Development of GI Tract Flashcards
Midgut remains conencted to yolk sac by _________
Vitelline duct
NOTE: The foregut and hindgut are blind-end tubes
Aside from the primitive gut, what are the other portions of the endorderm-lined cavity?
Yolk sac; allantois
NOTE: Both structures remain outside of the yolk sac
Septum transversum develops into connective tissue of ___________.
Diaphragm
REMEMBER: The diaphragm separates the thorax from the abdomen
What contributions does the endoderm make to the gut?
- Epithelial lining of the digestive tract
- Gives rise to the specific cells (parenchyma) of glands, such as hepatocytes and exocrine and endocrine cells of pancreas
What contributions does the visceral mesoderm give to the gut?
- Forms stroma (connective tissue) for glands
- Muscle
- Connective tissue
- Peritoneal components of the wall of the gut
Why is a portion of the endoderm-lined yolk sac cavity incorporated into the embryo?
As a result of craniocaudal and lateral folding of the embryo
There is a retinoic acid gradient established during folding. What is that gradient in regards to the cranial and caudal ends? What is the function?
- The cranial end is exposed to little or no retinoic acid, while the caudal end is exposed to a high concentration of retinoic acid
- This gradient causes characteris transcription factors to be expressed at each region of the gut tube
List the transciption factors that are expressed as a result of the retinoic acid gradient. What does each transcription factor lead to the production of?
- SOX2- Esophagus and stomach
- PDX1- Duodenum
- CDXC- Small intestine
- CDXA- Large intestine, rectum
The epithelial-mesenchymal interaction is initiated by ____________ expression throughout the gut tube.
Sonic Hedgehog (SHH)
SHH expression upregulates factors in the ___________that then determine the type of structure that forms from gut tube, such as stomach, duodenum, small intestine.
mesoderm
Blood supply to the foregut
Celiac trunk
What are the derivatives of the foregut?
- Distal esophagus
- Stomach
- First half of duodenum
- Liver
- Gallbladder
- Pancreas
NOTE: Though the spleen is not a deriverative of the foregut, it shares the same blood supply as foregut organs.
Blood supply to the midgut
Superior mesenteric artery
Derivatives of the midgut
- 2nd half of duodenum
- Jejunum
- Ileum
- Cecum
- Appendix
- Ascending colon
- Hepatic flexure of colon
- Transverse colon (proximal 2/3rd)
Blood supply to the hindgut
Inferior mesenteric artery
Derivatives of the hindgut
- Distal third of the transverse colon
- Descending colon
- Sigmoid colon
- Rectum
Mesentery
Folds of peritoneum; double layers of peritoneum that enclose an organ and conenct it to the body wall
NOTE: Mesenteries provide pathways for vessels, nerves, and lymphatics to and from abdominal viscera
The entire abdominal gut tube is suspended from the posterior wall by a __________.
Dorsal mesentery
NOTE: By the 5th week, the caudal part of the foregut, the midgut, and a major part of the hindgut are suspended from the abdominal wall by the dorsal mesentery.
Dorsal mesentery of the jejunal and ileal loops forms the ___________.
Mesentery proper
The ventral mesentary is derived from the ___________.
Septum transversum
Where does ventral mesentery exist?
Only in terminal part of esophagus, stomach, and upper part of the duodenum
Growth of the liver divides the ventral mesentery into:
- Lesser omentum
- Falciform ligament
What are the boundaries of the pharynx?
- Oropharyngeal membrane to respiratory diverticulum
NOTE: This is the first part of the foregut, the remainder of the foregut lies caudal to the pharyngeal tube and extends as far caudally as liver outgrowth
What is another name for the respiratory diverticulum?
Trachea-bronchial diverticulum
Trachea becomes partitioned from the esophagus by formation of the ____________.
Tracheoesophageal septum
*This leaves them connected only at the larynx
__________ prevents swallowing of amniotic fluid.
Atresia
NOTE: Esophageal atresia causes the esophagus to end in a blind-ended pouch rather than connecting normally to the stomach.
