. Splenorenal ligament
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Ligaments of the Midgut derived structures

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  1. Suspensory ligament of the duodenum (ligament of Treitz)
  2. Mesentery
  3. Transverse Mesocolon
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Ligaments of Hindgut derived structures

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  1. Transverse mesocolon (distal part)
  2. Sigmoid mesocolon

DMH Part 1 (35 decks)

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