Development+of+the+GI+System+I+&+II

=**Objectives**=

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 * 1. List the three major regions of the embryonic gut and define the boundaries (limits) of each in terms of embryonic structures and definitive structures.**

The foregut, midgut, and hindgut are the three major regions of the embryonic gut.

The primordial gut is closed at its cranial end by the oropharyngeal membrane and at the caudal end by the cloacal membrane. The epithelium at the cranial and caudal ends of the tract is derived from ectoderm of the stomodeum (mouth) and proctodeum (anal pit). The oropharyngeal and cloacal membrane eventually ruptures and becomes patent with the rest of the digestive tract.

The primordial gut is primarily lined by endoderm which forms the epithelium and glands of the digestive tract. The splanchnic mesenchyme surrounding the primordial gut forms the smooth muscle and connective tissue layers of the digestive tract.

The foregut gives rise to the esophagus, stomach, proximal half of the duodenum, liver, and pancreas.

The midgut gives rise to the distal half of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and the right 2/3 of the transverse colon.

The hindgut develops into the left 1/3 of the transverse colon, the descending colon, sigmoid colon and the rectum down to the ano-rectal line (endoderm-ectoderm junction).

**Foregut**

 * 2. List the structures derived from the foregut. What is the blood supply of the structures derived from the caudal portion of the foregut?**

The foregut gives rise to the structures from the oral cavity to the duodenum, including the salivary glands, tonsils, upper respiratory tract, esophagus, stomach, proximal half of the duodenum, liver, and pancreas. The blood supply of the structures derived from the caudal portion of the foregut is the celiac artery and its derivatives.


 * 3. What are the germ layers of origin for the epithelial structures derived from the foregut; the smooth muscle and connective tissue; vascular supply; nerve supply?**

The epithelium and glands derived from the foregut originate from endothelium. The smooth muscle and connective tissue are derived from splanchnic mesenchyme. The vascular supply is derived from splanchnic mesoderm. Neural crest cells are the origin of the sympathetic post ganglion cells that form the pre-vertebral and post-vertebral ganglia. Neural crest cells also are the origin of the post-ganglionic neurons called Auerbach’s and Meissner’s plexuses for parasympathetic innervation by the vagus nerve.


 * 4. Describe the septum transversum and the structures that are derived from it.**

The septum transversum is a thick plate of splanchnic mesoderm that occupies the space between the thoracic cavity and the yolk stalk and eventually develops into the central tendon of the diaphragm.


 * 5. Describe the results of the rotation of the stomach.**

During development of the stomach, it rotates clockwise 90 degrees (when viewing from above), causing the greater curvature to become positioned on the left-side of the stomach. As the stomach rotates, the dorsal mesogastrum (mesentery of the stomach) is pulled to the left, producing a cavity behind the stomach that becomes the lesser sac. The lesser sac expands superiorly and inferiorly while the dorsal mesogastrium continues to elongate inferiorly to form the greater omentum.

The duodenum also rotates clockwise during development at the same time as the stomach rotation, maintaining the S-shaped curve from stomach to duodenum. Additionally, the ventral pancreatic bud also sweeps clockwise to end up next to the dorsal pancreatic bud for subsequent fusion.


 * 6. List the derivatives of the hepatic diverticulum.**

The hepatic diverticulum is an evagination from the ventral surface of the caudal foregut that forms the liver, gall bladder, and biliary apparatus.

The hepatic diverticulum grows into the septum transversum where it forms the ventral mesentery. Proliferating endodermal cells form cords of hepatic cells and epithelia of the biliary system within the liver. The cords anastomose around the endothelium lined spaces that are forming the hepatic sinusoids. Connective tissue, endothelial cells, hemopoetic tissues, and kupffer cells arise from the splanchnic mesenchyme of the ventral mesentery.

The gall bladder and cystic duct form from the small caudal part of the hepatic diverticulum. The extrahepatic biliary apparatus is initially solid cords of epithelial cells that later canalize. Initially, the bile duct is attached to the ventral surface of the duodenal loop but rotation of the duodenum moves it to the posterior surface.


