Development of GI tract Flashcards
By what process are the primary germ layers formed?
Gastrulation - At the beginning of the third week, the embryo has implanted into the uterine wall.
The embryo is a flat disc, comprised of two cell layers:
Epiblast
Hypoblast
What are the 4 stages of gatrulation?
1) Epiblast cells in the mid-line of the embryo begin to ingress, starting from the caudal end - Visible as the Primitive Streak
2) Ingressing cells differentiate into Mesoderm - surrounding muscles, connective tissue and mesenteries and blood vessels.
3) Epiblast gives rise to Ectoderm (neural crest) - innervation of the gut (Enteric NS)
4) Hypoblast (and epiblast) gives rise to Endoderm - epithelium of the gut tube and glands
What occurs during the initial gut folding and tube formation?
The gut tube is formed by folding of sheets of cells in two directions
Folding towards the midline along the cranial-caudal axis
Folding towards the yolk sac at the cranial and caudal ends.
Summarise the formation of the gut tube.
- The embryo is initially a solid flat disk attached to the hemispherical yolk sac (& similarly to amnion).
- 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
- Within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the prochordal and cloacal plates
- 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)
What does the foregut consist of?
- Pharynx
- Oesophagus
- Stomach
- Cranial half of duodenum
- Ampulla of Vater
(joining of common bile duct and pancreatic duct)
What does the midgut consist of?
- Caudal duodenum (From duodenal papilla )
- Jejunum
- Ileum
- Caecum
- Appendix
- Ascending colon
- Proximal 2/3 of transverse colon
What does the hindgut consist of?
- Distal 1/3 of transverse colon
- Descending colon
- Rectum
What supplies the thoracic oesophagus with blood?
Arterial branches from descending aorta.
What supplies the foregut with blood?
Celiac artery
What supplies the midgut with blood?
The superior mesenteric artery.
What supplies the hindgut with blood?
The inferior mesenteric artery.
What sypathetic ganglia innervate each part of the gut?
- Celiac ganglion – foregut
- Superior mesenteric ganglion – midgut
- Inferior mesenteric ganglion – hindgut
Summarise gut development.
- Stomach arises by expansion and rotation
- During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen
- Initially concave ventral, convex dorsal
- 90o turn about cranio-caudal axis
- The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach
- Dorsal wall of stomach attached to body by mesentery: the dorsal mesogastrium (will form greater omentum)
- Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)
Summerise the formation of the liver.
- Inducing signal: heart to ventral gut endoderm
- Hepatic diverticulum grows into mesenchyme of septum transversum
- Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids
- Liver exceeds size of septum transversum, expands into ventral mesentery
- Remaining ventral mesentery gives rise to:
- falciform ligament between liver and body wall
- lesser omentum between liver and stomach
Summarise the formation of the pancreas.
- Two pancreatic buds
- Dorsal from duodenal endoderm (induced by notochord)
- Ventral from hepatic diverticulum (induced by hepatic mesoderm)
- As duodenum rotates, ventral and dorsal buds meet and fuse
