Development of the GI tract Flashcards
Give an overview of the development of the GI tract
GI tract development occurs during 3rd to 12th weeks embryonic age (5 to 14 weeks LMP)
Primary Germ Layer Formation
Gut tube formation
Regionalised changes caused by rotation, swelling, elongation
Organs and glands produced by budding from the gut tube
Describe the formation of the zygote
- Zygote to 16th cell
2. We then have a blastocyte including the inner cell mass, epiblast and hypoblast and trophectoderm
Describe the process of primary germ layer formation - 1
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:
1. Epiblast
2. Hypoblast
Describe the process of primary germ layer formation - 2
What does the epiblast do and what does it form?
What does the Mesoderm form?
What does the epiblast form?
What does the hypoblast form?
Epiblast cells in the mid-line of the embryo begin to ingress, 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 gives rise to Ectoderm (neural crest)
innervation of the gut (Enteric NS)
Hypoblast (and epiblast) gives rise to Endoderm
epithelium of the gut tube and glands
What is the purpose of gastrulation?
Gastrulation generates the three primary germ layers
Describe the process of initial gut folding and tube formation
What does the somatic mesoderm do?
What does the splanchnic mesoderm do?
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
Think of the Gut as a tube
The gut tube forms at 12-30 days. Look at slides of the structure of it.
Somatic mesoderm = voluntary
Splanchnic mesoderm = gut motility, nervous system
Give an overview of formation of the gut tube?
What is the gut tube made out of?
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)
Describe the regionalization and subdivision of the primary gut tube
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
How does arterial supply defines regions of the gut?
Foregut is up to the celiac artery, it branches anteriorly, and other branches going to the liver, spleen and stomach. Mouth to the second part of the duodenum
The midgut is up to the mesenteric artery. Third part of the duodenum to 2/3rds or transverse colon.
Hindgut is up to the mesenteric artery.
Describe the arterial blood supply to the GI tract
What does the arterial blood supply develop into?
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
How does sympathetic ganglia develop next to major branches of the aorta?
Post-ganglionic, sympathetic axons innervate the the same tissues that the arteries supply with blood
Celiac ganglion – foregut
Superior mesenteric ganglion – midgut
Inferior mesenteric ganglion – hindgut
Describe the development of the foregut - Stomach development
What does it arise by?
When does it arise and where?
What does the stomach attach to and form?
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:
- 90 degree 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)
Describe the mesenteric attachments from the stomach to the liver
Mesenteric arteries are arteries of the small intestine
It connects the liver and the stomach
Describe the foregut development
As the stomach rotates, the dorsal mesogastrium is drawn with 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
What is peritoneum?
The serous double membrane lining the cavity of the abdomen and covering the abdominal organs.
Not all organs are covered by it.
What is Pyloric Stenosis?
What does it cause?
What is the incidence level?
What occurs as a result of this abnormality?
An abnormality of foregut development.
Gastric outlet obstruction caused by smooth muscle hypertrophy
3 in 1000 incidence
Projectile vomiting shortly after feeding (not bile-stained)
Pyloric channel elongation, “railroad track”
L >16mm, wall >4mm, diameter >14mm
Describe Organ budding from the foregut - liver
What initiates it?
What does the Hepatic diverticulum grow?
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 sinusoid
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
Describe organ budding from the foregut - 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
Describe the formation of the midgut: intestines
Attached throughout length by dorsal mesentery (but not ventral mesentery)
The mesentery and gut grow at different rates, leading to stereotypical folding of the gut
With a very rapid increase in length, the intestines rotate around the SMA
Abdomen is too small to accommodate, so herniates into umbilical stalk at 6 or 7 weeks
By 10 weeks, the abdomen is bigger, and the intestines return
What is an Umbilical hernia?
Intestines return normally, but rectus abdominis fails to fuse around umbilicus: gut covered in skin
What is Omphalocele?
What causes it?
Failure of intestinal loops to return into abdomen:
hernia covered in amnion
(causes unknown, but associated with maternal obesity, alcohol/tobacco, SSRI use)
What is Gastroschisis?
What causes it?
Failure of ventral body wall to fuse: no covering Increasing incidence (1 in 3000). Marked association with young maternal age, low maternal BMI, recreational drugs (especially cocaine)
Give some more abnormalities of intestinal (midgut) development
Persistence of yolk duct (normally obliterated)
Most common intestinal abnormality
Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop (about 2 feet from ileocaecal junction)
Meckel’s diverticulum (2-4% population) usually asymptomatic
Can get inflamed (Meckel’s diverticulitis clinically indistinguishable from acute appendicitis)
May contain ectopic gastric cells: ulceration and lower GI bleeding
Can be connected to umbilicus by ligament
Gut rotation causes volvulus
Can form umbilical fistula
Abnormalities of intestinal development: Hirschsprung’s disease - describe this disease
Aganglionic megacolon
Primarily affects the hindgut
Dilatation of sections of the colon, with lack of tone and peristalsis, leading to profound constipation
Absence of parasympathetic ganglia
Caused by lack of neural crest cells
Describe the development of the hindgut: Anal development - the cloaca
The cloaca is the transient common end of digestive and urogenital systems, including the base of the allantois (urogenital sinus)
Covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum
Split by the urorectal septum
This gives rise to the urogenital membrane and anal membrane (perforate at 7-8 weeks)
Imperforate anus can be:
- Persistence of anal membrane
- Atresia of anal canal, rectum or both