 * 7. Describe the formation of the liver within the substance of the septum transversum.**

The hepatic diverticulum grows into the septum transversum where it forms the ventral mesentery. Proliferating endodermal cells form cords of hepatic cells and epithelia of the biliary system within the liver. The cords anastomose around the endothelium lined spaces that are forming the hepatic sinusoids. Connective tissue, endothelial cells, hemopoetic tissues, and kupffer cells arise from the splanchnic mesenchyme of the ventral mesentery.


 * 8. Describe the positional changes involved in the formation of the pancreas, especially the formation of the definitive duct system.**

The pancreas develops from 2 evaginations of the caudal foregut called the pancreatic buds. The neck, body, and tail is derived from the larger dorsal pancreatic bud. The smaller ventral bud froms the uncinate process (part of head behind the superior mesenteric vessels) and part of the head. During development, the ventral pancreatic bud rotates clockwise with the stomach and such that it ends up adjacent to the dorsal pancreatic bud with which it fuses with.

The main pancreatic duct is derived from the duct of the ventral bud and the distal part of the dorsal bud duct. The accessory pancreatic duct represents the proximal end of the duct of the dorsal bud.

The endodermal cells form both the exocrine and endocrine pancreatic tissues. The splanchnic mesenchyme forms the connective tissue and vascular elements of the pancreas.


 * 9. Define the terms: stenosis, atresia, hernias, polyhydramnios.**

__Stenosis__ Stenosis is the narrowing of the lumen of a tubular structure. This can happen when recanalization fails to occur properly, such as in the esophagus and duodenum.

__Artesia__ Failure of a structure to form completely, leaving a blind end and can occur in the esophagus, duodenum, and extrahepatic biliary apparatus.

__Hernias__ The protrusion of a tissue, structure, or part of an organ through the muscular tissue or membrane by which it is normally contained. For example, congential short esophagus can result in the hernation of the stomach through the esophageal hiatus of the diaphragm.

__Polyhydramnios__ A condition of excess amniotic fluid that can be cause by esophageal artesia or stenosis and other conditions where the embryo is unable to absorb amniotic fluid.


 * 10. Describe the roles of the dorsal and ventral mesentery in the development of the digestive system.**

The dorsal mesentery encloses the viscera of the foregut, midgut, and hindgut. During rotation of the stomach and duodenum, it lengthens inferiorly and expands to become the Greater Omentum and associated gastric ligaments. With some organs, the dorsal mesentery serves as an attachment to the dorsal abdominal wall, and may fuse, causing those organs to be fixed ( secondarily retroperitoneal).

The ventral mesentery degenerates in the midgut and hindgut, but remains in the foregut, where it may form specialized structures, such as the lesser omentum and its ligaments, the falciform ligament, and the visceral peritoneum of the liver.

**Midgut**

 * 1. List the definitive structures derived from the midgut.**

The midgut contains the distal part of the duodenum, the jejunum, ileum, the ascending colon (including cecum and appendix) and the proximal 2/3 of the transverse colon.


 * 2. What is the blood supply and autonomic innervation pattern of the structures formed from the embryonic midgut?**

The artery supplying the midgut is the Superior Mesenteric Artery. The vein is the Superior Mesenteric Vein, and the Lymph drains to the Superior Mesenteric Nodes. Parasympathetic innervation is supplied by the Vagus (CN X), and the sympathetic innervation is provided by the mesenteric plexuses of the thoracic splanchnic nerves.


 * 3. Describe the positional changes that take place during the process of midgut rotation.**

At week 6, growth in the midgut exceeds capacity of the intraembryonic coelom and the primordia of midgut organs herniates into the remnans of the communication with the extraembryonic coelom tin the proximal part of the umbilical cord. The cranially oriented portion of the midgut loop forms the small intestine, and the caudally oriented portion of the midgut loop forms the terminal part of the ileum and large intestine. After the physiological herniation, the intestinal primordia performs a 90-degree counterclockwise rotation about the Superior Mesenteric Artery. During continued growth, the intestinal contents again perform a 180-degree counterclockwise rotation about the SMA as the midgut loop recedes into the intraembryonic cavity. The ascending and descending colon contact the lateral walls, and their mesenteries fuse posteriorly against the posterior abdominal wall.