In esophageal atresia, amniotic fluid is not able to flow from the amniotic sac to the intestinal tract, causing a build up of excess fluid in the amniotic sac called ___________.
Polyhydramnios
Tracheoesphogeal fistula
An abnormal connection between the trachea and esophagus that allow regurgitated contents to contaminate lungs and abdomen with distend air.
REMEMBER: The tracheoesophageal septum is supposed to form between trachea and esophagus
How can positional changes of the stomach be explained?
By assuming that it rotates around a longitudinal and an anteroposterior axis.
How are the greater and lesser curvatures of the stomach formed?
- The stomach rotates 90 degrees clockwise around its longitudinal axis, causing its left side to face anteriorly and its right side to face posterior.
- During rotation, the original posterior wall grows faster than the anterior portion.
NOTE: The anterior portion forms the lesser curvature and the posterior portion forms the greater curvature
As a result of rotation of the stomach, the left vagus nerve innervates the ___________ of the stomach and the right vagus nerve innervates the _________ of the stomach.
Anterior wall; posterior wall
During rotation of the stomach, the caudal or pyloric part moves _____________, and cephalic or cardiac portion moves __________.
To the right and upward; to the left and slightly downward
Free margin of of the falciform ligament contains the _________.
Umbilical vein
NOTE: The umbilical vein is abloterated after brith to form the roud ligament of the liver
The round ligament is also known as the ________.
Ligamentum teres hepatis
What are the components of the hepatoduodenal ligament?
- Bile duct
- Portal vein
- Hepatic artery
The epiploic foramen joins which two structures?
The greater and lesser sac (omental bursa)
The 4-layered greater omentum fuses with the dorsal mesentery of the transverse colon, forming the ___________.
transverse mesocolon
The spleen forms within, and from, the _________.
dorsal mesentery
NOTE: During rotation of the stomach in the 5th week of development, the spleen appears as a mesodermal proliferation between the two leaves of dorsal mesogastrium.
The spleen is connected to the body wall in the region of the left kidney by the ___________ and to the stomach by the _________.
Lienorenal ligament; gastrolienal ligament
What is the landmark that separates the parts of the duodenum into foregut and midgut?
- Proximal to the bile duct: duodenum is considered foregut (supplied by the celiac trunk)
- Distal to the bile duct: Duodenum is considered midgut (supplied by the superior mesenteric artery)
How is a duodenal atresis diagnosable before birth?
Presents as a “double bubble” in ultrasound, along with polyhydramnios
NOTE: The duodenum undergoes a solid stage and then later recanalizes. Failure to recanalize produces a duodenal atresia
Components of the liver are derived from both the endoderm and mesoderm. Which parts respectively?
Endoderm: Hepatocytes
Mesoderm (of septum transversum): Sinusoids and hematopoietic tissue and central diaphragm
Liver and gall bladder connect to the duodenum via _________.
Bile duct
How is the diphragm formed?
- Septum transversum leaves two pericardio-peritoneal canals posteriorly
- These canals are gradually closed by the pleuroperitoneal membranes (folds)
NOTE: The left closes later than the right and is more often deficient.
The left pleuroperitoneal canal closes later than the right and is more often deficient. What can this result in?
Diaphragmatic herniation of the intestines into the lung space, causing pulmonary hypoplasia (incomplete development of the lungs)
Anything that decreases volume of the thoracic cavity may result in ____________.
Pulmonary hypoplasia
NOTE: Normal growth of the fetal lung depends on it being filled with adequate fluid, secreted by pulmonary epithelium.
What are the causes of pulmonary hypoplasia?
- Congenital diaphragmatic hernia
- Oligodramnios (bilateral reanl agenesis)
What signs will a newborn with severe diaphragmatic herniation present with?
- Severe dyspnea
- Bowel sounds over the chest
- Flattened abdomen
How is the final position of the pancreas determined?
Lengthening and fusion of dorsal mesogastrium to posterior body wall
How does the tail end up in a different region than the rest of pancreas?
