Development of the digestive system week 1 Flashcards
By the beginning of what week is the primitive gut formed?
The primitive gut is formed by the beginning of the Week 4, extends from the oropharyngeal membrane to the cloacal membrane. The digestive system is lined by an epithelial surface which varies according to the demands and functions of the area of the gastrointestinal tract.
____ ____ of the embryo forms the gut tube and body cavities.
What germ layer is the gut tube derived from?
The gut tube is derived from endoderm.
What are the 3 parts of the digestive tract? What arteries are they supplied by? What main artery do those arteries branch from?
foregut: supplied by the celiac trunk
midgut: superior mesenteric aa
hindgut: inferior mesenteric aa
all of these arteries are branches of the abdominal aa
Name the adult structures derived from the foregut, midgut, and hindgut.
What embryological pocess during development of the stomach causes formation of the greater and lesser curvatures?
What process occurs to place the stomach in its adult position?
What are the consequences of these 2 processes?
Differential growth rate results in the development of the greater and lesser curvatures; (the posterior portion of the primitive stomach grows at a substantially higher rate.)
2 Rotations:
• rotation along a longitudinal (cranial-caudal) axis for 90 degrees (posterior is now left; anterior is now right). This makes the right vagal trunk posterior and the left vagal trunk anterior.
• rotation along an anterior/posterior axis makes it look like the stomach is tilted so that the lesser curvature is “up” and the greater curvature is “down.”
Consequences of differential growth rates and rotations:
• Greater Curvature: former posterior part of the stomach now is on the left/inferior side.
• Lesser Curvature: former anterior part of the stomach now is on the right/superior side.
What 3 organs begin as invaginations from the foregu?
liver, gallbladder, and pancreas
What gives rise to both the liver and gallbladder (and associated ducts)? Which mesentery does the hepatic bud grow into?
The hepatic diverticulum gives rise to both the liver and gall bladder, and their associated ducts.
The hepatic bud grows into the ventral mesentery.
see slide 14 of notes.
Describe the development of the pancreas.
The pancreas develops from ventral and dorsal buds which evaginate from the gut tube. The ventral bud rotates around to fuse with the dorsal bud to form a single pancreas.
What parts of the pancreas to the dorsal and ventral buds form?
The dorsal pancreatic bud forms the neck, body, and tail of the pancreas.
The ventral pancreatic bud forms the head and uncinate process.
What is an annular pancreas? What are complications of it?
Tissue from the original ventral bud may persist surrounding the duodenum. This is referred to as an annular pancreas and causes an obstruction of the duodenum.
The 1st and 2nd parts of the duodenum mark the transition from the foregut. What structures is this transition near? What are the consequences as it pertains to vascular supply?
The 1st and 2nd parts of the duodenum mark the transition from foregut to midgut. This transition is near the entrance of the bile and pancreatic ducts into the duodenum. Thus, there is some overlapping of vascular supply; Celiac trunk (foregut )—Superior mesenteric artery (midgut.) Thus, the pancreas receives some blood supply from the superior mesenteric aa in addition to the celiac trunk.
Describe the processes of midgut herniation and reduction. When in development does midgut herniation occur?
Midgut herniation allows the small and large intestines to grow outside of the body and reduction of the midgut loop places everything into its adult position.
- Midgut herniates into the umbilical cord (Weeks 6-10 and undergoes rapid growth.
- Midgut reduction (return to the abdomen) requires a rotation of 90⁰ then 180⁰ degrees (total = 270⁰).
- Rotation is counterclockwise and around the axis of the superior mesenteric artery.
see slides 17-19 of notes
After reduction of the intestines, what structure grows over them?
greater omentum. slide 19 of notes
What is the end of the hindgut called? Describe the formation of the anal canal.
The formation of the anal canal requires a septum to separate the digestive system from urogenital outflow.
The end of the hindgut is called the Cloaca (sewer) where outflow from the digestive, urinary and reproductive systems occur. In order to separate these systems a urorectal septum grows forming the rectum and cranial part of anal canal (dorsal) and the urogenital sinus (ventral).