 * 4. Correlate the definitive organ relationships within the abdominal cavity with the development history of the region.**

The growth, extrusion, rotation, and subsequent re-entrance of the intestinal tract causes the positional relationships seen in the adult body. First, the outgrowth of the small and large intestine into the proximal part of the umbilical cord enters the remains of the extraembryonic coleom. After rotating 90 degrees counterclockwise(around the axis of the SMA), the small intestine returns to the intraembryonic coleom. As the large intestine follows, it rotates another 180 degrees counterclockwise, allowing the cecum and ascending colon to take up residence on the right side of the abdomen, and places the transverse colon horizontally. This rotational movement also places the ascending and descending colon, formerly intraperitoneal organs, into position to become secondarily retroperitoneal (their mesentery fuses with the posterior abdominal wall in their definitive locations in the abdominal cavity).


 * 5. What is Meckel's form of ileal diverticulum? Umbilico-ileal fistula? What is the congenital basis of these conditions?**

The Meckel's form of ileal diverticulum is an outpocketing of the ileum that may be attached to the umbilicus by a fibrous cord and may contain ectopic gastric tissue or pancreatic tissue. The umbilico-ileal fistula is a form of Meckel's ileal diverticulum where the yolk stalk is patent and there is communication between the umbilicus and the ileum. The congenital basis is due to the persistence of the yolk stalk, which is a communication between the yolk sac and the anterior-most portion of the midgut loop, at the level of the umbilicus.

Different forms of Meckel's Ileal Diverticulum: (1) Diverticulum with ulcer (2) Diverticulum with fibrous cord (3) Diverticulum with patent yolk stalk (vitelline duct/umbilico-ileal fistula) (4) Vitelline and umbilical systs, usually fluid filled (5) Volvulus of diverticulum (6) Persistent vitelline artery


 * 6. Briefly describe the positional changes that occur in the arteries and veins associated with the caudal foregut and midgut.**

//(note: I really don't understand this question well, so I'm not sure I've answered it)// The superior mesenteric artery goes from a transversely positioned vessel at the level of the umbilicus to a descending route within the mesentery of the small intestine(inferior path). the duodenum rotates so as to position its two inferior sections posterior to the SMA and SMV. The proximal portion of the duodenum and the stomach course anteriorly to the SMA and SMV as they join the aorta and IVC.


 * 7. Briefly describe the formation of enteric autonomic ganglia of the digestive system from neural crest cells.**

Neural Crest Cells migrate to form the sympathetic innervation of the digestive system as splanchnic nerves from para-and prevertebral ganglia. The meyenteric and submucosal ganglia, which form plexuses in the muscularis externa intrinsic to the GI tract, are a result of Vagus nerve innervation.


 * 8. Describe the developmental errors leading to:**

__Esophageal Atresia__ Failure of the esophagus to form, usually due to failure to recanalize during development.

__Duodenal and Pyloric Stenosis__ Narrowing of the duodenum or pyloris; can be due to incomplete recanalization.

__Biliary Atresia__ Failure of recanalization of the extrahepatic biliary apparatus during development.

__Anular Pancreas__ Bifurcation of the ventral bud of the pancreas that can result in stenosis of the duodenum.

__Omphalocoel__ Failure of portion of intestine to return to the abdominal cavity on week 10. Herniated mass is surrounded by epithelium of umbilical cord.

__Hiatal and Umbilical Hernias__ Hiatal hernias develop when the phrenicoesophageal ligament causes the stomach to herniated through the hiatus and into the thorax.

Umbilical hernias develop after successful return of interstine to the abdominal cavity due to defect in closure of umbilicus. Herniated mass is surrounded by skin.

__Segmental Duplication of the Intestinal Tract__ Defect in recanalization of the intestine results in duplication of the intestinal tract.

__Malrotation of the Hindgut Resulting in Subhepatic Cecum__ Cecum does not descend and become fixed to the liver.

__Malrotation of the Hindgut Resulting in Mobile Cecum__ Results from incomplete fixation of ascending colon and causes an increase risk of volvulus.

