Development of GI tract Flashcards
When does GI tract development occur
During 3rd to 12th week of embryonic age
In chronological order what is formed
Primary germ layer first
Then the gut tube formation
Then regionalised changes caused by rotation, swelling and elongation
How are organs and glands produced
By budding from the gut tube
What is the blastocyst made up of
Outer layer of cells = TROPHECTODERM/TROPHOBLAST
Middle = INNER CELL MASS (ICM)
What can the inner cell mass differentiate into
Epiblast
Hypoblast
Embryo is a flat disc comprised of 2 cell layers
Epiblast
Hypoblast
When are the primary layers formed
In grastulation
Ingressing cells give rise to
Mesoderm
Epiblast give rise to
Ectoderm
Hypoblast give rise to
Endoderm
How is the gut tube formed
Folding of sheets of cells in 2 directions
1) Folding towards the midline along the cranial caudal axis
2) Folding towards the yolk sac at the cranial caudal ends
What mesoderms surround the gut
Somatic mesoderm
Splanchnic mesoderm
Steps for the formation of gut tube
1) Embryo initially a solid flat disk attached to hemispherical yolk sac
2) Part of yolk sac cavity enclosed within the embryo by pinching off the yolk sac to form a yolk stalk and balloon like yolk sac
3) Within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the prochordal and cloacal plates
4) The primary gut tube made up of :
A sheet of endoderm which makes up the epithelia and glands. And surrounding mesoderm which makes muscle and connective tissue
Regions in foregut
Pharynx to Ampulla of Vater
Regions in midgut
Caudal duodenum to Proximal 2/3 of transverse colon
Regions in hindgut
Distal 1/3 of transverse colon to rectum
Where is the foregut arterial supply from
Celiac artery
Where is the midgut arterial supply from
Superior mesenteric artery
Where is the hindgut arterial supply from
Inferior mesenteric artery
What innervates the foregut
Celiac ganglion
What innervates the mid gut
Superior mesenteric ganglion
What innervates the hind gut
Inferior mesenteric ganglion
Abnormality of foregut development - Pyloric stenosis
Gastric outlet obstruction
Caused by smooth muscle hypertrophy - smooth muscle forming the pyloric sphincter overgrows
Causes projectile vomitting
Foregut stomach 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
- 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)
Mesogastrium
- As 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
Bursa
Protective layer, contains WBC to prevent infection if any gastric intestinal fluid spillages
Organ budding from the foregut - forming 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
Organ budding from foregut - forming of the pancreas
- Two pancreatic buds
1) Dorsal from duodenal endoderm (induced by notochord)
2) Ventral from hepatic diverticulum (induced by hepatic mesoderm) - As duodenum rotates, ventral and dorsal buds meet and fuse
- If ventral bud bifid (bi-lobed), and one rotates around duodenum, annular pancreas forms, which can obstruct duodenum
Formation of midgut intestines
- Attached throughout length by the dorsal mesentery – mesentery and gut grow at different rates leading to stereotypical folding of the gut
- Ventral branch of the aorta supplies the mid gut – superior mesenteric artery (SMA)
- Intestines rotate by 90 degrees around the SMA
- Abdomen is too small to accommodate, so herniates into umbilical stalk at 6 or 7 weeks – They rotate by 180 degrees
- By 10 weeks, the abdomen is bigger, and intestines return
3 kinds of hernia that can occur in new borns
Umbilical hernia
Omphalocele
Gastroschisis
Umbilical hernia
Intestines return normally but rectus abdominis fails to fuse around umbilicus - gut covered in skin
Omphalocele
Failure of intestinal loops to return to abdomen, hernia covered in amnion
Gastroschisis
Failure of the ventral body to fuse, no covering
Examples of more abnormalities of intestinal development
Persistence of yolk duct
Meckel’s diverticulum
Hirschprung’s disease
Persistence of yolk duct
Most common intestinal abnormality
Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop
Meckel’s diverticulum
- Usually asymptomatic
- May contain ectopic gastric cells: ulceration and lower GI bleeding
- Can be connected to umbilicus by ligament:
- Gut rotation causes volvulus - strangulation
Can form umbilical fistula - contents leak out
Hirschprung 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
Neural crest cells
- Neural crest cells populate developing gut and give rise to enteric ganglia
- Neural crest cells from occiptocervical region populate the entire gut
- Neural crest cells from sacral region populate the distal gut
Anal development - the CLOACA
- Cloaca – transient common end of the digestive and urogenital systems, including the base of the allantois (urogenital sinus)
- Covered by the cloacal membrane over an ectoderm depression, the proctodeum
- Split by the urorectal septum – give rise to urogenital membrane and anal membrane
- Imperforate anus can be:
- Persistence of an anal membrane
- Artesia of anal canal, rectum or both
3 steps to forming of the anal
1) Invagination
2) Septum formation
3) Separation
