Development of the GI tract Flashcards
When is folic acid needed?
→ In the first trimester
What is folic acid used for?
→helps with the fusion and the closure of the spinal cord.
What is spina bifida?
→when the spinal cord herniates out of the back.
When are the primary germ layers formed?
→ During the process of gastrulation
What happens at the beginning of the third week?
→embryo has implanted into the uterine wall
What is the structure of the embryo at 3 weeks?
→The embryo is a flat disc comprised of two cell layers
→Epiblast
→Hypoblast
What do the epiblast cells in the midline of the embryo start to do?
→ Begin to ingress starting from the caudal end
→Visible as the primitive streak
What do ingressing cells differentiate into and what do they surround?
→differentiate into Mesoderm
→Surrounding muscles, connective tissue, mesentery and blood vessels
What does the epiblast give rise to?
→Ectoderm
What does gastrulation give rise to?
→three primary germ layers
→ Ectoderm
→ mesoderm
→endoderm
How is the gut tube formed?
→by folding of sheets of cells in two directions
What are the two ways in which the gut tube folds?
→Folding towards the midline along the cranial-caudal axis
→Folding towards the yolk sac at the cranial and caudal ends
What does the buccopharyngeal membrane form?
→ mouth and throat
What does the cloacal membrane form?
→ urogenital tract
What is the septum tranversum?
→ area where the diaphragm is
What does the somatic mesoderm form?
→ The muscles
What does the Splanchnic mesoderm form?
→ autonomic nervous system
What is the mesentery and what does it do?
→ a folded membrane that anchors the GI tract to the posterior abdominal wall.
→It prevents the organs falling due to gravity.
What is the primary gut tube made up of?
→sheet of endoderm, which makes the epithelia and glands
→surrounding mesoderm, which makes muscle and connective tissue (including mesentery)
How is part of the yolk sac cavity enclosed?
→Part of the yolk sac cavity is enclosed within the embryo by pinching-off the yolk sac to form a yolk stalk and balloon-like yolk sac.
What makes up the foregut?
→Pharynx →Oesophagus →Stomach →Cranial half of duodenum →Ampulla of Vater (joining of bile ducts and pancreatic duct)
What makes up the midgut?
→Caudal duodenum →Jejunum →Ileum →Caecum →Appendix →Ascending colon →Proximal ⅔ of transverse colon
What makes up the hindgut?
→Distal ⅓ of transverse colon
→Descending colon
→Rectum
What artery supplies the foregut?
→ Celiac artery
What artery supplies the midgut?
→ Superior mesenteric artery
What artery supplies the hindgut?
→ Inferior mesenteric artery
What is the gut surrounded by?
→plexus of blood vessels joining vitelline (relating to or associated with the yolk of an egg) vessels to aorta
What does the plexus resolve to form?
→Plexus resolves to form the arteries that supply the GI tract from the aorta
How many arterial branches are there to the thoracic oesophagus?
→5
What innervates the foregut?
→ celiac ganglion
What innervates the midgut?
→ superior mesenteric ganglion
What innervates the hindgut?
→ inferior mesenteric ganglion
How does the stomach arise?
→ expansion
→rotation
What is the stomach initially?
→concave ventral
→convex dorsal
What happens at week 4?
→at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen
What establishes the curvature of the stomach?
→90o turn about craniocaudal axis
→The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach
How is the dorsal wall of the stomach attached?
→Attached to body by mesentery
→The dorsal mesogastrium (will form greater omentum)
How is the ventral wall of the stomach attached?
→attached by ventral mesentery, which includes the liver (will form lesser omentum)
What is the longest nerve in the body?
→ Sciatic nerve
What does the sciatic nerve follow?
→ Most of the oeosophagus
What is the peritoneum for?
→ Membrane designed to protect organs
What are the two types of peritoneum?
→Parietal and visceral peritoneum
What does the falciform ligament do?
→falciform ligament keeps the liver in place
What does the omentum do?
→Omentum protects the bowel from getting inflamed
What happens as the stomach rotates?