__Malrotation of the Hindgut Resulting in Volvulus__ Incomplete rotation results in the cecum being located just inferior to the pyloris of the stomach. Connective tissue bands pass over the duodenum to anchor the cecum. This condition of mixed rotation results in volvulus of the small intestine.

**Hindgut**

 * 1. Define/Describe:**

__Allantois__ An extension of the yolk sac that is continuous with the region of the urogenital tract that will become the urinary bladder.

__Cloaca__ Terminal part of the hindgut lined by endoderm.

__Cloacal Membrane__ Cloacal membrane is in contact with the ectodermal surface that seals off the cloaca during hindgut development.

__Anal Pit__ Another name for proctodeum

__Proctodeum__ The region of the surface ectoderm overlying the cloacal membrane.

__Urorectal Septum__ Subdivides the cloaca into a ventral urogenital region and a dorsal anal-rectal region.

The proctodeum is the region of the surface embryonic ectoderm overlying the cloacal membrane..
 * 2. Describe the process by which the proctodeum (anal pit) is formed.**


 * 3. Define/Describe the location of the tissue that will form the urorectal septum.**

The urorectal septum is comprised of folds of embryonic mesodermal tissue that fuse and grow towards the cloacal membrane, separating the allantois and the cloaca.


 * 4. List the major structures (organs) that are definitively situated anterior to the tissue plane created by the urorectal septum; those situated posterior to the plane.**

The organs and muscles of the urogenital triangle are anterior to the urorectal septum (urinary bladder, male and female gonadal organs, etc). The organs and muscles of the rectoanal triangle are derived from the area posterior to the septum (rectum, anal canal, anus).


 * 5. Describe the vascular supply (blood and lymphatic) and innervation of the portion of the anal canal derived from the hindgut; the portion of the anal canal derived from the proctodeum.**

The portion of the anal canal derived from the hindgut is above the pectinate line. The portion of the anal canal derived from the proctodeum is below the pectinate line.


 * Relaton to Pectinate Line || Blood Supply || Lymphatics || Innervation ||
 * Above (Hindgut Derived) || Branches of the inferior mesenteric artery || Lumbar lymph nodes || Autonomics; stretch sensation supplied by sympathetics ||
 * Below (Proctodeum Derived || Branches of the internal iliac || Inguinal lymph nodes || Somatic; Pain sensation supplied by spinal nerves ||


 * 6. Explain:**

__Congenital Megacolon__ Excessively dilated large intestine. Hirschsprung disease caused by the failure of neural crest cells to populate the large intestine, which results in the absence of myenteric ganglion and causing congenital megacolon.

__Imperforate Anus__ Absence of normal anal opening.

__Fistulae Between Rectum and Urethra in Male__ Communication between the rectum and urethra in a male (rectourethral fistula) and can occur with anorectal agenesis.

__Anoperineal Fistula__ Communication between the anal canal with the perineum (instead of connecting with the anal pit) that can occur with anal agenesis.

__Anal Stenosis__ Narrowing of anus and anal canal due to dorsal deviation of the urorectal septum.

__Anorectal Agenesis__ Anus and rectum fail to form, often resulting from a fistula (such as rectourethral fistula), causing a communicataion between the rectum and another structure.

**Challenge**

 * 1. Summarize the vascular supply of each portion of the gut, (stomodeum, supradiphragmatic foregut, infradiaphragmatic foregut, midgut, hindgut, proctodeum).**

Stomodeum - Supradiaphragmatic foregut - Infradiaphragmatic foregut - Celiac Trunk and IVC Midgut - Superior Mesenteric Vessels and portal venous system Hindgut -Inferior Mesenteric Vessels and portal venous system Proctodeum


 * 2. Summarize the innervation of the portions of the gut mentioned above.**

Parasympathetic innervation is provided by the vagus nerve and its trunks and plexuses (CNX) forming the Myenteric and Submucosal plexuses intrinsic to the GI tract. Sympathetic innervation is a result of the splanchnic nerves, which are comprised of contributions from mixed spinal nerves of T5-12, descending via the paravertebral ganglia without synapsing, then traveling to a prevertebral ganglion where it synapses, then hitches a ride on an artery to its target tissue.