- Initially, the pancreas grows into dorsal mesoduodenum, but eventually its tail extends into the dorsal mesogastrium
- Since this portion of the dorsal mesogastrium fuses with the dorsal body wall, the tail of the pancreas lies against this region
Which position does the pancreas reside in, intraperitoneal or retroperitoneal?
Retroperitoneal
When the duodenum rotates the dorsal and ventral pancreatic buds fuse to form the ___________.
Uncinate process
Main pancreatic duct is fusion of both ducts; proximal portion of orginal dorsal duct may persist as an __________.
Accessory pancreatic duct
Both exocrine and endocrine pancreatic cells derive from _________.
Endoderm
Pancreas divisum
Failure of ducts to fuse, resulting in small, separate duct systems that can lead to pancreatis
Annular pancreas
Ventral bud may migrate in both anterior and posterior directions around duodenum, creating a ring that may obstruct the duodenum
NOTE: Annular pancreas may be difficult to distinguish from duodenal atresia
Midgut and colon development
- Growth of GI exceeds the volume of the abdominal cavity, whichs causes the tube to herniate through the umbilicus
- While herniated, gut undergoes primary rotation
- This corresponds with rotation of the stomach
- Results in positioning of appendix to the left
- Brings the right vagus to the front, changing name to anterior vagus nerve
3. With growth of embryo, abdominal cavity expands thus drawing gut tube back within abdominal cavity and causing secondary rotation
- Appendix is then positioned to the right
__4. Once in the abdominal cavity, colon continues to grow in length, pushing appendix to its final position in the lower right quadrant
The site of the vitelline duct is in ______.
Distal ileum
Nonrotation can result in____
Entire small intesine on the right side
Malrotation can result in..
- Cecum remains subhepatic
- Subhepatic appendix makes it difficult to diagnose appendicitis
- Mesentary is deficient and may twist; leading to necrosis
Reversed rotation can result in…
- Midgut rotates in opposite direction, so large intestine enters abdomen first, positioning the colon posterior to the duodenum
Omphalocele
- Herniation through enlarged umbilical ring
- Organs lie within cord, covered with amnion
- Associated with serious chromosomal defects and other malformations
Gastroschisis
Bowel herniates through body wall lateral to umbilicus
*Organs are not covered with membrane
Umbilical hernia
Protrusion of bowel through umbilical ring, but covered with skin
*Usually closes with maturation
Merkel’s Diverticulum is usually found on ___________
Ileum
Merkel’s Diverticulum
If vitelline duct persists, it may form a ligament, a cyst or a fistula
In cases of merkel’s diverticulum, intestinal loops may be caught by ligament, causing ________.
Volvulus
The cloaca is divided into ___ and ____.
Urogenital sinus; anorectal canal
What is the mesoderm between the allantois and the hindgut called?
Urorectal septum
Rupture of the ________ forms the anal canal.
Anal membrane
Mechanism by which hidgut is partioned
- Hindgut initially enters the cloaca.
- Mesoderm between the allantois and hindgut grows toward the cloacal membrane and divides it into urogenital sinus and anorectal canal.
The anal canal is divided into superior and inferior portions. Where do they join?
At the pectinate line
*•demarks junction between different types of epithelium and different blood and nerve supplies
What is each portion of the anal canal derived from?
- Superior portion: Endoderm (since it comes from the cloaca)
- Inferior portion: Ectoderm (from the proctodeum)
Hindgut abnormalities
- Urorectal fistula
- Rectovaginal fistula
- Rectoperineal fistula
- Imperforate anus
What is Hirschsprung Disease?
Hirschsprung’s disease is a birth defect in which nerves are missing from parts of the intestine. The most prominent symptom is constipation.
Cause of Hirshsprung Disease
Results from failure of neural crest cells to migrate to form enteric ganglia
NOTE: Without ganglia, the wall fails to relax and comes constricted. The area proximal to the aganglionic area becomes distended.