What is the pectinate line?
What is the anocutaneous line (aka white line)?
The pectinate line marks the end of the original hindgut. It demarcates the transition of epithelium from simple columnar to nonkeratinized stratified sqaumous.
The anocutaneous line (aka white line) marks the transition of epithelium from nonkeratinized stratified sqaumous to keratinzed stratified.
The original mesenterires are called the ventral and dorsal mesogastrium. What structures do these mesenteries support? What omentum/other mesenteries do they contribute to?
What is the purpose of the falciform ligament?
Ventral Mesentery (mesogastrium): • suspends the foregut from the ventral body wall. • does not extend inferior to the liver . • contributes to the lesser omentum which connects the liver to the stomach and duodenum.
Dorsal Mesentery (mesogastrium): • suspends the foregut, midgut and hindgut from the dorsal body wall. • contributes to the greater omentum, the mesentery of the small intestine and the gastro-splenic and splenorenal ligaments. parts of the dorsal mesentery disappear and other parts remain in the adult.
falciform ligament: connects the liver to the anterior body wall.
Explain the formation of the lesser and greater sac.
What connects the 2 sacs? Where can this be found in the body?
- Recall that the digestive tract starts with a simple tube suspended from the body wall by the ventral and dorsal mesogastrium. (mesogastrium = mesentery)
- The large area surrounding the tube in the abdomen is the primitive peritoneal cavity.
- As the gut tube develops and undergoes rotation, mesenteries are rearranged, spaces are created and structures are pressed against the posterior body wall.
- With rotation (lesser omentum rotates to right. see attached pic) the peritoneal cavity is subdivided into a lesser sac (behind the stomach) and the greater sac (the larger of the 2 subdivisions.) the lesser sac is also referred to as the omental bursa.
- A small opening referred to as the Epiploic Foramen of Winslow connects these two sacs . This opening can be located at the right edge of the hepatoduodenal ligament.
What is the significance of the lesser sac as it pertains to ulcers and cancer?
If someone has an ulcer in the posterior portion of the stomach, blood can collect in the lesser sac. People can also develop cancer in the lesser sac.
What does it mean for an organ to be retroperitoneal? How does this process occur? Give examples of retroperitoneal organs.
- Parts of the dorsal mesentery fuse abdominal structures to the posterior body wall rendering them immobile.
- These structures are considered to be retroperitoneal (i.e. not within the peritoneal cavity.)
- Examples include the ascending and descending parts of the large intestine and part of the pancreas (which is considered secondarily retroperitoneal.)
Organs that are fused to the abdominal wall are done so because they are organs that should not have mobility.
What is omphalocele?
Omphalocele - occurs when the midgut loop fails to return to the abdominal cavity and remains in the umbilical stalk. The viscera herniate through the umbilical ring and are contained in a shiny sac of amnion at the base of the umbilical cord. Omphalocele is often associated with multiple anomalies of the heart and nervous system with a high mortality rate (25%).
What is gastroschisis?
Gastroschisis - occurs when the abdominal viscera herniate through the body wall directly into the amniotic cavity, usually to the right of the umbilicus. This is a defect in the closure of the lateral body folds and a weakness of the anterior wall at the site of absorption of the right umbilical vein. Note that the viscera do not protrude through the umbilical ring and are not enclosed in a sac of amnion.
What is Ileal (Meckel) diverticulum?
Ileal (Meckel) Diverticulum - occurs when a remnant of the vitelline duct persists, forming a blind pouch on the antimesenteric border of the ileum where feces stasis can occur. They are found 2 feet from the ileocecal junction, are 2 inches long, and appear in 2% of the population. Thus, 2, 2 and 2 is a good way to remember this!
What is Colonic Aganglionosis (Hirschsprung Disease)?
results from the failure of neural crest cells to form the myenteric plexus in the sigmoid colon and rectum (neural crest cells form ganglia in the walls of organs)
• loss of peristalsis and immobility of the hindgut
• fecal retention
• abdominal distention of the transverse colon (megacolon).
Can be fixed by resectioning affected portion of the colon.
