Development Of The GI Tract Flashcards
When does the GI tract develop in vivo?
3-12 weeks
When are the primary germ layers formed?
Gastrulation
What is the embryo made of at the beginning of the third week?
Epiblasts and hypoblasts
What are the steps in gastrulation?
- epiblast cells in the midline begin to ingress, starting from the caudal end
- ingressing cells differentiate into the mesoderm
- epiblast gives rise to the ectoderm
- hypoblast and epiblast gives rise to the endoderm
What are the ingressing epiblast cells visible as?
The primitive streak
What types of cells do the ingressing cells differentiate into?
Surrounding muscles, connective tissue, mesenteries and blood vessels
What does the epiblast form?
Innervation of the gut (enteric nervous system)
What does the hypoblast form?
Epithelium of the gut tubes and glands
How many primary layers does gastrulation generate?
3
How is the gut tube formed initially?
Folding of sheets of cells in two directions
What are the two directions that the sheets of cells are folding in?
Along the midline (cranial-caudal axis)
Towards the yolk sac at the cranial and caudal ends
What part of the nervous system is important in gut motility?
Splanchnic nervous system
What does Vitelline mean?
Relating to, or associated with the yolk of an egg
How many arterial branches are there to the thoracic oesophagus?
5
Where does the sympathetic ganglia develop next to?
Major branches of the aorta
What nerve innervates the foregut?
Coeliac ganglion
What nerve innervates the midgut?
Superior mesenteric ganglion
What nerve innervates the hindgut?
Inferior mesenteric ganglion
How does the stomach arise?
Expansion and rotation
When does stomach development start?
Week 4
What happens to the stomach in week 4?
Tube dilates forming an enlarged lumen
What are the steps in stomach development?
90 deg turn about the cranial caudal axis -> dorsal border grows more rapidly than the ventral
What fixes the mature gut in place?
Fusion of the mesenteries with the posterior abdominal wall
How is the lesser sac formed?
As the stomach rotates, the dorsal mesogastrium is drawn with it and encloses a space (the lesser sac)
What is a pyloric stenosis?
Gastric outlet obstruction caused by smooth muscle hypertrophy
What are the symptoms of pyloric stenosis?
Projectile vomiting shortly after feeding
What is the inducing signal for the formation of the liver?
Heart to ventral gut endoderm
How does liver formation start?
Hepatic diverticulum grows into the mesenchyme of septum transversum
What happens when the liver exceeds the size of the septum transversum?
Expands into the ventral mesentery
What does the remaining ventral mesentery give rise to?
The falciform ligament between the liver and body wall
Lesser omentum between liver and stomach
Where does pancreas formation originate from?
2 pancreas buds
What are the two pancreas buds called and where are they found?
Dorsal from the duodenal endoderm
Ventral from the hepatic diverticulum
What is the dorsal pancreatic bud induced by?
Notochord
What is the ventral pancreatic bud induced by?
Hepatic mesoderm
What happens to the two pancreas buds as the duodenum rotates?
Ventral and dorsal buds meet and fuse
What does bifid mean?
Bi-lobed
What happens if the ventral bud is bifid and one rotates anteriorly?
An annular pancreas forms, which can obstruct the duodenum
What mesentery is attached to the intestine?
Dorsal mesentery
What causes the folding of the gut?
The gut grows faster than the mesentery
What are the first 3 steps in intestine development?
- Intestines rotate around the superior mesenteric artery
- herniate into umbilical stalk at 6/7 weeks
- intestines return to body at 10 weeks?
Why do the intestines herniate into the umbilical stalk?
Abdomen is too small to accommodate
What is an umbilical hernia?
When the rectus abdominis (muscle) fails to fuse around the umbilicus after the intestines have retuned to the abdomen. Skin forms over the top
What is omphalocele?
Failure of intestinal loops to return into abdomen - hernia covered in amnion
What is the cause of omphalocele?
Unknown, but related to maternal obesity, alcohol/tobacco, antidepressant use
What is gastrochisis?
Failure of the ventral body wall to fuse (no covering on the GI tract)
What is gastroschisis associated with?
Young maternal age, low maternal BMI and recreational drug use
If the yolk duct persists, where is it attached?
Ileum, near the ileocecal junction- apex of midgut loop (2ft from ileocecal junction)
What are the symptoms of meckels diverticulum?
Usually asymptomatic- can get inflamed (indistinguishable from acute appendicitis)
What is meckels diverticulum?
Persistence of the yolk duct
What happens if the meckels diverticulum contains ectopic gastric cells?
Ulceration and lower GI bleeding
What happens if the meckels diverticulum is connected to the umbilicus by a ligament?
- gut rotation can cause a volvulus
- umbilical fistula
What is a volvulus?
Strangulation- like a ballooning effect
What’s an umbilical fistula?
Connection from one epithelial surface to another- means the contents can leak out
What are the symptoms of Hirschsprung’s disease?
Dilation of sections of the the colon, with lack of tone and peristalsis
What is hirschprungs disease caused by?
Lack of neural crest cells
What are neural crest cells?
Move through the embryo to give rise to the enteric ganglia
Where do the occipitocervical (vagal) region populate?
Whole gut
Where do neural crest cells from the sacral region populate?
Distal gut
What is the cloaca?
Transient common end of the digestive and urogenital systems including the base of the urogenital sinus
What is the cloaca split by (and when in development)?
Urorectal septum at 7-8 weeks