→ The dorsal mesogastrium is drawn with it
What does the mesogastrium enclose?
→ a space
→ the omental bursa
What does the folded mesogastrium do?
→ Grows to form the greater omentum
→The folds fuse and obliterate the bursa
What are retroperitoneal structures?
→oesophagus
→ rectum
→ kidney
Where does the greater omentum extend from?
→ Greater curvature
Where does the lesser omentum come from?
→ Lesser curvature
What is the epiploic foramen?
→ passage between the greater and lesser sac
→ allows communication
What is gastric outlet obstruction caused by?
→ Smooth muscle hypertrophy
What happens with pyloric stenosis?
→Projectile vomiting shortly after feeding
→Pyloric channel elongation ‘railroad track’
What does the pylorus do?
→ Controls what can enter the duodenum
What are the dimensions in pyloric stenosis?
→L > 16mm
→Wall > 4mm
→Diameter > 14mm
What is the inducing signal for the liver?
→heart to ventral gut endoderm
Where does the hepatic diverticulum grow?
→grows into mesenchyme of septum transversum
What parts of the liver proliferate?
→ Chords of hepatic endoderm
→ Bile drainage ducts
→ Blood vessels: arranged as sinusoids
How does the liver expand?
→ Liver exceeds size of septum transversum and expands into ventral mesentery.
What does the remaining ventral mesentery give rise to?
→Falciform ligament between liver and body wall
→Lesser omentum between liver and stomach
What are the two pancreatic buds and what are they induced by?
→Dorsal from duodenal endoderm (induced by notochord)
→Ventral from hepatic diverticulum (induced by hepatic mesoderm)
What happens as the duodenum rotates?
→ventral and dorsal buds meet and fuse
How does annular pancreas occur?
→develops from bi-lobed ventral buds that migrate in opposite directions.
What can an annular pancreas do?
→Annular pancreas can constrict the duodenum
How are the intestines attached?
→Attached throughout length by dorsal mesentery
What leads to the folding of the gut?
→Mesentery and gut grow at different rates, leading to folding of the gut
What branch of the aorta supplies the midgut?
→Ventral branch of the aorta supplies the midgut : superior mesenteric artery
What do the intestines rotate around?
→With a rapid increase in length the intestines rotate around the SMA
Why do the intestines herniate?
→Abdomen is too small to accommodate so it herniates into the umbilical stalk at 6-7 weeks
When do the intestines return?
→By 10 weeks the abdomen is bigger and the intestines
return.
What is umbilical hernia?
→Intestines return normally but rectus abdominis fails to fuse around umbilicus : gut covered in skin
What is omphalocele?
→Failure of intestinal loops to return into abdomen (associated with obesity, alcohol/tobacco or SSRI)
What is gastroschisis?
→Failure of ventral body wall to fuse : no covering
What is the persistence of the yolk duct?
→Most common intestinal abnormality
→Yolk duct attached to ileum near ileocecal junction - apex of midgut loop
What can Meckel’s diverticulum cause?
→Gut rotation causes volvulus
→Can form umbilical fistula
What can Meckel’s diverticulum contain and what does this cause?
→ ectopic gastric cells : ulceration and lower GI bleeding
What is Hirschsprung’s disease?
→Aganglionic megacolon (no ganglia) →Primarily affects the hindgut →Absence of parasympathetic ganglia →Caused by lack of neural crest cells →Ganglia present in dilated/hypertrophic region →Aganglionic segment shows contraction
What does Hirschsprung’s disease cause?
→Dilatation of sections of the colon, with lack of tone and peristalsis, leading to profound constipation
What is the cloacal membrane?
→ Transient common end of the digestive and urogenital systems including the base of the allantois (urogenital sinus)
What is the cloaca covered by?
→Covered by cloacal (proctodeal) membrane over ectoderm depression
What is the cloaca split by and what does this give rise to?
→Split by the urorectal septum
→This gives rise to the urogenital membrane and anal membrane (perforate at 7-8 weeks)
What can imperforate anus be?
→Persistence of anal membrane
→Atresia of anal canal, rectum or both
