Development of the GI Tract Flashcards
How does a blastocyst form?
Primary germ layer formation
The primary germs layers are formed during the process of 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
Epiblast cells in the mid-line of the embryo begin to ingress (invaginate), starting from the caudal end; visible as the primitive streak
Ingressing cells differentiate into Mesoderm; surrounding muscles, connective tissue and mesenteries and blood vessels
Epiblast divides into the Ectoderm (neural crest); innervation of the gut (Enteric NS)
Hypoblast (and epiblast) gives rise to Endoderm; epithelium of the gut tube and glands
This is what this process of gastrulation is; we have these epiblasts forming ectoderm on the outside, mesoderm invaginating and coming in one the inside and endoderm round to help to supply tissue sites for the lumen
How is the gut folded?
The gut tube is formed by folding the sheets of the cells in two direction
Folding towards the midline along the cranial-caudal axis
Folding towards the yolk sac at the cranial and caudal ends
Think of a gut as a tube, because it is a tube…
What features does the (sagittal view) of gut folding have?
Within the gut folding itself, at the cranial end we have all these things happening such as the buccopharyngeal membrane, septum transversum etc. all happening simultaneously
The cloacal membrane is important when we look at the hindgut
How is the gut formed?
The embryo is initially a solid flat disk attached to the hemispherical yolk sac (and 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, within that gut opening
Surrounding mesoderm, which makes muscle and connective tissue (including mesentery)
How is the primary gut tube divided?
Foregut: - Pharynx - Oesophagus - Stomach - Cranial half of duodenum - Ampulla of Vater- (joining of common bile duct and pancreatic duct) Midgut: - Caudal duodenum (From duodenal papilla ) - Jejunum - Ileum - Caecum - Appendix - Ascending colon - Proximal 2/3 of transverse colon Hindgut: - Distal 1/3 of transverse colon - Descending colon - Rectum Separated like this because each one has a different major blood supply
What are the different arterial supplies to different regions of your gut?
The celiac artery is your first big anterior branch as you enter the abdominal cavity; that will give off many branches including to your foregut
The mesentery is a layer of peritoneum, a membranous layer that enwraps and engulfs the organs
The mesenteric artery supplies your midgut
The inferior mesenteric artery supplies everything else; your hindgut
What arteries define the borders of the gut?
Arterial blood supply to the GI tract
Gut surrounded by plexus of blood vessels, joining vitelline (relating to, or associated with the yolk of an egg) vessels to aorta
Plexus resolves to form the arteries that supply the GI tract from the aorta
These define the boundaries of the gut:
- 5 arterial branches to the thoracic oesophagus (from the descending aorta)
- Celiac artery to the foregut
- Superior mesenteric artery to the midgut
- Inferior mesenteric artery to the hindgut
Why does sympathetic ganglia develop next to major branches of the aorta?
Need a nervous supply for the ‘rest and digest’
Post-ganglionic sympathetic axons innervate the same tissues that the arteries supply with blood
Celiac ganglion- foregut
Superior mesenteric ganglion- midgut
Inferior mesenteric ganglion- hindgut
How does the stomach develop?*
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
- 900 turn about cranio-caudal axis
- The dorsal border grows more rapidly than the ventral, which establishes the greater curvature of the stomach
Dorsal wall of the stomach attached to the body by mesentery; the dorsal mesogastrium (will form greater omentum)
Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)
Stomach expansion and rotation*
Why me
How are the stomach and liver attached to the peritoneum?*
We all have attached parts of your organs to the peritoneum
Dorsal (behind) mesogastrium attaches stomach to peritoneum then the lesser omentum, between the liver and stomach
Attached to that lesser curvature
From the liver we have a remnant called the falciform ligament; attaches directly to the anterior wall (keeps the liver in place)
How does the dorsal mesogastrium change with stomach development?
As the stomach rotates, the dorsal mesogastrium is drawn to it
This mesogastrium encloses a space, the omental bursa (lesser sac)
The folded mesogastrium grows to form the greater omentum, the folds fusing to obliterate the bursa
Double folded peritoneum
What are the different types of attachement to the peritoneum?
Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in place
There are aspects that are not actually covered by this peritoneum, those aspects are called extraperitoneal like the kidneys
Secondary means some parts are covered and parts aren’t
The rectum and anus are retroperitoneal
What is pyloric stenosis?
An abnormality of foregut development- pyloric stenosis
Gastric outlet obstruction caused by smooth muscle hypertrophy- so that smooth muscle layer has thickened
3 in 1000 incidence
Projectile vomiting shortly after feeding (not bile-stained)
Pyloric channel elongation “railroad track”
L > 16mm, wall >4mm, diameter >14mm
(This also closes when you are vomiting)
What happens in the case of organ budding from the foregut- pancreas?
Organ budding from the foregut- 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 (pentagonal shaped structures in the liver)
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
